SAS/C Library Reference, Vol. 1

atof -- Convert a String to Floating Point

SYNOPSIS

 #include <stdlib.h>

 double atof(const char *p);

DESCRIPTION

atof converts the character string p to a double-precision, floating-point number after skipping any leading white space (such as blanks, tabs, and new-line characters). The conversion stops at the first unrecognized character.

The argument string may contain a decimal point and may be followed by an e or an E and a signed integer exponent. A leading minus sign indicates a negative number. White space is not allowed between the minus sign and the number or between the number and the exponent.

RETURN VALUE

atof returns a value of type double. If no initial segment of the string is a valid number, the return value is 0.

DIAGNOSTICS

No indication is returned to the program to specify whether the string contains a valid number, so you should validate the string before calling atof.

If the floating-point value is outside the range of valid 370 floating-point numbers, +-HUGE_VAL is returned if the correct value is too large, or 0.0 if the correct value is too close to 0.

EXAMPLE

  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <ctype.h>

  main() {
     char input[80];
     int int_format;
     long int_value;
     double dbl_value;

     puts("Enter a valid C numeric constant (without any suffixes)");
     gets(input);
        /* If there's a decimal point, it's a double.   */
     if (strchr(input, '.')) int_format = 0;
        /* If it starts 0x, it's an integer.            */
     else if((input[0] == '0' && tolower(input[1]) == 'x') ||
             ((input[0] == '+' || input[0] == '-') &&
              input[1] == '0' && tolower(input[2]) == 'x'))
        int_format = 1;
        /* If it has an E and isn't hex, it's a double. */
     else if (strpbrk(input, "eE")) int_format = 0;
       /* Doubles must have either "." or "e".          */
     else int_format = 1;

        /* Convert to integer (errors ignored).         */
     if (int_format) {
        int_value = strtol(input, NULL, 0);
        printf("Your input appears to be the integer %dn",
              int_value);
     }
        /* Convert to double (errors ignored).          */
     else {
        dbl_value = atof(input);
        printf("Your input appears to be the double %.16gn",
              dbl_value);
     }
     exit(0);
  }

RELATED FUNCTIONS

strtod