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C++ Programming Code Examples

C++ > Code Snippets Code Examples

Throwing Multiple Basic

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/* Throwing Multiple Basic */ #include <fstream> #include <iostream> #include <vector> #include <string> #include <exception> using namespace std; void readIntegerFile(const string& fileName, vector<int>& dest) { ifstream istr; int temp; istr.open(fileName.c_str()); if (istr.fail()) { throw exception(); } while (istr >> temp) { dest.push_back(temp); } if (istr.eof()) { istr.close(); } else { istr.close(); throw exception(); } } int main(int argc, char** argv) { vector<int> myInts; const string fileName = "IntegerFile.txt"; try { readIntegerFile(fileName, myInts); } catch (const exception& e) { cout << "Unable either to open or to read " << fileName << endl; exit (1); } for (size_t i = 0; i < myInts.size(); i++) { cout << myInts[i] << " "; } cout << endl; return (0); }
main() Function in C++
A program shall contain a global function named main, which is the designated start of the program in hosted environment. main() function is the entry point of any C++ program. It is the point at which execution of program is started. When a C++ program is executed, the execution control goes directly to the main() function. Every C++ program have a main() function.
Syntax for main() Function in C++
void main() { ............ ............ }
void
void is a keyword in C++ language, void means nothing, whenever we use void as a function return type then that function nothing return. here main() function no return any value.
main
main is a name of function which is predefined function in C++ library. In place of void we can also use int return type of main() function, at that time main() return integer type value. 1) It cannot be used anywhere in the program a) in particular, it cannot be called recursively b) its address cannot be taken 2) It cannot be predefined and cannot be overloaded: effectively, the name main in the global namespace is reserved for functions (although it can be used to name classes, namespaces, enumerations, and any entity in a non-global namespace, except that a function called "main" cannot be declared with C language linkage in any namespace). 3) It cannot be defined as deleted or (since C++11) declared with C language linkage, constexpr (since C++11), consteval (since C++20), inline, or static. 4) The body of the main function does not need to contain the return statement: if control reaches the end of main without encountering a return statement, the effect is that of executing return 0;. 5) Execution of the return (or the implicit return upon reaching the end of main) is equivalent to first leaving the function normally (which destroys the objects with automatic storage duration) and then calling std::exit with the same argument as the argument of the return. (std::exit then destroys static objects and terminates the program). 6) (since C++14) The return type of the main function cannot be deduced (auto main() {... is not allowed). 7) (since C++20) The main function cannot be a coroutine.
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/* simple code example by main() function in C++ */ #include <iostream> using namespace std; int main() { int day = 4; switch (day) { case 1: cout << "Monday"; break; case 2: cout << "Tuesday"; break; case 3: cout << "Wednesday"; break; case 4: cout << "Thursday"; break; case 5: cout << "Friday"; break; case 6: cout << "Saturday"; break; case 7: cout << "Sunday"; break; } return 0; }
#include Directive in C++
#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program. This directive is read by the preprocessor and orders it to insert the content of a user-defined or system header file into the following program. These files are mainly imported from an outside source into the current program. The process of importing such files that might be system-defined or user-defined is known as File Inclusion. This type of preprocessor directive tells the compiler to include a file in the source code program.
Syntax for #include Directive in C++
#include "user-defined_file"
Including using " ": When using the double quotes(" "), the preprocessor access the current directory in which the source "header_file" is located. This type is mainly used to access any header files of the user's program or user-defined files.
#include <header_file>
Including using <>: While importing file using angular brackets(<>), the the preprocessor uses a predetermined directory path to access the file. It is mainly used to access system header files located in the standard system directories. Header File or Standard files: This is a file which contains C/C++ function declarations and macro definitions to be shared between several source files. Functions like the printf(), scanf(), cout, cin and various other input-output or other standard functions are contained within different header files. So to utilise those functions, the users need to import a few header files which define the required functions. User-defined files: These files resembles the header files, except for the fact that they are written and defined by the user itself. This saves the user from writing a particular function multiple times. Once a user-defined file is written, it can be imported anywhere in the program using the #include preprocessor. • In #include directive, comments are not recognized. So in case of #include <a//b>, a//b is treated as filename. • In #include directive, backslash is considered as normal text not escape sequence. So in case of #include <a\nb>, a\nb is treated as filename. • You can use only comment after filename otherwise it will give error.
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/* using #include directive in C language */ #include <stdio.h> int main() { /* * C standard library printf function * defined in the stdio.h header file */ printf("I love you Clementine"); printf("I love you so much"); printf("HappyCodings"); return 0; }
Vector Library push_back() Function in C++
Add element at the end. Adds a new element at the end of the vector, after its current last element. The content of val is copied (or moved) to the new element. This effectively increases the container size by one, which causes an automatic reallocation of the allocated storage space if -and only if- the new vector size surpasses the current vector capacity. push_back() function is used to push elements into a vector from the back. The new value is inserted into the vector at the end, after the current last element and the container size is increased by 1.
Syntax for Vector push_back() Function in C++
#include <vector> void push_back (const value_type& val); void push_back (value_type&& val);
val
Value to be copied (or moved) to the new element. Member type value_type is the type of the elements in the container, defined in vector as an alias of its first template parameter (T). This function does not return any value. If a reallocation happens, the storage is allocated using the container's allocator, which may throw exceptions on failure (for the default allocator, bad_alloc is thrown if the allocation request does not succeed).
Complexity
Constant (amortized time, reallocation may happen). If a reallocation happens, the reallocation is itself up to linear in the entire size.
Iterator validity
If a reallocation happens, all iterators, pointers and references related to the container are invalidated. Otherwise, only the end iterator is invalidated, and all iterators, pointers and references to elements are guaranteed to keep referring to the same elements they were referring to before the call.
Data races
The container is modified. If a reallocation happens, all contained elements are modified. Otherwise, no existing element is accessed, and concurrently accessing or modifying them is safe.
Exception safety
If no reallocations happen, there are no changes in the container in case of exception (strong guarantee). If a reallocation happens, the strong guarantee is also given if the type of the elements is either copyable or no-throw moveable. Otherwise, the container is guaranteed to end in a valid state (basic guarantee). If allocator_traits::construct is not supported with val as argument, it causes undefined behavior.
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/* vector::push_back() is a library function of "vector" header, it is used to insert/add an element at the end of the vector, it accepts an element of the same type and adds the given element at the end of the vector and increases the size of the vector. */ //C++ STL program code example to demonstrate example of vector::push_back() function #include <iostream> #include <vector> using namespace std; int main() { //vector declaration vector<int> v1; //inserting elements and printing size cout << "size of v1: " << v1.size() << endl; v1.push_back(10); cout << "size of v1: " << v1.size() << endl; v1.push_back(20); v1.push_back(30); v1.push_back(40); v1.push_back(50); cout << "size of v1: " << v1.size() << endl; //printing all elements cout << "elements of vector v1..." << endl; for (int x : v1) cout << x << " "; cout << endl; return 0; }
IOS Library eof() Function in C++
Check whether eofbit is set. Returns true if the eofbit error state flag is set for the stream. This flag is set by all standard input operations when the End-of-File is reached in the sequence associated with the stream. Note that the value returned by this function depends on the last operation performed on the stream (and not on the next). Operations that attempt to read at the End-of-File fail, and thus both the eofbit and the failbit end up set. This function can be used to check whether the failure is due to reaching the End-of-File or to some other reason.
Syntax for IOS eof() Function in C++
bool eof() const;
This function does not accept any parameter. Function returns true if the stream's eofbit error state flag is set (which signals that the End-of-File has been reached by the last input operation). false otherwise.
Data races
Accesses the stream object. Concurrent access to the same stream object may cause data races.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the stream.
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/* The eof() method of ios class in C++ is used to check if the stream is has raised any EOF (End Of File) error. It means that this function will check if this stream has its eofbit set. */ // C++ code example to demonstrate the working of eof() function #include <iostream> #include <fstream> int main () { std::ifstream is("example.txt"); char c; while (is.get(c)) std::cout << c; if (is.eof()) std::cout << "[EoF reached]\n"; else std::cout << "[error reading]\n"; is.close(); return 0; }
String Library c_str() Function in C++
Get C string equivalent. Returns a pointer to an array that contains a null-terminated sequence of characters (i.e., a C-string) representing the current value of the string object. The basic_string::c_str() is a builtin function in C++ which returns a pointer to an array that contains a null-terminated sequence of characters representing the current value of the basic_string object. This array includes the same sequence of characters that make up the value of the basic_string object plus an additional terminating null-character at the end. This array includes the same sequence of characters that make up the value of the string object plus an additional terminating null-character ('\0') at the end.
Syntax for String c_str() Function in C++
#include <string> const char* c_str() const noexcept;
This function does not accept any parameter. Function returns a pointer to the c-string representation of the string object's value. The pointer returned points to the internal array currently used by the string object to store the characters that conform its value. Both string::data and string::c_str are synonyms and return the same value. The pointer returned may be invalidated by further calls to other member functions that modify the object.
Complexity
Constant
Iterator validity
No changes
Data races
The object is accessed
Exception safety
No-throw guarantee: this member function never throws exceptions.
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/* c_str() function returns a pointer to an array that contains a null-terminated sequence of characters (i.e., a C-string) representing the current value of the string object. */ /* Get C string equivalent by c_str() function code example */ #include <bits/stdc++.h> #include <string> using namespace std; int main() { // declare a example string string s1 = "HappyCodings"; // check if the size of the string is same as the // size of character pointer given by c_str if (s1.size() == strlen(s1.c_str())) { cout << "s1.size is equal to strlen(s1.c_str()) " << endl; } else { cout << "s1.size is not equal to strlen(s1.c_str())" << endl; } // print the string printf("%s \n", s1.c_str()); }
Exceptions Try/Catch in C++
When executing C++ code, different errors can occur: coding errors made by the programmer, errors due to wrong input, or other unforeseeable things. When an error occurs, C++ will normally stop and generate an error message. The technical term for this is: C++ will throw an exception (throw an error). An exception is a problem that arises during the execution of a program. A C++ exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero. Exceptions provide a way to transfer control from one part of a program to another. C++ exception handling is built upon three keywords: try, catch, and throw. The try and catch keywords come in pairs:
Syntax for Try/Catch Statement in C++
try { // Block of code to try throw exception; // Throw an exception when a problem arise } catch () { // Block of code to handle errors }
try
The try statement allows you to define a block of code to be tested for errors while it is being executed. The code which can throw any exception is kept inside(or enclosed in) atry block. Then, when the code will lead to any error, that error/exception will get caught inside the catch block.
throw
The throw keyword throws an exception when a problem is detected, which lets us create a custom error. It is used to throw exceptions to exception handler i.e. it is used to communicate information about error. A throw expression accepts one parameter and that parameter is passed to handler. throw statement is used when we explicitly want an exception to occur, then we can use throw statement to throw or generate that exception.
catch
The catch statement allows you to define a block of code to be executed, if an error occurs in the try block. catch block is intended to catch the error and handle the exception condition. We can have multiple catch blocks to handle different types of exception and perform different actions when the exceptions occur. For example, we can display descriptive messages to explain why any particular excpetion occured. If you do not know the throw type used in the try block, you can use the "three dots" syntax (...) inside the catch block, which will handle any type of exception:
try { int age = 15; if (age >= 18) { cout << "Access granted - you are old enough."; } else { throw 505; } } catch (...) { cout << "Access denied - You must be at least 18 years old.\n"; }
There are some standard exceptions in C++ under <exception> which we can use in our programs. They are arranged in a parent-child class hierarchy which is depicted below: • std::exception - Parent class of all the standard C++ exceptions. • logic_error - Exception happens in the internal logical of a program. domain_error - Exception due to use of invalid domain. invalid argument - Exception due to invalid argument. out_of_range - Exception due to out of range i.e. size requirement exceeds allocation. length_error - Exception due to length error. • runtime_error - Exception happens during runtime. range_error - Exception due to range errors in internal computations. overflow_error - Exception due to arithmetic overflow errors. underflow_error - Exception due to arithmetic underflow errors. • bad_alloc - Exception happens when memory allocation with new() fails. • bad_cast - Exception happens when dynamic cast fails. • bad_exception - Exception is specially designed to be listed in the dynamic-exception-specifier. • bad_typeid - Exception thrown by typeid.
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/* Simple code example to show exception handling in C++. The output of program explains flow of execution of try/catch blocks. */ #include <iostream> using namespace std; int main() { int x = -1; // Some code cout << "Before try \n"; try { cout << "Inside try \n"; if (x < 0) { throw x; cout << "After throw (Never executed) \n"; } } catch (int x ) { cout << "Exception Caught \n"; } cout << "After catch (Will be executed) \n"; return 0; }
For Loop Statement in C++
In computer programming, loops are used to repeat a block of code. For example, when you are displaying number from 1 to 100 you may want set the value of a variable to 1 and display it 100 times, increasing its value by 1 on each loop iteration. When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop. A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.
Syntax of For Loop Statement in C++
for (initialization; condition; update) { // body of-loop }
initialization
initializes variables and is executed only once.
condition
if true, the body of for loop is executed, if false, the for loop is terminated.
update
updates the value of initialized variables and again checks the condition. A new range-based for loop was introduced to work with collections such as arrays and vectors.
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/* For Loop Statement in C++ Language */ // C++ program to find the sum of first n natural numbers // positive integers such as 1,2,3,...n are known as natural numbers #include <iostream> using namespace std; int main() { int num, sum; sum = 0; cout << "Enter a positive integer: "; cin >> num; for (int i = 1; i <= num; ++i) { sum += i; } cout << "Sum = " << sum << endl; return 0; }
Namespaces in C++ Language
Consider a situation, when we have two persons with the same name, jhon, in the same class. Whenever we need to differentiate them definitely we would have to use some additional information along with their name, like either the area, if they live in different area or their mother's or father's name, etc. Same situation can arise in your C++ applications. For example, you might be writing some code that has a function called xyz() and there is another library available which is also having same function xyz(). Now the compiler has no way of knowing which version of xyz() function you are referring to within your code. A namespace is designed to overcome this difficulty and is used as additional information to differentiate similar functions, classes, variables etc. with the same name available in different libraries. Using namespace, you can define the context in which names are defined. In essence, a namespace defines a scope.
Defining a Namespace
A namespace definition begins with the keyword namespace followed by the namespace name as follows:
namespace namespace_name { // code declarations }
To call the namespace-enabled version of either function or variable, prepend (::) the namespace name as follows:
name::code; // code could be variable or function.
Using Directive
You can also avoid prepending of namespaces with the using namespace directive. This directive tells the compiler that the subsequent code is making use of names in the specified namespace.
Discontiguous Namespaces
A namespace can be defined in several parts and so a namespace is made up of the sum of its separately defined parts. The separate parts of a namespace can be spread over multiple files. So, if one part of the namespace requires a name defined in another file, that name must still be declared. Writing a following namespace definition either defines a new namespace or adds new elements to an existing one:
namespace namespace_name { // code declarations }
Nested Namespaces
Namespaces can be nested where you can define one namespace inside another name space as follows:
namespace namespace_name1 { // code declarations namespace namespace_name2 { // code declarations } }
• Namespace is a feature added in C++ and not present in C. • A namespace is a declarative region that provides a scope to the identifiers (names of the types, function, variables etc) inside it. • Multiple namespace blocks with the same name are allowed. All declarations within those blocks are declared in the named scope. • Namespace declarations appear only at global scope. • Namespace declarations can be nested within another namespace. • Namespace declarations don't have access specifiers. (Public or private) • No need to give semicolon after the closing brace of definition of namespace. • We can split the definition of namespace over several units.
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/* namespaces in C++ language */ // A C++ code to demonstrate that we can define // methods outside namespace. #include <iostream> using namespace std; // Creating a namespace namespace ns { void display(); class happy { public: void display(); }; } // Defining methods of namespace void ns::happy::display() { cout << "ns::happy::display()\n"; } void ns::display() { cout << "ns::display()\n"; } // Driver code int main() { ns::happy obj; ns::display(); obj.display(); return 0; }
Vectors in C++ Language
In C++, vectors are used to store elements of similar data types. However, unlike arrays, the size of a vector can grow dynamically. That is, we can change the size of the vector during the execution of a program as per our requirements. Vectors are part of the C++ Standard Template Library. To use vectors, we need to include the vector header file in our program.
Declaration for Vectors in C++
std::vector<T> vector_name;
The type parameter <T> specifies the type of the vector. It can be any primitive data type such as int, char, float, etc.
Initialization for Vectors in C++
// Vector initialization method 1 // Initializer list vector<int> vector1 = {1, 2, 3, 4, 5};
We are initializing the vector by providing values directly to the vector. vector1 is initialized with values 1, 2, 3, 4, 5.
// Vector initialization method 2 vector<int> vector3(5, 12);
Here, 5 is the size of the vector and 8 is the value. This code creates an int vector with size 5 and initializes the vector with the value of 8. So, the vector is equivalent to
vector<int> vector2 = {8, 8, 8, 8, 8};
The vector class provides various methods to perform different operations on vectors. Add Elements to a Vector: To add a single element into a vector, we use the push_back() function. It inserts an element into the end of the vector. Access Elements of a Vector: In C++, we use the index number to access the vector elements. Here, we use the at() function to access the element from the specified index. Change Vector Element: We can change an element of the vector using the same at() function. Delete Elements from C++ Vectors: To delete a single element from a vector, we use the pop_back() function. In C++, the vector header file provides various functions that can be used to perform different operations on a vector. • size(): returns the number of elements present in the vector. • clear(): removes all the elements of the vector. • front(): returns the first element of the vector. • back(): returns the last element of the vector. • empty(): returns 1 (true) if the vector is empty. • capacity(): check the overall size of a vector. Vector iterators are used to point to the memory address of a vector element. In some ways, they act like pointers.
Syntax for Vector Iterators in C++
vector<T>::iterator iteratorName;
We can initialize vector iterators using the begin() and end() functions. The begin() function returns an iterator that points to the first element of the vector. The end() function points to the theoretical element that comes after the final element of the vector.
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/* Vectors in C++ language */ // C++ program to illustrate the capacity function in vector #include <iostream> #include <vector> using namespace std; int main() { vector<int> myvector; for (int i = 1; i <= 5; i++) myvector.push_back(i); cout << "Size : " << myvector.size(); cout << "\nCapacity : " << myvector.capacity(); cout << "\nMax_Size : " << myvector.max_size(); // resizes the vector size to 4 myvector.resize(4); // prints the vector size after resize() cout << "\nSize : " << myvector.size(); // checks if the vector is empty or not if (myvector.empty() == false) cout << "\nVector is not empty"; else cout << "\nVector is empty"; // Shrinks the vector myvector.shrink_to_fit(); cout << "\nVector elements are: "; for (auto it = myvector.begin(); it != myvector.end(); it++) cout << *it << " "; return 0; }
Vector Library size() Function in C++
Return size. Returns the number of elements in the vector. This is the number of actual objects held in the vector, which is not necessarily equal to its storage capacity. vector::size() is a library function of "vector" header, it is used to get the size of a vector, it returns the total number of elements in the vector. The dynamic array can be created by using a vector in C++. One or more elements can be inserted into or removed from the vector at the run time that increases or decreases the size of the vector. The size or length of the vector can be counted using any loop or the built-in function named size().
Syntax for Vector size() Function in C++
#include <vector> size_type size() const noexcept;
This function does not accept any parameter. Function returns the number of elements in the container. Member type size_type is an unsigned integral type.
Complexity
Constant
Iterator validity
No changes
Data races
The container is accessed. No contained elements are accessed: concurrently accessing or modifying them is safe.
Exception safety
No-throw guarantee: this member function never throws exceptions.
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/* get the size of a vector, it returns the total number of elements in the vector by vector::size() library function. */ #include <bits/stdc++.h> using namespace std; int main() { // Initializing a vector of string type vector<string> vec = { "Happy", "8)", "Codings" }; // Clearing the vector // Now size is equal to 0 vec.clear(); // Typecasting vec.size() to int for (int i = 0; i < (int)vec.size() - 1; i++) cout << vec[i] << ' '; cout << "Happy8)Codings"; return 0; }
Ifstream open() Function in C++
Open file. Opens the file identified by argument filename, associating it with the stream object, so that input/output operations are performed on its content. Argument mode specifies the opening mode. If the stream is already associated with a file (i.e., it is already open), calling this function fails. The file association of a stream is kept by its internal stream buffer: Internally, the function calls rdbuf()->open(filename,mode) The function clears the stream's state flags on success (setting them to goodbit). In case of failure, failbit is set.
Syntax for open() Function in C++
#include <fstream> void open (const char* filename, ios_base::openmode mode = ios_base::in); void open (const string& filename, ios_base::openmode mode = ios_base::in);
filename
String with the name of the file to open. Specifics about its format and validity depend on the library implementation and running environment.
mode
Flags describing the requested i/o mode for the file. This is an object of the bitmask member type openmode that consists of a combination of the following member constants: • in * input File open for reading: the internal stream buffer supports input operations. • out output File open for writing: the internal stream buffer supports output operations. • binary binary Operations are performed in binary mode rather than text. • ate at end The output position starts at the end of the file. • app append All output operations happen at the end of the file, appending to its existing contents. • trunc truncate Any contents that existed in the file before it is open are discarded. These flags can be combined with the bitwise OR operator (|). * in is always set for ifstream objects (even if explicitly not set in argument mode). Note that even though ifstream is an input stream, its internal filebuf object may be set to also support output operations. If the mode has both trunc and app set, the opening operation fails. It also fails if trunc is set but out is not. Function returns none. If the function fails to open a file, the failbit state flag is set for the stream (which may throw ios_base::failure if that state flag was registered using member exceptions).
Data races
Modifies the ifstream object. Concurrent access to the same stream object introduce data races.
Exception safety
Basic guarantee: if an exception is thrown, the stream is in a valid state. It throws an exception of member type failure if the function fails (setting the failbit state flag) and member exceptions was set to throw for that state.
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/* ifstream data type represents the input file stream and is used to read information from files. */ // print the content of a text file. #include <iostream> // std::cout #include <fstream> // std::ifstream int main () { std::ifstream ifs; ifs.open ("test.txt", std::ifstream::in); char c = ifs.get(); while (ifs.good()) { std::cout << c; c = ifs.get(); } ifs.close(); return 0; }
If Else Statement in C++
In computer programming, we use the if statement to run a block code only when a certain condition is met. An if statement can be followed by an optional else statement, which executes when the boolean expression is false. There are three forms of if...else statements in C++: • if statement, • if...else statement, • if...else if...else statement,
Syntax for If Statement in C++
if (condition) { // body of if statement }
The if statement evaluates the condition inside the parentheses ( ). If the condition evaluates to true, the code inside the body of if is executed. If the condition evaluates to false, the code inside the body of if is skipped.
Syntax for If...Else Statement
if (condition) { // block of code if condition is true } else { // block of code if condition is false }
The if..else statement evaluates the condition inside the parenthesis. If the condition evaluates true, the code inside the body of if is executed, the code inside the body of else is skipped from execution. If the condition evaluates false, the code inside the body of else is executed, the code inside the body of if is skipped from execution. The if...else statement is used to execute a block of code among two alternatives. However, if we need to make a choice between more than two alternatives, we use the if...else if...else statement.
Syntax for If...Else...Else If Statement in C++
if (condition1) { // code block 1 } else if (condition2){ // code block 2 } else { // code block 3 }
• If condition1 evaluates to true, the code block 1 is executed. • If condition1 evaluates to false, then condition2 is evaluated. • If condition2 is true, the code block 2 is executed. • If condition2 is false, the code block 3 is executed. There can be more than one else if statement but only one if and else statements. In C/C++ if-else-if ladder helps user decide from among multiple options. The C/C++ if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the C else-if ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
Syntax for If Else If Ladder in C++
if (condition) statement 1; else if (condition) statement 2; . . else statement;
Working of the if-else-if ladder: 1. Control falls into the if block. 2. The flow jumps to Condition 1. 3. Condition is tested. If Condition yields true, goto Step 4. If Condition yields false, goto Step 5. 4. The present block is executed. Goto Step 7. 5. The flow jumps to Condition 2. If Condition yields true, goto step 4. If Condition yields false, goto Step 6. 6. The flow jumps to Condition 3. If Condition yields true, goto step 4. If Condition yields false, execute else block. Goto Step 7. 7. Exits the if-else-if ladder. • The if else ladder statement in C++ programming language is used to check set of conditions in sequence. • This is useful when we want to selectively executes one code block(out of many) based on certain conditions. • It allows us to check for multiple condition expressions and execute different code blocks for more than two conditions. • A condition expression is tested only when all previous if conditions in if-else ladder is false. • If any of the conditional expression evaluates to true, then it will execute the corresponding code block and exits whole if-else ladder.
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/* If Else Statement in C++ Language */ #include <iostream> using namespace std; int main () { // local variable declaration: int a = 100; // check the boolean condition if( a < 20 ) { // if condition is true then print the following cout << "a is less than 20;" << endl; } else { // if condition is false then print the following cout << "a is not less than 20;" << endl; } cout << "value of a is : " << a << endl; return 0; }
While Loop Statement in C++
In while loop, condition is evaluated first and if it returns true then the statements inside while loop execute, this happens repeatedly until the condition returns false. When condition returns false, the control comes out of loop and jumps to the next statement in the program after while loop. The important point to note when using while loop is that we need to use increment or decrement statement inside while loop so that the loop variable gets changed on each iteration, and at some point condition returns false. This way we can end the execution of while loop otherwise the loop would execute indefinitely. A while loop that never stops is said to be the infinite while loop, when we give the condition in such a way so that it never returns false, then the loops becomes infinite and repeats itself indefinitely.
Syntax for While Loop Statement in C++
while (condition) { // body of the loop }
• A while loop evaluates the condition • If the condition evaluates to true, the code inside the while loop is executed. • The condition is evaluated again. • This process continues until the condition is false. • When the condition evaluates to false, the loop terminates. Do not forget to increase the variable used in the condition, otherwise the loop will never end!
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/* While Loop Statement in C++ language */ // program to find the sum of positive numbers // if the user enters a negative number, the loop ends // the negative number entered is not added to the sum #include <iostream> using namespace std; int main() { int number; int sum = 0; // take input from the user cout << "Enter a number: "; cin >> number; while (number >= 0) { // add all positive numbers sum += number; // take input again if the number is positive cout << "Enter a number: "; cin >> number; } // display the sum cout << "\nThe sum is " << sum << endl; return 0; }
IOS Library fail() Function in C++
Check whether either failbit or badbit is set. Returns true if either (or both) the failbit or the badbit error state flags is set for the stream. At least one of these flags is set when an error occurs during an input operation. failbit is generally set by an operation when the error is related to the internal logic of the operation itself; further operations on the stream may be possible. While badbit is generally set when the error involves the loss of integrity of the stream, which is likely to persist even if a different operation is attempted on the stream. badbit can be checked independently by calling member function bad.
Syntax for IOS fail() Function in C++
bool fail() const;
This function does not accept any parameter. Function returns true if badbit and/or failbit are set. false otherwise. iostate value indicates functions to check state flags (member constant) good() eof() fail() bad() rdstate() goodbit No errors (zero value iostate) true false false false goodbit eofbit End-of-File reached on input operation false true false false eofbit failbit Logical error on i/o operation false false true false failbit badbit Read/writing error on i/o operation false false true true badbit eofbit, failbit and badbit are member constants with implementation-defined values that can be combined (as if with the bitwise OR operator). goodbit is zero, indicating that none of the other bits is set. Reaching the End-of-File sets the eofbit. But note that operations that reach the End-of-File may also set the failbit if this makes them fail (thus setting both eofbit and failbit). This function is a synonym of ios::operator!.
Data races
Accesses the stream object. Concurrent access to the same stream object may cause data races.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the stream.
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/* The fail() method of ios class in C++ is used to check if the stream is has raised any fail error. It means that this function will check if this stream has its failbit set. */ /* check whether either failbit or badbit is set by ios::fail function code example */ #include <iostream> #include <fstream> #include <cstdlib> int main() { std::ifstream file("test.txt"); if(!file) // operator! is used here { std::cout << "File opening failed\n"; return EXIT_FAILURE; } // typical C++ I/O loop uses the return value of the I/O function // as the loop controlling condition, operator bool() is used here for(int n; file >> n; ) { std::cout << n << ' '; } std::cout << '\n'; if (file.bad()) std::cout << "I/O error while reading\n"; else if (file.eof()) std::cout << "End of file reached successfully\n"; else if (file.fail()) std::cout << "Non-integer data encountered\n"; }
exit() Function in C++
The exit function terminates the program normally. Automatic objects are not destroyed, but static objects are. Then, all functions registered with atexit are called in the opposite order of registration. The code is returned to the operating system. An exit code of 0 or EXIT_SUCCESS means successful completion. If code is EXIT_FAILURE, an indication of program failure is returned to the operating system. Other values of code are implementation-defined.
Syntax for exit() Function in C++
void exit (int status);
status
Status code. If this is 0 or EXIT_SUCCESS, it indicates success. If it is EXIT_FAILURE, it indicates failure. Calls all functions registered with the atexit() function, and destroys C++ objects with static storage duration, all in last-in-first-out (LIFO) order. C++ objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit() are called in the reverse order of their registration. A function registered with atexit(), before an object obj1 of static storage duration is initialized, will not be called until obj1's destruction has completed. A function registered with atexit(), after an object obj2 of static storage duration is initialized, will be called before obj2's destruction starts. Normal program termination performs the following (in the same order): • Objects associated with the current thread with thread storage duration are destroyed (C++11 only). • Objects with static storage duration are destroyed (C++) and functions registered with atexit are called. • All C streams (open with functions in <cstdio>) are closed (and flushed, if buffered), and all files created with tmpfile are removed. • Control is returned to the host environment. Note that objects with automatic storage are not destroyed by calling exit (C++). If status is zero or EXIT_SUCCESS, a successful termination status is returned to the host environment. If status is EXIT_FAILURE, an unsuccessful termination status is returned to the host environment. Otherwise, the status returned depends on the system and library implementation. Flushes all buffers, and closes all open files. All files opened with tmpfile() are deleted. Returns control to the host environment from the program. exit() returns no values.
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/* terminate the process normally, performing the regular cleanup for terminating programs by exit() function code example */ #include<iostream> using namespace std; int main() { int i; cout<<"Enter a non-zero value: "; //user input cin>>i; if(i) // checks whether the user input is non-zero or not { cout<<"Valid input.\n"; } else { cout<<"ERROR!"; //the program exists if the value is 0 exit(0); } cout<<"The input was : "<<i; }
Vector Library Operator Index [] in C++
Access element. Returns a reference to the element at position n in the vector container. A similar member function, vector::at, has the same behavior as this operator function, except that vector::at is bound-checked and signals if the requested position is out of range by throwing an out_of_range exception. Portable programs should never call this function with an argument n that is out of range, since this causes undefined behavior.
Syntax for Vector Operator Index [] in C++
#include <vector> reference operator[] (size_type n); const_reference operator[] (size_type n) const;
n
Position of an element in the container. Notice that the first element has a position of 0 (not 1). Member type size_type is an unsigned integral type. Function returns the element at the specified position in the vector. If the vector object is const-qualified, the function returns a const_reference. Otherwise, it returns a reference. Member types reference and const_reference are the reference types to the elements of the container (see vector member types).
Complexity
Constant
Iterator validity
No changes
Data races
The container is accessed (neither the const nor the non-const versions modify the container). The reference returned can be used to access or modify elements. Concurrently accessing or modifying different elements is safe.
Exception safety
If the container size is greater than n, the function never throws exceptions (no-throw guarantee). Otherwise, the behavior is undefined.
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/* Returns a reference to the element at specified location pos. No bounds checking is performed. Unlike std::map::operator[], this operator never inserts a new element into the container. Accessing a nonexistent element through this operator is undefined behavior. */ /* Access element from a vector by vector::operator[] code example */ #include <iostream> #include <vector> int main () { std::vector<int> myvector (10); // 10 zero-initialized elements std::vector<int>::size_type sz = myvector.size(); // assign some values: for (unsigned i=0; i<sz; i++) myvector[i]=i; // reverse vector using operator[]: for (unsigned i=0; i<sz/2; i++) { int temp; temp = myvector[sz-1-i]; myvector[sz-1-i]=myvector[i]; myvector[i]=temp; } std::cout << "myvector contains:"; for (unsigned i=0; i<sz; i++) std::cout << ' ' << myvector[i]; std::cout << '\n'; return 0; }
C++ Files and Streams
In C++ programming we are using the iostream standard library, it provides cin and cout methods for reading from input and writing to output respectively. To read and write from a file we are using the standard C++ library called fstream. Let us see the data types define in fstream library is: • ofstream: This data type represents the output file stream and is used to create files and to write information to files. • ifstream: This data type represents the input file stream and is used to read information from files. • fstream: This data type represents the file stream generally, and has the capabilities of both ofstream and ifstream which means it can create files, write information to files, and read information from files. To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source file.
Opening a File
A file must be opened before you can read from it or write to it. Either ofstream or fstream object may be used to open a file for writing. And ifstream object is used to open a file for reading purpose only. File streams in C++ are basically the libraries that are used in the due course of programming. The programmers generally use the iostream standard library in the C++ programming as it provides the cin and cout methods that are used for reading from the input and writing to the output respectively. In order to read and write from a file, the programmers are generally using the standard C++ library that is known as the fstream. Following is the standard syntax for open() function, which is a member of fstream, ifstream, and ofstream objects.
void open(const char *filename, ios::openmode mode);
Here, the first argument specifies the name and location of the file to be opened and the second argument of the open() member function defines the mode in which the file should be opened. ios::app Append mode. All output to that file to be appended to the end. ios::ate Open a file for output and move the read/write control to the end of the file. ios::in Open a file for reading. ios::out Open a file for writing. ios::trunc If the file already exists, its contents will be truncated before opening the file.
Closing a File
When a C++ program terminates it automatically flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination. Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects.
void close();
Writing to a File
While doing C++ programming, you write information to a file from your program using the stream insertion operator (<<) just as you use that operator to output information to the screen. The only difference is that you use an ofstream or fstream object instead of the cout object.
Reading from a File
You read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an ifstream or fstream object instead of the cin object. 'ifstream' data type of 'fstream' library is used to read the files of C++. But before reading, there are several tasks which are performed sequentially like opening the file, reading and closing it. Different data types are used for the specific purpose. We can simply read the information from the file using the operator ( >> ) with the name of the file. We need to use the fstream or ifstream object in C++ in order to read the file. Reading of the file line by line can be done by simply using the while loop along with the function of ifstream 'getline()'.
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/* C++ files and streams */ /* writing to a file and reading from a file in C++ language */ #include <fstream> #include <iostream> using namespace std; int main () { char data[100]; // open a file in write mode. ofstream outfile; outfile.open("afile.dat"); cout << "Writing to the file" << endl; cout << "Enter your name: "; cin.getline(data, 100); // write inputted data into the file. outfile << data << endl; cout << "Enter your age: "; cin >> data; cin.ignore(); // again write inputted data into the file. outfile << data << endl; // close the opened file. outfile.close(); // open a file in read mode. ifstream infile; infile.open("afile.dat"); cout << "Reading from the file" << endl; infile >> data; // write the data at the screen. cout << data << endl; // again read the data from the file and display it. infile >> data; cout << data << endl; // close the opened file. infile.close(); return 0; }


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