fgets - c - reverse string comparison (palindrome)

Question

I want to check an entered text for a palindrome.

However, when I enter a palindrome, I always get that it is NOT a palindrome. Have I done something wrong with the stored linefeed character?

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

int main()
{
char text[256], reverse[256];
int i, j;

    printf("Type a text: ");
    fgets(text, 255, stdin);

    j = strlen(text)-1;
    
    for (i=0; i<=j; ++i)
        {
            if(text[i] >= 'A' && text[i] <= 'Z')
            {
            text[i] += 32;
            }
            if(text[i] == '
')
            {
            text[i] = i - 1;
            }
        }

    i = strlen(text)-1;

    for (i = i, j = 0 ;i >= 0 ; --i, ++j)
        {
        reverse[j] = text[i];
        }

    printf("Text: %s
", text);
    printf("Reverse: %s
", reverse);

    if (strcmp(text, reverse) == 0)
        {
        printf("The entered text "%s" is a palindrome!
", text);
        }
    else
        {
        printf("The entered text "%s" is NOT a palindrome!
", text);
        }
}

Answer 1

This if statement

      if(text[i] == '
')
        {
        text[i] = i - 1;
        }

does not make a sense. Moreover as the new line character ' ' is a single character in an entered string then keeping it within the string will result that the string is not a palindrome.

You need to substitute it for the zero character ''.

And the array reverse does not contain a string after this loop

for (i = i, j = 0 ;i >= 0 ; --i, ++j)
    {
    reverse[j] = text[i];
    }

Also using the function strlen is redundant and inefficient.

The program can look the following way.

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

int main(void) 
{
    char text[256], reverse[256];

    printf( "Type a text: " );
    
    fgets( text, sizeof( text ), stdin );
    
    size_t n = 0;
    
    while ( text[n] != '' )
    {
        if ( text[n] == '
' )
        {
            text[n] = '';
        }
        else
        {
            text[n] = tolower( ( unsigned char )text[n] );
            ++n;
        }
    }
    
    reverse[n] = '';

    for ( size_t i = 0; n-- != 0; i++ )
    {
        reverse[n] = text[i];
    }
    
    printf( "Text: "%s"
", text );
    printf( "Reverse: "%s"
", reverse);

    if ( strcmp( text, reverse ) == 0 )
    {
        printf( "The entered text "%s" is a palindrome!
", text );
    }
    else
    {
        printf( "The entered text "%s" is NOT a palindrome!
", text );
    }
    
    return 0;
}

The program output might look like

Type a text: ABC cba
Text: "abc cba"
Reverse: "abc cba"
The entered text "abc cba" is a palindrome!

In general the used by you approach when you need an auxiliary array and when the source string is being changed is not good.

Your could write a simple function as it is shown in this demonstrative program.

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

int is_palindrome( const char *s )
{
    size_t n = strlen( s );
    size_t i = 0;
    
    while ( i < n / 2 && tolower( ( unsigned char )s[i] ) == 
                         tolower( ( unsigned char )s[n -i -1] ) )
    {
        ++i;
    }
    
    return i == n / 2;
}

int main(void) 
{
    char text[256];

    printf( "Type a text: " );
    
    fgets( text, sizeof( text ), stdin );
    
    text[ strcspn( text, "
" ) ] = '';

    printf( "Text: "%s"
", text );

    if ( is_palindrome( text ) )
    {
        printf( "The entered text "%s" is a palindrome!
", text );
    }
    else
    {
        printf( "The entered text "%s" is NOT a palindrome!
", text );
    }
    
    return 0;
}

The program output might look like

Type a text: ABC cba
Text: "ABC cba"
The entered text "ABC cba" is a palindrome!

As you see the original string was not changed except the new line character ' ' was substituted for the zero character ''.