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C++ Programming Code Examples

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Swap characters in a file.

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/* Swap characters in a file. */ #include <iostream> #include <fstream> using namespace std; int main(int argc, char *argv[]) { if(argc!=2) { cout << "usage: SWAP <filename>\n"; return 1; } // open file for input/output fstream io(argv[1], ios::in | ios::out | ios::binary); if(!io) { cout << "Cannot open file.\n"; return 1; } char ch1, ch2; long i; for(i = 0 ; !io.eof(); i+=2) { io.seekg(i, ios::beg); io.get(ch1); if(io.eof()) continue; io.get(ch2); if(io.eof()) continue; io.seekg(i, ios::beg); io.put(ch2); io.put(ch1); } io.close(); return 0; }
seekg() Function in C++
Set position in input sequence. Sets the position of the next character to be extracted from the input stream. Sets the position of the next character to be extracted from the input stream. seekg() is a function in the iostream library (part of the standard library) that allows you to seek to an arbitrary position in a file. It is used in file handling to sets the position of the next character to be extracted from the input stream from a given file. Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to true). Then (if good), it calls either pubseekpos (1) or pubseekoff (2) on its associated stream buffer object (if any). Finally, it destroys the sentry object before returning. Calling this function does not alter the value returned by gcount.
Syntax for seekg() Function in C++
#include <fstream> //(1) istream& seekg (streampos pos); //(2) istream& seekg (streamoff off, ios_base::seekdir way);
pos
New absolute position within the stream (relative to the beginning). streampos is an fpos type (it can be converted to/from integral types).
off
Offset value, relative to the way parameter. streamoff is an offset type (generally, a signed integral type).
way
Object of type ios_base::seekdir. It may take any of the following constant values: ios_base::beg beginning of the stream ios_base::cur current position in the stream ios_base::end end of the stream Function returns the istream object (*this). Errors are signaled by modifying the internal state flags: eofbit - failbit Either the construction of sentry failed, or the internal call to pubseekpos (1) or pubseekoff (2) failed (i.e., either function returned -1). badbit Another error occurred on the stream (such as when the function catches an exception thrown by an internal operation). When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation. If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.
Data races
Modifies the stream object. Concurrent access to the same stream object may cause data races.
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state. It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state. Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.
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/* sets the position of the next character to be extracted from the input stream from a given file by seekg() function code example */ // CPP Prpgram to demonstrate the seekg function in file handling #include <fstream> #include <iostream> using namespace std; // Driver Code int main(int argc, char** argv) { // Open a new file for input/output operations fstream myFile("test.txt", ios::in | ios::out | ios::trunc); // Add the characters "Hello World" to the file myFile << "Hello World"; // Seek to 6 characters from the beginning of the file myFile.seekg(6, ios::beg); // Read the next 5 characters from the file into a // buffer char A[6]; myFile.read(A, 5); // End the buffer with a null terminating character A[5] = 0; // Output the contents read from the file and close it cout << A << endl; myFile.close(); }
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; }
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; }
Iostream Library get() Function in C++
Get characters. Extracts characters from the stream, as unformatted input. The get() function is used to read a character(at a time) from a file. The classes istream and ostream define two member functions get(), put() respectively to handle the single character input/output operations. There are two types of get() functions. Both get(char *) and get(void) prototype can be used to fetch a character including the blank space,tab and newline character. The get(char *) version assigns the input character to its argument and the get(void) version returns the input character. Since these functions are members of input/output Stream classes, these must be invoked using appropriate objects.
Syntax for get() Function in C++
#include <iostream> //single character (1) int get(); istream& get (char& c); //c-string (2) istream& get (char* s, streamsize n); istream& get (char* s, streamsize n, char delim); //stream buffer (3) istream& get (streambuf& sb); istream& get (streambuf& sb, char delim);
c
The reference to a character where the extracted value is stored.
s
Pointer to an array of characters where extracted characters are stored as a c-string. If the function does not extract any characters (or if the first character extracted is the delimiter character) and n is greater than zero, this is set to an empty c-string.
n
Maximum number of characters to write to s (including the terminating null character). If this is less than 2, the function does not extract any characters and sets failbit. streamsize is a signed integral type.
delim
Explicit delimiting character: The operation of extracting successive characters stops as soon as the next character to extract compares equal to this.
sb
A streambuf object on whose controlled output sequence the characters are copied. • (1) single character Extracts a single character from the stream. The character is either returned (first signature), or set as the value of its argument (second signature). • (2) c-string Extracts characters from the stream and stores them in s as a c-string, until either (n-1) characters have been extracted or the delimiting character is encountered: the delimiting character being either the newline character ('\n') or delim (if this argument is specified). The delimiting character is not extracted from the input sequence if found, and remains there as the next character to be extracted from the stream (see getline for an alternative that does discard the delimiting character). A null character ('\0') is automatically appended to the written sequence if n is greater than zero, even if an empty string is extracted. • (3) stream buffer Extracts characters from the stream and inserts them into the output sequence controlled by the stream buffer object sb, stopping either as soon as such an insertion fails or as soon as the delimiting character is encountered in the input sequence (the delimiting character being either the newline character, '\n', or delim, if this argument is specified). Only the characters successfully inserted into sb are extracted from the stream: Neither the delimiting character, nor eventually the character that failed to be inserted at sb, are extracted from the input sequence and remain there as the next character to be extracted from the stream. The function also stops extracting characters if the end-of-file is reached. If this is reached prematurely (before meeting the conditions described above), the function sets the eofbit flag. Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to true). Then (if good), it extracts characters from its associated stream buffer object as if calling its member functions sbumpc or sgetc, and finally destroys the sentry object before returning. The number of characters successfully read and stored by this function can be accessed by calling member gcount. The first signature returns the character read, or the end-of-file value (EOF) if no characters are available in the stream (note that in this case, the failbit flag is also set). All other signatures always return *this. Note that this return value can be checked for the state of the stream (see casting a stream to bool for more info). Errors are signaled by modifying the internal state flags: • eofbit The function stopped extracting characters because the input sequence has no more characters available (end-of-file reached). • failbit Either no characters were written or an empty c-string was stored in s. • badbit Error on stream (such as when this function catches an exception thrown by an internal operation). When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation. If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.
Data races
Modifies c, sb or the elements in the array pointed by s. Modifies the stream object. Concurrent access to the same stream object may cause data races, except for the standard stream object cin when this is synchronized with stdio (in this case, no data races are initiated, although no guarantees are given on the order in which extracted characters are attributed to threads).
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state. It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state. Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.
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/* reads a single character from the associated stream by get() function code example. */ /* It is used to fetch a character from the file and continues to do so until the end-of-the file condition is reached. */ //C++ Reading the content to a file using ifstream class and file mode ios::in #include<iostream> #include<ifstream> using namespace std; int main() { //Creating an input stream to read a file ifstream ifstream_ob; //Opening a file named File1.txt to read its content ifstream_ob.open("File1.txt", ios::in); char ch; //Reading the file using get() function and displaying its content while(ifstream_ob) { ch = ifstream_ob.get(); cout<<ch; } //Closing the input strea ifstream_ob.close(); 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; }
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; }
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; }
Ostream Library put() Function in C++
Put character. Inserts character c into the stream. Internally, the function accesses the output sequence by first constructing a sentry object. Then (if good), it inserts c into its associated stream buffer object as if calling its member function sputc, and finally destroys the sentry object before returning.
Syntax for Ostream put() Function in C++
ostream& put (char c);
c
Character to write Function returns the ostream object (*this). Errors are signaled by modifying the internal state flags: • eofbit - • failbit May be set if the construction of sentry failed. • badbit Either the insertion on the stream failed, or some other error happened (such as when this function catches an exception thrown by an internal operation). When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation. If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure. Through the previous study, we know that C++ programs generally use the cout output stream object of the ostream class and the << output operator to achieve output, and the cout output stream has a corresponding buffer in memory. But sometimes users have special output requirements, such as outputting only one character. In this case, you can use the put() member method provided by this class to achieve.
Data races
Modifies the stream object. Concurrent access to the same stream object may cause data races, except for the standard stream objects (cout, cerr, clog) when these are synchronized with stdio (in this case, no data races are initiated, although no guarantees are given on the order in which characters from multiple threads are inserted).
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state. It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state. Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.
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/* It is used to inserts character c into the stream.this function accesses the output sequence by first constructing a sentry object. Then (if good), it inserts c into its associated stream buffer object as if calling its member function sputc, and finally destroys the sentry object before returning. */ //C++ code example to Writing data to a file using put() function and ios::out mode #include<iostream> #include<fstream> #include<cstring> using namespace std; int main() { //Creating an output stream to write data to a file ofstream ofstream_ob; //Opens/creates a file named File2.txt ofstream_ob.open("File2.txt", ios::out); char arr[100] = "Hello World. We wish you best in everything. Never give up!"; int length = strlen(arr); char ch; //Reading the char array i.e. a character at a time and writing it to the file for(int i=0; i<length; i++) { ch = arr[i]; ofstream_ob.put(ch); //Writing a character to file, by using put() function } //Closing the output stream ofstream_ob.close(); return 0; }
Assignment Operators in C++
As the name already suggests, these operators help in assigning values to variables. These operators help us in allocating a particular value to the operands. The main simple assignment operator is '='. We have to be sure that both the left and right sides of the operator must have the same data type. We have different levels of operators. Assignment operators are used to assign the value, variable and function to another variable. Assignment operators in C are some of the C Programming Operator, which are useful to assign the values to the declared variables. Let's discuss the various types of the assignment operators such as =, +=, -=, /=, *= and %=. The following table lists the assignment operators supported by the C language:
=
Simple assignment operator. Assigns values from right side operands to left side operand
+=
Add AND assignment operator. It adds the right operand to the left operand and assign the result to the left operand.
-=
Subtract AND assignment operator. It subtracts the right operand from the left operand and assigns the result to the left operand.
*=
Multiply AND assignment operator. It multiplies the right operand with the left operand and assigns the result to the left operand.
/=
Divide AND assignment operator. It divides the left operand with the right operand and assigns the result to the left operand.
%=
Modulus AND assignment operator. It takes modulus using two operands and assigns the result to the left operand.
<<=
Left shift AND assignment operator.
>>=
Right shift AND assignment operator.
&=
Bitwise AND assignment operator.
^=
Bitwise exclusive OR and assignment operator.
|=
Bitwise inclusive OR and assignment operator.
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/* Assignment operators are used to assigning value to a variable. The left side operand of the assignment operator is a variable and right side operand of the assignment operator is a value. The value on the right side must be of the same data-type of the variable on the left side otherwise the compiler will raise an error. */ // C++ program to demonstrate working of Assignment operators #include <iostream> using namespace std; int main() { // Assigning value 10 to a // using "=" operator int a = 10; cout << "Value of a is "<<a<<"\n"; // Assigning value by adding 10 to a // using "+=" operator a += 10; cout << "Value of a is "<<a<<"\n"; // Assigning value by subtracting 10 from a // using "-=" operator a -= 10; cout << "Value of a is "<<a<<"\n"; // Assigning value by multiplying 10 to a // using "*=" operator a *= 10; cout << "Value of a is "<<a<<"\n"; // Assigning value by dividing 10 from a // using "/=" operator a /= 10; cout << "Value of a is "<<a<<"\n"; return 0; }
Continue Statement in C++
Continue statement is used inside loops. Whenever a continue statement is encountered inside a loop, control directly jumps to the beginning of the loop for next iteration, skipping the execution of statements inside loop's body for the current iteration. The continue statement works somewhat like the break statement. Instead of forcing termination, however, continue forces the next iteration of the loop to take place, skipping any code in between. For the for loop, continue causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, program control passes to the conditional tests.
Syntax for Continue Statement in C++
loop-statement{ continue; }
As the name suggest the continue statement forces the loop to continue or execute the next iteration. When the continue statement is executed in the loop, the code inside the loop following the continue statement will be skipped and next iteration of the loop will begin.
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/* continue statement skips the execution of further statements in the block and continues with the next iteration. */ // program to calculate positive numbers till 50 only // if the user enters a negative number, // that number is skipped from the calculation // negative number -> loop terminate // numbers above 50 -> skip iteration #include <iostream> using namespace std; int main() { int sum = 0; int number = 0; while (number >= 0) { // add all positive numbers sum += number; // take input from the user cout << "Enter a number: "; cin >> number; // continue condition if (number > 50) { cout << "The number is greater than 50 and won't be calculated." << endl; number = 0; // the value of number is made 0 again continue; } } // display the sum cout << "The sum is " << sum << endl; 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; }
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; }


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