Trying to check if a number is a palindrome through the use of strings

Question

I am trying to check if an input number is a palindrome. I am doing it through strings rather than ints. So, I am taking in a string and reversing it into another string. However, when I use the string compare function it does not give me 0, stating that the strings are not the same. Even when I put in for example "1001", both the input and reverse strings displays 1001. I have figured it out with other methods but am trying to understand what is wrong with this one in specific.

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

int main(void)
{
    char input[100];
    char reverse[100];

    int numLen = 0;

    printf("Enter a number\n");
    fgets(input, 100, stdin);
    printf("The number is: %s\n", input);

    numLen = strlen(input) - 1;
    printf("Length of string is: %d\n", numLen);

    for (int i = 0; i < numLen; i++)
    {
        reverse[i] = input[numLen - 1 - i];

        if (i == numLen - 1)
        {
            reverse[i + 1] = '\0';
        }
    }

    printf("The reverse number is: %s\n", reverse);
    printf("The original number is: %s\n", input);

    int result = strcmp(input, reverse);
    printf("Result of strcmp gives us: %d\n", result);

    if (strcmp(input, reverse) == 0)
    {
        printf("These numbers are palindromes\n");
    }
    else
    {
        printf("These numbers are not palindromes\n");
    }

    return 0;
}

Answer 1

The problem is you are not handling the strings properly. You should overwrite the '\n' with \0.

    ...
    char input[100];
    char reverse[100];

    int numLen = 0;

    printf("Enter a number\n");
    fgets(input, 100, stdin);
    printf("The number is: %s\n", input);
    input[strcspn(input,"\n")]='\0'; // getting the length of the 
                                     // string without `\n` 
                                     // and overwriting with `\0`
    numLen = strlen(input) ;  // now you don't need to put the -1
    printf("Length of string is: %d\n", numLen);

    for (int i = 0; i < numLen; i++)
    {
        ....

Apart from these two changes everything else remains the same. You were reversing it all right. And then you used strcmp right way. But the extra \n is removed in the code I have shown.

(still) Why it works?

Now to give you a better idea. You formed the reversed string alright. But the original string has \n within itself.

printf("The reverse number is: (%s)\n", reverse);
printf("The original number is: (%s)\n", input);

In the previous program you just do write these two lines. You will understand where you went wrong.

On giving input 1001Enter it gives this output.

The reverse number is: (1001)
The original number is: (1001
)

---

What is strcspn doing?

I have using strcspn function got the length without \n and overwriting it with \0.

0 1 2 3  4  5   --> indices
1 0 0 1 \n \0   --> strcspn(input,"\n") returns 4.
1 0 0 1 \0 \0   --> input[strcspn(input,"\n")]='\0'

---

You can do simply like this without the copying and everything.

Without extra memory - in place palindrome checking

bool checkPal(const char *s){
    for(int i = 0, j= strlen(s)-1; i< strlen(s) && j>=0 ; i++)
        if(s[i] != s[j])
            return false;
    return true;
}
int main(void)
{
    char input[100];
    char reverse[100];

    
    printf("Enter a number\n");
    if( fgets(input, 100, stdin) )
    printf("The number is: %s\n", input);
    
    input[strcspn(input,"\n")]='\0';
    int numLen = strlen(input) ;
    printf("Length of string is: %d \n", numLen);
  
    printf("These numbers are %spalindromes\n", checkPal(input)?"not ":"");
    return 0;
}

A more succinct way to write the checkPal() would be,

bool checkPal(const char *first){
    const char *last = first + strlen(first);
    while (first < last) {
       if (*first++ != *--last) {
           return false;
       }
   }
   return true;
}

last points to the \0 character. Subtraction is necessary before we start doing comparison. To get a clear idea of what happens you have to know the precedence and few rules.

The first<last part is obvious. We are comparing till we reach a point where we first > last (For even length strings) or first = last (for odd length strings).

The if is a bit tricky. *first++ there are two operators involved. * (indirection) and ++(post increment).

And precedence of ++ is higher than de-reference *.

So *first++ will be - first is incremented. Then you might think that we are missing one character very first time but that's not the case. Value of a postfix expression is the value before we do first++. So now you have the first character.

Same way *--last will have the same effect except the value of the prefix expression is the value after the operation. So you are considering the last character.

If they matches we continue. first and last already contain the modified value. We repeat the same logic for rest of the characters in the smaller sub-string.

If a mismatch occurs then we return immediately. (Because it's not a palindrome).