Check if input is integer type in C

Question

The catch is that I cannot use atoi or any other function like that (I'm pretty sure we're supposed to rely on mathematical operations).

 int num; 
 scanf("%d",&num);
 if(/* num is not integer */) {
  printf("enter integer");
  return;
 }

I've tried:

(num*2)/2 == num
num%1==0
if(scanf("%d",&num)!=1)

but none of these worked.

Any ideas?

Answer 1

I found a way to check whether the input given is an integer or not using atoi() function .

Read the input as a string, and use atoi() function to convert the string in to an integer.

atoi() function returns the integer number if the input string contains integer, else it will return 0. You can check the return value of the atoi() function to know whether the input given is an integer or not.

There are lot more functions to convert a string into long, double etc., Check the standard library "stdlib.h" for more.

Note : It works only for non-zero numbers.

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

int main() {
    char *string;
    int number;

    printf("Enter a number :");
    scanf("%s", string);

    number = atoi(string);

    if(number != 0)
        printf("The number is %d\n", number);
    else
        printf("Not a number !!!\n");
    return 0;
}