C library - atof() function

The C stdlib library atof() function is used to convert a string into a floating-point number and represent the converted floating point number to its corresponding double value.

A floating-point number is a type of integer that includes a fractional part, represented using a decimal point. For example, numbers like 10.05 and 5.5005 are floating-point numbers.

Syntax

Following is the C library syntax of the atof() function −

double atof(const char *str)

Parameters

This function accepts a single parameter −

• str − It is a pointer to a null-terminated sting, which represent the a floating point number.

Return Value

This function returns a floating-point number that corresponds to its double representation. If the input string is not a valid floating-point number, it returns 0.0.

Example 1

The following is the basic c example that demonstrate the use of atof() function.

#include <stdlib.h>
#include <stdio.h>
int main(void)
{
   double res;
   char *str; 
   str = "-1509.10E-10";
   res = atof(str); 
   printf("res = %.4e\n", res);
}

Output

Following is the output −

res = -1.5091e-007

Example 2

In this example, we concatenated two string and then convert the resulting string into a floating point number using the atof() function.

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

int main() {
	
    // Define two strings to concatenate
    char str1[] = "123.456";
    char str2[] = "789";
    
    //calculate the length of string first + second
    int length = strlen(str1) + strlen(str2) + 1;

    // Allocate memory for the concatenated string
    char *concatenated = malloc(length);

    // check memory allocation if null return 1.
    if(concatenated == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }

    // Concatenate str1 and str2
    strcpy(concatenated, str1);
    strcat(concatenated, str2);

    // Convert concatenated string into a floating point number.
    // use the atof() function
    double number = atof(concatenated);

    printf("The concatenated string is: %s\n", concatenated);
    printf("The floating point number is: %f\n", number);

    // at the last free the alocated memory
    free(concatenated);
    return 0;
}

Output

Following is the output −

The concatenated string is: 123.456789
The floating point number is: 123.456789

Example 3

The following is the another example, here we convert the both numerical string and character string into the floating-point number.

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

int main () {
   float res;
   char str[20];
   
   //define a string   
   strcpy(str, "151413.10e");
   
   //convert into float
   //use atof() function
   res = atof(str);
   
   printf("String value = %s\n" , str);
   printf("Float value = %f\n", res);

   strcpy(str, "tutorialspoint.com");
   
   //use atof() function
   res = atof(str);
   printf("String value = %s\n", str);
   printf("Float value = %f\n", res);

   return(0);
}

Output

Following is the output −

String value = 151413.10e
Float value = 151413.093750
String value = tutorialspoint.com
Float value = 0.000000