This blog post will teach you how to convert a string to an integer in C. Almost programming language provides the library function to convert a string to an integer. The atoi() function converts a string to an integer in the C programming language.

So let’s see the first example using the atoi(). But the atoi() function has some limitations, so if you are not familiar with the atoi() function, then it is my suggestion that you should first read the atoi library function. You can check the other post “How to use atoi() and how to make own atoi() “.

Using the atoi():

The atoi() function converts a string to the int representation. It takes a string as an argument and returns its integer value.

Consider the below example where the atoi function is converting a numeric string to an integer.

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

int main()
{
    const char* str1 = "2706";

    int num = atoi(str1);

    printf("%d",num);

    return 0;
}

Output: 2706

Using the strtol():

The strtol() function converts a string to the long int representation. But if no conversion could be performed, zero is returned.

The strtol() function discard all leading whitespace character at the beginning of the string until the first non-whitespace character is found. And after it starts conversion until it finds the first character that is not a number.

If the converted value is outside the range of long int, function returns LONG_MAX or LONG_MIN and errno is set to ERANGE.

strtol() Syntax:

long int strtol(const char *restrict nptr, char **restrict endptr, int base);

Parameters:

nptr – pointer to the null-terminated byte string to be interpreted
endptr– is a pointer to indicate where the conversion stops (could be null).
base – set of valid values for base is {0,2,3,…,36}.

Now I think you have sufficient knowledge of strtol, so let’s see the example of how to strtol converts a string to int.

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


int main()
{
    const char str[] = "2324";
    char *endPtr;

    const long int d = strtol(str, &endPtr, 10);

    printf("%ld\n",d);

    return 0;
}

Output: 2324

Using the strtoimax() or strtoumax():

The strtoimax() and strtoumax() functions are equivalent to the strtol, strtoll, strtoul, and strtoull functions, except that the initial portion of the string is converted to intmax_t and uintmax_t representation, respectively.

Syntax:

#include <inttypes.h>

intmax_t strtoimax(const char * restrict nptr, char ** restrict endptr, int base);

uintmax_t strtoumax(const char * restrict nptr, char ** restrict endptr, int base);

Parameters:

nptr – pointer to the null-terminated byte string to be interpreted
endptr– is a pointer to indicate where the conversion stops (could be null).
base – set of valid values for base is {0,2,3,…,36}.

#include <stdio.h>
#include <inttypes.h>


int main()
{
    char* endptr1;
    char* endptr2;

    /* base 10*/
    printf("%ld\n", strtoimax("2324",&endptr1,10));

    /* base 10*/
    printf("%lu\n", strtoumax("2324",&endptr2,10));

    return 0;
}

Output:
2324
2324

Using own implemented method:

#include<stdio.h>

#define Is_NUMERIC_STRING(d) (*(char*)d >= 48) && (*(char*)d<= 57)

int StringToInt(const char *pStr)
{
    int result = 0; // variable to store the result
    int sign = 1; //Initialize sign as positive

    if(pStr == NULL) //If pointer is null
        return 0;

    // if whitespaces then ignore.
    while ((*pStr) == ' ')
    {
        ++pStr;
    }

    //If number is negative, then update sign
    if((*pStr) == '-')
    {
        sign = -1;
        ++pStr; //Increment the pointer
    }

    while(Is_NUMERIC_STRING(pStr)) //check string validity
    {
        result = (result*10)+ (*pStr-48);
        pStr++; //Increment the pointer
    }
    return (sign * result);
}

int main()
{
    const char str[] = "2324";

    const int d = StringToInt(str);

    printf("%d\n",d);

    return 0;
}

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