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C++ Programming Code Examples

C++ > Strings Code Examples

Program to Implement the Program Used in grep/egrep/fgrep

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/* Program to Implement the Program Used in grep/egrep/fgrep This is a C++ Program to implement grep linux command. */ #include<stdio.h> #include<dirent.h> int main() { char fn[10], pat[10], temp[200]; FILE *fp; printf("\n Enter file name : "); scanf("%s", fn); printf("Enter the pattern: "); scanf("%s", pat); fp = fopen(fn, "r"); while (!feof(fp)) { fgets(temp, sizeof(fp), fp); if (strcmp(temp, pat)) printf("%s", temp); } fclose(fp); return 1; }
gets() Function in C++
Get string from stdin. Reads characters from the standard input (stdin) and stores them as a C string into str until a newline character or the end-of-file is reached. The newline character, if found, is not copied into str. A terminating null character is automatically appended after the characters copied to str. Notice that gets is quite different from fgets: not only gets uses stdin as source, but it does not include the ending newline character in the resulting string and does not allow to specify a maximum size for str (which can lead to buffer overflows).
Syntax for gets() Function in C++
#include <cstdio> char * gets ( char * str );
str
Pointer to a block of memory (array of char) where the string read is copied as a C string. On success, the function returns str. If the end-of-file is encountered while attempting to read a character, the eof indicator is set (feof). If this happens before any characters could be read, the pointer returned is a null pointer (and the contents of str remain unchanged). If a read error occurs, the error indicator (ferror) is set and a null pointer is also returned (but the contents pointed by str may have changed). The most recent revision of the C standard (2011) has definitively removed this function from its specification. The function is deprecated in C++ (as of 2011 standard, which follows C99+TC3).
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/* The gets() function in C++ reads characters from stdin and stores them until a newline character is found or end of file occurs. */ /* Get string from stdin by gets() function code example */ //Program demonstrating the difference between gets and cin #include<iostream.h> #include<stdio.h> int main() { char str1[80],str2[80]; cout<<"\n Enter a string using gets(): "; gets(str1); cout<<"\n Enter a string using cin object: "; cin>>str2; cout<<"\n String input with gets(): "; cout<<str1; cout<<"\n String input with cin object: "; cout<<str2; 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; }
strcmp() Function in C++
Compare two strings. Compares the C string str1 to the C string str2. This function starts comparing the first character of each string. If they are equal to each other, it continues with the following pairs until the characters differ or until a terminating null-character is reached. This function performs a binary comparison of the characters. For a function that takes into account locale-specific rules, see strcoll. The strcmp() function in C++ compares two null-terminating strings (C-strings). The comparison is done lexicographically. It is defined in the cstring header file.
Syntax for strcmp() Function in C++
int strcmp ( const char * str1, const char * str2 );
str1
pointer to the C-string that needs to be compared.
str2
pointer to the C-string that needs to be compared. Here, str1 stands for left hand side, str2 stands for right hand side. Function returns a positive value if the first differing character in str1 is greater than the corresponding character in str2. Function returns a negative value if the first differing character in str1 is less than the corresponding character in str2. Function returns 0 if str1 and str2 are equal. The strcmp() compares the contents of str1 and str2 lexicographically. The sign of the result is the sign of difference between the first pairs of characters that differ in str1 and str2.
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/* compare two strings by strcmp() string function code example */ #include <cstring> #include <iostream> using namespace std; int main() { char str1[] = "HappyCodings"; char str2[] = "Happy"; // returns -1 because "HappyCodings" < "Happy" lexicographically int result = strcmp(str1, str2); cout << "Comparing " << str1 << " and " << str2 << ": " << result << endl; // returns 1 because "Happy" > "HappyCodings" lexicographically result = strcmp(str2, str1); cout << "Comparing " << str2 << " and " << str1 << ": " << result << endl; // returns 1 because "HappyCodings" = "HappyCodings" lexicographically result = strcmp(str1, str1); cout << "Comparing " << str1 << " and " << str1 << ": " << result; 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; }
Strings in C++ Language
Strings are objects that represent sequences of characters. The standard string class provides support for such objects with an interface similar to that of a standard container of bytes, but adding features specifically designed to operate with strings of single-byte characters. The string class is an instantiation of the basic_string class template that uses char (i.e., bytes) as its character type, with its default char_traits and allocator types. Note that this class handles bytes independently of the encoding used: If used to handle sequences of multi-byte or variable-length characters (such as UTF-8), all members of this class (such as length or size), as well as its iterators, will still operate in terms of bytes (not actual encoded characters).
Declaration for Strings in C++
char str[4] = "C++ Programming"; char str[] = {'C','+','+','\0'}; char str[4] = {'C','+','+','\0'};
In C programming, the collection of characters is stored in the form of arrays. This is also supported in C++ programming. Hence it's called C-strings. C-strings are arrays of type char terminated with null character, that is, \0 (ASCII value of null character is 0). • A character array is simply an array of characters that can be terminated by a null character. A string is a class that defines objects that be represented as a stream of characters. • The size of the character array has to be allocated statically, more memory cannot be allocated at run time if required. Unused allocated memory is wasted in the case of the character array. In the case of strings, memory is allocated dynamically. More memory can be allocated at run time on demand. As no memory is preallocated, no memory is wasted. • There is a threat of array decay in the case of the character array. As strings are represented as objects, no array decay occurs. • Implementation of character array is faster than std:: string. Strings are slower when compared to implementation than character array. • Character arrays do not offer many inbuilt functions to manipulate strings. String class defines a number of functionalities that allow manifold operations on strings.
String Functions in C++
• int compare(const string& str): It is used to compare two string objects. • int length(): It is used to find the length of the string. • void swap(string& str): It is used to swap the values of two string objects. • string substr(int pos,int n): It creates a new string object of n characters. • int size(): It returns the length of the string in terms of bytes. • void resize(int n): It is used to resize the length of the string up to n characters. • string& replace(int pos,int len,string& str): It replaces portion of the string that begins at character position pos and spans len characters. • string& append(const string& str): It adds new characters at the end of another string object. • char& at(int pos): It is used to access an individual character at specified position pos. • int find(string& str,int pos,int n): It is used to find the string specified in the parameter. • int find_first_of(string& str,int pos,int n): It is used to find the first occurrence of the specified sequence. • int find_first_not_of(string& str,int pos,int n ): It is used to search the string for the first character that does not match with any of the characters specified in the string. • int find_last_of(string& str,int pos,int n): It is used to search the string for the last character of specified sequence. • int find_last_not_of(string& str,int pos): It searches for the last character that does not match with the specified sequence. • string& insert(): It inserts a new character before the character indicated by the position pos. • int max_size(): It finds the maximum length of the string. • void push_back(char ch): It adds a new character ch at the end of the string. • void pop_back(): It removes a last character of the string. • string& assign(): It assigns new value to the string. • int copy(string& str): It copies the contents of string into another. • char& back(): It returns the reference of last character. • Iterator begin(): It returns the reference of first character. • int capacity(): It returns the allocated space for the string. • const_iterator cbegin(): It points to the first element of the string. • const_iterator cend(): It points to the last element of the string. • void clear(): It removes all the elements from the string. • const_reverse_iterator crbegin(): It points to the last character of the string. • const_char* data(): It copies the characters of string into an array. • bool empty(): It checks whether the string is empty or not. • string& erase(): It removes the characters as specified. • char& front(): It returns a reference of the first character. • string& operator+=(): It appends a new character at the end of the string. • string& operator=(): It assigns a new value to the string. • char operator[](pos): It retrieves a character at specified position pos. • int rfind(): It searches for the last occurrence of the string. • iterator end(): It references the last character of the string. • reverse_iterator rend(): It points to the first character of the string. • void shrink_to_fit(): It reduces the capacity and makes it equal to the size of the string. • char* c_str(): It returns pointer to an array that contains null terminated sequence of characters. • const_reverse_iterator crend(): It references the first character of the string. • reverse_iterator rbegin(): It reference the last character of the string. • void reserve(inr len): It requests a change in capacity. • allocator_type get_allocator();: It returns the allocated object associated with the string.
Non-member Function Overloads
• operator+ Concatenate strings (function ) • relational operators Relational operators for string (function ) • swap Exchanges the values of two strings (function ) • operator>> Extract string from stream (function ) • operator<< Insert string into stream (function ) • getline Get line from stream into string (function )
Operators used for String Objects
• =: assignment • +: concatenation • ==: Equality • !=: Inequality • <: Less than • <=: Less than or equal • >: Greater than • >=: Greater than or equal • []: Subscription • <<: Output • >>: Input
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/* C++ String Library */ /* The C-Style Character String */ // C++ Program to demonstrate the working of getline(), push_back() and pop_back() #include <iostream> #include <string> // for string class using namespace std; // Driver Code int main() { // Declaring string string str; // Taking string input using getline() getline(cin, str); // Displaying string cout << "The initial string is : "; cout << str << endl; // Inserting a character str.push_back('s'); // Displaying string cout << "The string after push_back operation is : "; cout << str << endl; // Deleting a character str.pop_back(); // Displaying string cout << "The string after pop_back operation is : "; cout << str << endl; return 0; }
fclose() Function in C++
Close file. Closes the file associated with the stream and disassociates it. Closes the given file stream. Any unwritten buffered data are flushed to the OS. Any unread buffered data are discarded. Whether or not the operation succeeds, the stream is no longer associated with a file, and the buffer allocated by std::setbuf or std::setvbuf, if any, is also disassociated and deallocated if automatic allocation was used. All internal buffers associated with the stream are disassociated from it and flushed: the content of any unwritten output buffer is written and the content of any unread input buffer is discarded. Even if the call fails, the stream passed as parameter will no longer be associated with the file nor its buffers.
Syntax for fclose() Function in C++
#include <cstdio> int fclose ( FILE * stream );
stream
Pointer to a FILE object that specifies the stream to be closed. If the stream is successfully closed, a zero value is returned. On failure, EOF is returned.
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/* The fclose() function takes a single argument, a file stream which is to be closed. All the data that are buffered but not written are flushed to the OS and all unread buffered data are discarded. Even if the operation fails, the stream is no longer associated with the file. If the file pointer is used after fclose() is executed, the behaviour is undefined. */ /* Close file by fclose() function code example */ #include <iostream> #include <cstdio> using namespace std; int main() { FILE *fp; fp = fopen("file.txt","w"); char str[20] = "Hello World!"; if (fp == NULL) { cout << "Error opening file"; exit(1); } fprintf(fp,"%s",str); fclose(fp); cout << "File closed successfully"; return 0; }
sizeof() Operator in C++
The sizeof() is an operator that evaluates the size of data type, constants, variable. It is a compile-time operator as it returns the size of any variable or a constant at the compilation time. The size, which is calculated by the sizeof() operator, is the amount of RAM occupied in the computer. The sizeof is a keyword, but it is a compile-time operator that determines the size, in bytes, of a variable or data type. The sizeof operator can be used to get the size of classes, structures, unions and any other user defined data type.
Syntax for sizeof() Operator in C++
sizeof(data_type);
data_type
data type whose size is to be calculated The data_type can be the data type of the data, variables, constants, unions, structures, or any other user-defined data type. If the parameter of a sizeof() operator contains the data type of a variable, then the sizeof() operator will return the size of the data type. sizeof() may give different output according to machine, we have run our program on 32 bit gcc compiler.
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/* The sizeof() is an operator in C and C++. It is an unary operator which assists a programmer in finding the size of the operand which is being used. */ #include <iostream> using namespace std; int main() { int arr[]={10,20,30,40,50}; std::cout << "Size of the array 'arr' is : "<<sizeof(arr) << std::endl; cout << "Size of char : " << sizeof(char) << endl; cout << "Size of int : " << sizeof(int) << endl; cout << "Size of short int : " << sizeof(short int) << endl; cout << "Size of long int : " << sizeof(long int) << endl; cout << "Size of float : " << sizeof(float) << endl; cout << "Size of double : " << sizeof(double) << endl; cout << "Size of wchar_t : " << sizeof(wchar_t) << 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; }
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; }
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; }
fgets() Function in C++
Get string from stream. Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or the end-of-file is reached, whichever happens first. A newline character makes fgets stop reading, but it is considered a valid character by the function and included in the string copied to str. A terminating null character is automatically appended after the characters copied to str. Notice that fgets is quite different from gets: not only fgets accepts a stream argument, but also allows to specify the maximum size of str and includes in the string any ending newline character.
Syntax for fgets() Function in C++
#include <cstdio> char * fgets ( char * str, int num, FILE * stream );
str
Pointer to an array of chars where the string read is copied.
num
Maximum number of characters to be copied into str (including the terminating null-character).
stream
Pointer to a FILE object that identifies an input stream. stdin can be used as argument to read from the standard input. On success, the function returns str. If the end-of-file is encountered while attempting to read a character, the eof indicator is set (feof). If this happens before any characters could be read, the pointer returned is a null pointer (and the contents of str remain unchanged). If a read error occurs, the error indicator (ferror) is set and a null pointer is also returned (but the contents pointed by str may have changed).
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/* The fgets() function in C++ reads a specified maximum number of characters from the given file stream. */ /* Get string from stream by fgets() function code example */ #include <iostream> #include <cstdio> using namespace std; int main() { int count = 10; char str[10]; FILE *fp; fp = fopen("file.txt","w+"); fputs("An example file\n", fp); fputs("Filename is file.txt\n", fp); rewind(fp); while(feof(fp) == 0) { fgets(str,count,fp); cout << str << endl; } fclose(fp); return 0; }