Why does it tell me its an Armstrong number when I enter strings?

Question

This is the code for finding an Armstrong number of 3 digits. But when i enter strings or any other special character it categorizes it as an armstrong number while it should be other way around.

#include <stdio.h>

#include <stdlib.h>

int main() {
  int a, original, rev, rem;
  printf("Enter the number : \n");
  scanf("%d", & a);

  original = a;

  rev = 0;
  while (a != 0) {

    rem = a % 10;
    rev = rev + (rem * rem * rem);
    a /= 10;
  }
  if (rev == original) {
    printf("Its an Armstrong number\n");
  } else {
    printf("Its not an Armstrong number \n");
  }

}

For me it works: `Enter the number : aaaa Its not an Armstrong number` — Caridorc, Feb 19 '23 at 01:45
`scanf` is expecting an integer so I guess it will cause undefined behaivour if the input is a string. — xihtyM, Feb 19 '23 at 01:49
[Please do not upload images of code/data/errors.](//meta.stackoverflow.com/q/285551) — Ken White, Feb 19 '23 at 02:02
[**Always** check the return values of `scanf()`](https://stackoverflow.com/q/10084224/995714) — phuclv, Feb 19 '23 at 02:07

score 3 · Answer 1 · answered Feb 19 '23 at 02:04

3

It is Undefined Behaviour as you use not initialized variable a.

You should have checked for the incorrect input:

  if(scanf("%d", &a) != 1)
  {
    printf("Invalid input\n");
    return 1;
  }

answered Feb 19 '23 at 02:04

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Yes, need to check the input value is number or not. – tzztson Feb 19 '23 at 02:18

score 3 · Accepted Answer · answered Feb 19 '23 at 02:25

You need to check the result of scanf - it will return the number of items successfully converted and assigned.

if ( scanf( "%d", &a ) != 1 )
  // bad input
else
  // check if a is an armstrong #

An input like nnnniii is not a valid integer, so the read fails and a is not updated.

While the initial value of auto variables is indeterminate, it's possible that a has an initial value of 0, so your test passes by accident.

score 1 · Answer 3 · answered Feb 19 '23 at 02:17

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

int main() {
  int a, original, rev, rem;
  printf("Enter the number : \n");
  if(scanf("%d", &a) != 1)
  {
    printf("This is not number\n");
    return 1;
  }
 
  original = a;
  rev = 0;
  while (a != 0) {
    rem = a % 10;
    rev = rev + (rem * rem * rem);
    a /= 10;
  }

 if (rev == original) {
    printf("Its an Armstrong number\n");
  } else {
    printf("Its not an Armstrong number \n");
  }

}

When you get value, you need to check it is number or not.

Why does it tell me its an Armstrong number when I enter strings?

3 Answers3