C Program to Check for Palindrome String - c

Question

I wrote two sample programs to check for a palindrome string. But in both I am getting output like, its not a palindrome number. What I am missing?
I strictly assume somehow code is executing my if statement and put flag in to 1. May be because of that length calculation. Anyone has a better idea?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
int main(void) {
setbuf(stdout,NULL);
char name[100];
int i,length,flag=0,k;
printf("Enter your name");
/*scanf("%s",name);*/
gets(name);
length=strlen(name);
for(i=0;i<=length-1;i++)
{
for(k=length-1;k>=0;k--)
{
if(name[i]!=name[k])
{
flag=1;
break;
}
}
}
if(flag==0)
{
printf("Give word is a palindrome");
}
if(flag==1)
{
printf("This is NOT a palindrome word");
}
return 0;
}
and
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
int main(void) {
setbuf(stdout,NULL);
char name[100];
int i,length,flag=0;
printf("Enter your name");
/*scanf("%s",name);*/
gets(name);
length=strlen(name);
for(i=0;i<=length/2;i++)
{
if(name[i]!=name[length-1])
{
flag=1;
}
}
if(flag==0)
{
printf("Give word is a palindrome");
}
if(flag==1)
{
printf("This is NOT a palindrome word");
}
return 0;
}

Answer 1

First Algorithm
The algorithm you are using in the first program involves comparing each letter to every other letter which does not help in determining if the number is a palindrome and it does not seem fixable.
Second Algorithm
The problem with the second approach, however, is you are always comparing name[i] to name[length]. Instead change it to length-i-1. This will start comparing from length-1 and decrement the length of the character by 1 for every next iteration:
for(i = 0;i <= length / 2;i++)
{
if(name[i] != name[length-i-1])
{
flag=1;
break;
}
}
gets() and buffer overflow
Do not use gets. This method is susceptible to a buffer overflow. If you enter a string longer than 100 characters, it will result in undefined behavior. Use fgets instead for deterministic behavior:
fgets(name, sizeof(name), stdin);
This takes in the size of the buffer and only reads up to sizeof(name) characters.
Full code
Ideally, you should consider wrapping the logic to check if the string is a palindrome in a function:
int is_palindrome(char*);
int main(void)
{
char name[100];
setbuf(stdout,NULL);
printf("Enter your name");
fgets(name, sizeof(name), stdin);
if(is_palindrome(name))
{
printf("The given word is a palindrome");
}
else
{
printf("This is NOT a palindrome word");
}
return 0;
}
int is_palindrome(char* name)
{
int length = strlen(name);
int flag = 0, i;
for(i = 0;i <= length / 2; i++)
{
if(name[i]!=name[length-i-1])
{
return 0;
}
}
return 1;
}

Answer 2

There is plenty wrong with both your attempts. I strongly suggest using a debugger to investigate how your code works (or doesn't).
Your first attempt performs length2 (incorrect) comparisons, when clearly only length / 2 comparisons are required. The second performs length / 2 comparisons but the comparison is incorrect:
name[i] != name[length-1] ;
should be:
name[i] != name[length - i - 1] ;
Finally you iterate exhaustively when you could terminate the comparison as soon as you know they are not palindromic (on first mismatch).
There may be other errors - to be honest I did not look further than the obvious, because there is a better solution.
Suggest:
#include <stdbool.h>
#include <string.h>
bool isPalindrome( const char* str )
{
bool is_palindrome = true ;
size_t rev = strlen( str ) - 1 ;
size_t fwd = 0 ;
while( is_palindrome && fwd < rev )
{
is_palindrome = (str[fwd] == str[rev]) ;
fwd++ ;
rev-- ;
}
return is_palindrome ;
}
In use:
int main()
{
const char* test[] = { "xyyx", "xyayx", "xyxy", "xyaxy" } ;
for( size_t t = 0; t < sizeof(test)/sizeof(*test); t++ )
{
printf("%s : %s palindrome\n", test[t],
isPalindrome( test[t] ) ? "Is" : "Is not" ) ;
}
return 0;
}
Output:
xyyx : Is palindrome
xyayx : Is palindrome
xyxy : Is not palindrome
xyaxy : Is not palindrome

Answer 3

Try this
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
char text[100];
int begin, middle, end, length = 0;
printf("enter the name: ");
scanf("%s",text);
while ( text[length] != '\0' ){
length++;}
end = length - 1;
middle = length/2;
for ( begin = 0 ; begin < middle ; begin++ ) {
if ( text[begin] != text[end] ) {
printf("Not a palindrome.\n");
break;
}
end--;
}
if( begin == middle )
printf("Palindrome.\n");
return 0;
}
The problem with the first piece of code is you are comparing it more than required, compare it with length-i-1.
The main problem with the second code is you are comparing it with only the last letter of a word.
Hope you understood your mistake