In this section, we will learn what the atoi() function is and how to use it in C.

String to int in C: atoi() Function

If we have a character-string that contains only numbers, we can use the `atoi()` function to get this number as an integer type.

The function’s prototype is in `stdlib.h` header file and so we need to include this header file in order to use the function.

atoi() Function Syntax

Here’s the prototype of the `atoi()` function:

int atoi(const char *str)

atoi() Function Parameters

This function takes one argument and that is the address of the character-string that we want to get its integer value.

atoi() Function Return Value

The return value of this function is the integer value of the character-string or 0 if the operation was un-successful (in case the character-string didn’t contain number for example).

Example: convert String to int c

#include <stdio.h>
#include <stdlib.h>
int main() {

    char * pointer = "234234";

    int result = atoi(pointer);

    printf("The result is: %d",result);

    return  0;
}

Output:

The result is: 234234

Example: c string to integer

#include <stdio.h>
#include <stdlib.h>
int main() {

    char * pointer = "f234234tes";

    int result = atoi(pointer);

    printf("The result is: %d",result);

    return  0;
}

Output:

The result is: 0

Note: in the `stdlib.h` header file, there are two other functions named `atof()` and `atol()` by which we can convert a string to a double and long type, respectively.

Both of the functions have the same prototype as the `atoi()` function has.

Example: convert string to double in c

#include <stdio.h>
#include <stdlib.h>
int main() {

    char * pointer = "234.33";

    double result = atof(pointer);

    printf("The result is: %f",result);

    return  0;
}

Output:

The result is: 234.33