c - 反向字符串比较（回文）

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] == '\n')
            {
            text[i] = i - 1;
            }
        }

    i = strlen(text)-1;

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

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

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

Answer 1

Yes, your treatment of the linefeed character is wrong.

You replaced the linefeed character with some other (or maybe same) character, but it means there will be an extra character in the string and the extra character will prevent the string from being jugded as a palindrome.

You should replace the linefeed character to '\0' (NUL character), which is used as string termination mark in C.

Answer 2

This if statement

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

does not make a sense. Moreover as the new line character '\n' 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 '\0' .

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] != '\0' )
    {
        if ( text[n] == '\n' )
        {
            text[n] = '\0';
        }
        else
        {
            text[n] = tolower( ( unsigned char )text[n] );
            ++n;
        }
    }
    
    reverse[n] = '\0';

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

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

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

    if ( is_palindrome( text ) )
    {
        printf( "The entered text \"%s\" is a palindrome!\n", text );
    }
    else
    {
        printf( "The entered text \"%s\" is NOT a palindrome!\n", 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 '\n' was substituted for the zero character '\0' .