atof() in C – Syntax, Use and Example

Write a C program to input string representation of number and convert string to double using atof() library function. How to convert string to double using atof() library function in C programming?

atof() library function

atof() function converts string to floating point number. The library function is defined in stdlib.h header file.

Syntax of atof() function

double atof (const char* str);
  • It accepts pointer constant to character str which is string representation of floating pointer number.
  • On success, it returns converted floating point number as double type. If the given string str has no possible conversion then returns 0.0.

Important note:

Example program to use atof() function

/**
 * C program to convert string to double using atof() library function.
 */

#include <stdio.h>
#include <stdlib.h>     // Used for atof()


int main()
{
    char number[30];
    double decimalNumber;


    /* Input string representation of floating pointer number from user. */
    printf("Enter any number: ");
    fgets(number, 30, stdin);


    /* Convert string representation of number to double */
    decimalNumber = atof(number);


    /* Use %f, %lf, %llf to print floating point number */
    printf("Converted floating point number = %lf\n", decimalNumber);


    return 0;
}

Output

Enter any number: 1.5
Converted floating point number = 1.500000

Enter any number: 1
Converted floating point number = 1.000000

Enter any number: 01
Converted floating point number = 1.000000

Enter any number: 2.5
Converted floating point number = 2.500000

Enter any number: a2
Converted floating point number = 0.000000

Enter any number: 22.2a
Converted floating point number = 22.200000

Enter any number: 2.2.2.2
Converted floating point number = 2.200000

Enter any number: 9999999999999999999999999999999999999999999999999999
Converted floating point number = 99999999999999991000000000000.000000

Happy coding 😉

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