Palindromes in C - c

Question

I am programming a palindrome code to check whether a given string is a palindrome or not. Eg: mom, 1001 ...
MY_CODE:
#include <stdio.h>
#include <stdlib.h>
#include <stdio_ext.h>
int main()
{
int i,n;
char p[999];
char flag;
printf("number of characters in the strings");
scanf("%d",&n);
printf("Enter string: ");
for (i=0;i<n;i++)
{
printf("\n");
__fpurge(stdin);
scanf("%c",&p[i]);
}
for (i=0;i<n;i++)
{
if (p[i]==p[n-1-i])
{
flag=0;
break;
}
else flag=1;
}
if (flag==1)
printf("It's not a palindrome");
if (flag==0)
printf("It's a palindrome.");
return 0;
}
I am trying to do that with the idea that if the last and first characters are matched and so on for the next characters. If they are all matched the string is a palindrome otherwise it is not,as simple as that but my output
shows 123;mom; and every nonsense, a palindrome ("even the word 'nonsense' :D).
Can someone guide me?
P.S.: I am a newbie and learning C. My OS: Ubuntu 15.10.

Answer 1

You are checking if characters are the same and is so, setting flag=0, to say it is a palindrome, but that will happen for the 1st match - if other matches do not occur, the flag is never set to not be a palindrome.
The better way is assume it is a palindrome and then if anything provides otherwise, set it as false.
#include <stdio.h>
#include <string.h>
int main()
{
char p[999];
printf("Enter string: ");
if(fgets(p, sizeof(p), stdin))
{
int palindrome = 1;
int i;
int n;
n = strlen(p);
/* handle new line at end of fgets input */
if(p[n-1] == '\n')
p[n-1] = '\0';
n = strlen(p);
/* 0 length string (after newline removed) - not a palindrome */
if(n == 0) palindrome = 0;
for(i=0;i<n/2;i++)
{
/* look for a chatacter pair that doesn't match - if find, then we don't have a palindrome */
if(p[i] != p[n-1-i])
palindrome = 0;
}
if(palindrome)
printf("is a palindrome\n");
else
printf("is not a palindrome\n");
}
return 0;
}

Answer 2

From your code, I have a couple of things to say as follows.
#include <string.h>
#include <stdio.h>
// #include <stdlib.h> // this is not necessary.
int main()
{
// int i,n;
// char p[999];
// char flag;
// ######################################################
// You literally don't need to input the number of characters since you can get the string's length from `strlen`.
// In addition, different inputs yield different lengths,
// therefore, it is not applicable if the input becomes extremely long, for example.
//
// printf("number of characters in the strings");
// scanf("%d",&n);
// printf("Enter string: ");
// for (i=0;i<n;i++)
// {
// printf("\n");
// __fpurge(stdin);
// scanf("%c",&p[i]);
// }
// ######################################################
char str[100];
printf( "Enter a value :");
gets( str );
int i=0;
int n;
n = strlen(str)-1;
while (i<n)
{
if (str[i++] != str[n--])
{
printf("%s is Not palidrome\n", str);
return 0;
}
}
printf("%s is palidrome\n", str);
// for (i=0;i<n;i++)
// {
// if (p[i]==p[n-1-i])
// {
// flag=0;
// break;
// }
// else flag=1;
// }
// if (flag==1)
// printf("It's not a palindrome");
// if (flag==0)
// printf("It's a palindrome.");
return 0;
}
With a slightly modification as above, that code works well.
Note: The palindrome problem is a classical problem, and you literally can find many solutions from the internet. You can refer the solution code in C at here.
Update: Here is my update with fgets and while-loop.
#include <string.h>
#include <stdio.h>
int main()
{
char str[100];
printf( "Enter a string (char/value) :");
fgets (str, 100, stdin);
int i=0;
int n;
n = strlen(str)-1;
while (i<n)
{
if (str[i++] != str[n--])
{
printf("%s is Not palidrome\n", str);
return 0;
}
}
printf("%s is palidrome\n", str);
return 0;
}