How to convert String to Integer in C

C-String-Programs

In this article, we are going to learn different techniques of how to convert string to integer in C language. The techniques that we are going to learn in this article are:

How to convert string to Integer in C

  • atoi() Function to Convert a String to an Integer in C
  • strtol() Function to Convert a String to an Integer in C
  • strtoumax() Function for Convert String to an Integer in C

1. atoi() Function to Convert a String to an Integer in C

In this example, we are going to make use of the atoi() function from the C standard library. This function can be used by just including the header file in your program. This function accepts a char input and returns the integer.Let us understand this function used in the below code example to convert a string to an integer number.If we have to convert integer to string

Syntax

int atoi(const char *str);
  • *str is a pointer to a string to be converted to an integer
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main (void) 
{
    int Num;
    char str[20];
    strcpy(str,"55");

    Num = atoi(str);
    printf("The String input = %s is equal to integer = %d\n", str, Num);

    return(0);
}

Output

The String input = 55 is equal to integer = 55

2. strtol() Function to Convert a String to an Integer in C

In this example, we are going to make use of the strol() function from the C Standard library. This function can be used by just including the header file in your program. This function accepts a char input and returns the number equivalent in a long integer.

Syntax

long int strtol(const char *str, char **end,int base);

Parameters

  • *str = is a pointer to a string to be converted to a long integer.
  • **end = is a pointer to indicate where the conversion stops.
  • base = is the base representation with the range of [2, 36]. For example 2 is binary, 8 is octal etc.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    char str[10];
    char *ptr;
    long Num;
    strcpy(str, "55");
    Num = strtol(str, &ptr, 10);

    printf("The String input = %s is equal to Base 10(Decimal) long integer = %ld\n", str, Num);

    return 0;
}

Output

he String input = 55 is equal to Base 10(Decimal) long integer = 55

3. strtoumax() Function for Convert String to an Integer in C

In this example, we are going to make use of the strtoumax() function from the C Standard library. This function can be used by just including header file in your program.This function accepts a char input and return the number equivalent in integer.

Syntax

int strtoumax(const char* str, char** end, int base);

Parameters

  • *str = is a pointer to a string to be converted to a long integer.
  • **end = is a pointer to indicate where the conversion stops.
  • base = is the base representation with the range of [2, 36]. For example 2 is binary, 8 is octal etc.

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

int main()
{
    char str[10];
    char *ptr;
    int Num;
    strcpy(str, "55");

    Num = strtoumax(str, &ptr,10);

    printf("The String input = %s is equal to integer = %d\n", str, Num);  

    return 0;
}

Output

The String input = 55 is equal to integer = 55

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