C atof() Function | Convert String to Double

Introduction

The atof() function in C is a standard library function that converts a string to a double. It is part of the C standard library (stdlib.h) and is commonly used to convert string representations of floating-point numbers into their corresponding double values.

atof() Function Syntax

The syntax for the atof() function is as follows:

double atof(const char *str);

Parameters:

• str: A C string that contains the representation of a floating-point number.

Returns:

• The function returns the converted double value. If no valid conversion could be performed, it returns 0.0.

Examples

Converting a Simple String to Double

To demonstrate how to use atof() to convert a string to a double, we will write a simple program.

Example

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

int main() {
    const char *str = "123.45";
    double num;

    // Convert string to double
    num = atof(str);

    // Print the converted value
    printf("The converted value is: %f\n", num);

    return 0;
}

Output:

The converted value is: 123.450000

Handling Invalid Input

This example shows how atof() behaves with invalid input.

Example

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

int main() {
    const char *str = "abc123";
    double num;

    // Convert string to double
    num = atof(str);

    // Print the converted value
    printf("The converted value is: %f\n", num);

    return 0;
}

Output:

The converted value is: 0.000000

Real-World Use Case

Converting User Input to Double

In real-world applications, the atof() function can be used to convert user input, provided as a string, into a double for further numerical processing.

Example

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

int main() {
    char input[100];
    double value;

    // Prompt the user for input
    printf("Enter a floating-point number: ");
    fgets(input, sizeof(input), stdin);

    // Convert input to double
    value = atof(input);

    // Print the converted value
    printf("You entered: %f\n", value);

    return 0;
}

Output (example user input "45.67"):

Enter a floating-point number: 45.67
You entered: 45.670000

Conclusion

The atof() function is useful for converting strings that represent floating-point numbers into double values. This function is essential when working with numerical data stored as strings.