Programming Tutorials

atoi(), itoa() in C++

By: Manoj Kumar in C++ Tutorials on 2007-09-17  

stdlib is something of a miscellaneous collection of functions that did not fit into the other libraries. It includes simple integer math functions, sorting functions (including qsort(), one of the fastest sorts available), and text conversions for moving from ASCII text to integers, long, float, and so forth.

The functions in stdlib you are likely to use most often include atoi(), itoa(), and the family of related functions. atoi() provides ASCII to integer conversion. atoi() takes a single argument: a pointer to a constant character string. It returns an integer (as you might expect). Listing below illustrates its use.

Using atoi() and related functions.

1:     #include <stdlib.h>
2:     #include <iostream.h>
4:     int main()
5:     {
6:        char buffer[80];
7:        cout << "Enter a number: ";
8:        cin >> buffer;
10:       int number;
11:       // number = buffer; compile error
12:       number = atoi(buffer);
13:       cout << "Here's the number: " << number << endl;
15:       // int sum = buffer + 5;
16:       int sum = atoi(buffer) + 5;
17:       cout << "Here's sum: " << sum << endl;
18:     return 0;
19: }

Output: Enter a number: 9
Here's the number: 9
Here's sum: 14

Analysis: On line 6 of this simple program, an 80-character buffer is allocated, and on line 7 the user is prompted for a number. The input is taken as text and written into the buffer.
On line 10, an int variable, number, is declared, and on line 11 the program attempts to assign the contents of the buffer to the int variable. This generates a compile-time error and is commented out.

On line 12, the problem is solved by invoking the standard library function atoi(), passing in the buffer as the parameter. The return value, the integer value of the text string, is assigned to the integer variable number and printed on line 13.

On line 15, a new integer variable, sum, is declared, and an attempt is made to assign to it the result of adding the integer constant 5 to the buffer. This, too, fails and is solved by calling the standard function atoi().


NOTE: Some compilers implement standard conversion procedures (such as atoi()) using macros. You can usually use these functions without worrying about how they are implemented. Check your compiler's documentation for details.

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