How to convert string to int without using library functions in c

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How to convert string to int without using library functions in c programming language

#include<stdio.h>

int stringToInt(char[] );

int main(){

  

    char str[10];

    int intValue;

    printf("Enter any integer as a string: ");

    scanf("%s",str);

   intValue = stringToInt(str);

    printf("Equivalent integer value: %d",intValue);

    return 0;

}

int stringToInt(char str[]){

    int i=0,sum=0;

    while(str[i]!='\0'){

         if(str[i]< 48 || str[i] > 57){

             printf("Unable to convert it into integer.\n");

             return 0;

         }

         else{

             sum = sum*10 + (str[i] - 48);

             i++;

         }

    }

    return sum;

}

Sample output:

Enter any integer as a string: 123

Equivalent integer value: 123

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