to find if a given string is palindrome or is not palindrome

Question

I made a program to find if a entered string is palindrome or not palindrome but it always says that its not a palindrome

#include <conio.h> 
#include <graphics.h> 
#include <string.h>
void main(void)
{
    int i,len,halflen,flag=1;
    char str[50];
    clrscr();
    printf("Enter a string:\n");
    gets(str);
    len=strlen(str);
    halflen=len/2;
    for(i=0;i<halflen;i++)
    {
        if(str[i]!=str[i+halflen])
            flag=0;
        break;

    }
    if(flag)
        printf("It is a Palindrome.");
    else
        printf("It is not a Palindrome.");
    getch();
}

Answer 1

To give you a clue I've done some tidier indenting of a bit of your code:

for(i=0;i<halflen;i++)
    {
        if(str[i]!=str[i+halflen])
            flag=0;
        break;
    }

Answer 2

Your central comparison is flawed:

if (str[i] != str[i+halflen]) 

This isn't comparing the two characters you think it is.

Try entering "HelloHello" into your program, it will say it is a palindrome!

You need to compare these two:

if (str[i] != str[len-i-1])

(and fix the braces, as suggested in the other answer)

Answer 3

From the 2005 version of myself:

bool isAlphaNumeric(char c)
{
    return (iswalpha(c) || iswdigit(c));
}

bool isPalindrome(char *str)
{
    /* A man, a plan, Anal Panama!!! */
    if(*str == '\0')
    {
        return false;
    }

    int len = strlen(str);
    if(len <= 1) return true;

    char *start = str;
    char *end = start + len - 1;

    while(start < end)
    {
        if(!isAlphaNumeric(*start))
        {
            *start++;
            continue;
        }
        if(!isAlphaNumeric(*end))
        {
            *end--;
            continue;
        }
        if(towlower(*start) != towlower(*end))
        {
            return false;
        }
        *start++;
        *end--;
    }
    return true;
}

Answer 4

Here you want something like...

    if(str[i]!=str[strlen (str) - i - 1])
    {
        flag = 0;
        break;
    }

The break needs to go in the if block otherwise it will always get executed. Initialising flag at some point would be a good idea, too. If I might be permitted an observation, ALWAYS enclose the if-block and else block in curly brackets even if there is only one statement; it would save you several of the problems you've got here.

Later - edited per Mr Rodriguez' comments below.