check whether a string is palindrome

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C++

#include<stdio.h>
#include<stdlib.h>
int main()
{
 char a[20];
 int n;
 printf("enter the size of the string  ");
 scanf("%d",&n);
 printf("enter the string ");
 scanf("%s",&a[20]);
 for(int i=0;i<(n-1)/2;i++)
     {
        if(a[i]==a[n-1-i])
            {
                 printf("string is palindrome");
            }
        else
         printf("string is not palindrome");

     }
     return 0;
}

when i am running this program i am getting stack exception...is my code wrong?...i mean is the process wrong? or is the error due to some kind of buffer overflow for using scanf ? Anyone pls help..

================
Edit: Matt T Heffron (code moved from comment below):
i fixed the logic,checked overflow problems but its still not executing...can someone explain...

C++

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{
 char a[20];
 int n,c;
 c=0;
 printf("enter the size of the string  ");
 scanf("%d",&n);
 printf("enter the string ");
 fgets(a,n,stdin);

 for(int i=0;i<(n-1)/2;i++)
	 {
		if(a[i]==a[n-1-i])
			{
			 c=0;
			}
		else
		{
		  c=1;
		  break;
		}
	 }

	 if(c==0)
	  printf("string is palindrome");
	 else
	  printf("string is not palindrome");

	 return 0;

	 }

Posted 11-Nov-15 0:20am

Sam Dean

Updated 12-Nov-15 10:30am

Matt T Heffron

v3

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Comments

W∴ Balboos, GHB 11-Nov-15 7:22am

With the fixes to your solution, below, do not forget that when comparing character that case is important: convert your string to all UC or all LC before commencing the test.

Also, if the string is a sentence, you need to remove punctuation (spaces) before your test, at least for many definitions of palindrome that allow the spacing to be neglected.

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OriginalGriff · Answer 1 · 2015-11-11T00:28:00

Solution 1

Um... try changing this:

C++

scanf("%s",&a[20]);

To this:

C++

scanf("%s",a);

Or even:

C++

scanf("%s",&a[0]);

Posted 11-Nov-15 0:28am

OriginalGriff

Krunal Rohit · Answer 2 · 2015-11-11T02:21:00

C++

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

int main(){
    char string1[20];
    int i, length;
    int flag = 0;
    
    printf("Enter a string:");
    scanf("%s", string1);
    
    length = strlen(string1);
    
    for(i=0;i < length ;i++){
        if(string1[i] != string1[length-i-1]){
            flag = 1;
            break;
	   }
	}
    
    if (flag) {
        printf("%s is not a palindrome", string1);
    }    
    else {
        printf("%s is a palindrome", string1);
    }
    return 0;
}

Or with the help of string functions like strrev, strcmp:

C++

#include <stdio.h>
#include <string.h>
 
int main()
{
   char a[100], b[100];
 
   printf("Enter the string to check if it is a palindrome\n");
   gets(a);
 
   strcpy(b,a);
   strrev(b);
 
   if (strcmp(a,b) == 0)
      printf("Entered string is a palindrome.\n");
   else
      printf("Entered string is not a palindrome.\n");
 
   return 0;
}

-KR

CPallini · Answer 3 · 2015-11-11T00:58:00

Solution 2

In addition of the mistake pointed by our Original, your code is logically flawed (it can assert the string is palindrome before the whole string is actually checked).
Try

C

 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #define MAX_LEN 20
int main()
{
  char a[MAX_LEN+1];

  printf("enter the string ");
  if ( fgets( a, MAX_LEN+1, stdin) )
  {
    size_t len = strlen(a)-1; // get rid of '\n'
    size_t i;
    printf("n = %d\n", len);
    for(i=0;i<len/2;i++)
    {
      if (a[i]!=a[len-1-i] ) break;
    }
    if ( i == len/2)
      printf("\nstring is palindrome");
    else
      printf("\nstring is not palindrome");

    printf("\n");
  }
  return 0;
}

Posted 11-Nov-15 0:58am

CPallini

Updated 11-Nov-15 2:33am

v2

Comments

Patrice T 11-Nov-15 11:16am

I fear the program fail if string length is odd.

CPallini 11-Nov-15 12:00pm

I guess it works. :-)

Patrice T 11-Nov-15 12:18pm

Ok, the integer division trick :)
I avoid it when I am not in optimising phase.

Sam Dean 11-Nov-15 14:46pm

thanks @CPallini...my logic is same as yours..checking the 1st element with the last..excepts that it won't check through the whole string..

Mohibur Rashid 11-Nov-15 19:05pm

Why do you think that?

Matt T Heffron 11-Nov-15 19:24pm

So at this point you should be using your debugger to step through the program execution, look at the values of variables and figure out what it is doing that is not what you intend.
Knowing how to use the debugger is a valuable skill!

Sam Dean 11-Nov-15 23:26pm

@Matt T Heffron, THANK YOU MATT, i am new to programming..this may sound silly but can you tell me how to use a debugger?

Sam Dean 12-Nov-15 10:05am

i fixed the logic,checked overflow problems but its still not executing...can someone explain...

(Edit: Matt T Heffron -- code moved to question)

jeron1 12-Nov-15 14:05pm

"still not executing", what does that mean exactly?

Matt T Heffron 12-Nov-15 16:35pm

This isn't readily readable for us (code in the comments loses the formatting).
Use the "Improve question" above when you are showing the work in progress.
(I edited this and moved it to the "Question" above.)

Andreas Gieriet · Answer 4 · 2015-11-12T11:43:00

Now for something completely different :-)
How about a recursive approach?

C++

#include <stdio.h>
#include <string.h>
#define MAXCHARS 20 // excluding trailing zero character
#define STRINGIZE(a) #a
#define MAX_CHARS_STRING_FMT(len) "%" STRINGIZE(len) "s"

const char* read_string()
{
    static char buffer[MAXCHARS+1]; // allow for a trailing zero character
    scanf(MAX_CHARS_STRING_FMT(MAXCHARS), buffer);
    buffer[MAXCHARS] = '\0'; // paranoid...
    return buffer;
}

int is_palindrom(const char* buffer, size_t len)
{
    return (len < 2) || ((buffer[0] == buffer[len-1]) && is_palindrom(buffer+1, len-2));
}

int main()
{
    const char* buffer = read_string();
    size_t len = strlen(buffer);
    printf("'%s' is %sa palindrom\n", buffer, is_palindrom(buffer, len) ? "": "not "); 
    return 0;
}

Have fun! :-)
Cheers
Andi

Patrice T · Answer 5 · 2015-11-11T05:28:00

A varying solution from the one of KrunaRohit

C++

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

int main(){
    char string1[20];
    int i, j, length;
    
    printf("Enter a string:");
    scanf("%s", string1);
    
    length = strlen(string1);
    
    int flag = 1;
    for(i=0, j=length-1; i < j ;i++, j--){
        if(string1[i] != string1[j]){
            flag = 0;
            break;
	   }
	}
    
    if (flag) {
        printf("%s is a palindrome", string1);
    }    
    else {
        printf("%s is not a palindrome", string1);
    }
    return 0;
}

Using traditionnal 8 bits chars will fail if the palindrome include some utf-8 chars.

check whether a string is palindrome

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