C Program to Check for Palindrome String

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In this article, you will learn and get about checking whether the given string is palindrome or not. Here are the list of programs available here:

Palindrome String Program in C
Palindrome String Program in C without using any Library function (string function)
Palindrome String Program in C using user-defined function

But before going through these programs. Let's understand about the meaning of palindrome string.

What is Palindrome String ?

Those string whose all the opposite characters are same is palindrome. And if any of the opposite character mismatched, then the string is not palindrome. For example:

codoc is a palindrome string, because:
- first character (c) is equal to last character (c)
- second character (o) is equal to second last character (0)
- third character (d) is equal to third last character (d)
codssdoc is a palindrome string, because:
- first character (c) is equal to last character (c)
- second character (o) is equal to second last character (o)
- third character (d) is equal to third last character (d)
- fourth character (s) is equal to fourth last character (s)
codes is not a palindrome string, because:
- first character (c) is not equal to last character (s)
codescracker is not a palindrome string, because:
- first character (c) is not equal to last character (r)

In other words, you can say that, if the reverse of a string is equal to its original, then that string can be called as a palindrome string.

Palindrome String Program in C

The question is, write a program in C to enter any string at run-time and check whether it is palindrome or not. The answer to this question is:

#include<stdio.h>
#include<conio.h>
#include<string.h>
int main()
{
    char str[50];
    int i, j, len, chk=0;
    printf("Enter any String (word): ");
    scanf("%s", str);
    len = strlen(str);
    for(i=0, j=(len-1); i<=(len-1); i++, j--)
    {
        if(str[i] != str[j])
        {
            chk=1;
            break;
        }
    }
    if(chk==1)
        printf("\nIt's not a Palindrome String");
    else
        printf("\nIt's a Palindrome String");
    getch();
    return 0;
}

The above program is written under CodeBlocks IDE. Here is the output after build and run. This is the first screenshot of first sample run:

And here is the second screenshot of first sample run:

As the reverse of codescracker is rekcarcsedoc which is not equal to the original string, that is codescracker, therefore the string is not palindrome.

Here is the first screenshot of second sample run:

Here in this case, the reverse of codoc is codoc which is same as the original string, that is codoc, therefore the string is palindrome.

Program Explained

First we have to count the length of string using strlen() function. For example, let's suppose the length of string is 6.
Create a loop to compare first character of the string to the last, and then second to the second last and so on. Therefore here first character is compared to the sixth character, and then second to the fifth, and third to the fourth. As indexing starts from 0, therefore, here we have to minus one from all. That is, 0^th to the length-1.
If any of the above comparison goes wrong, or if any character mismatched, then we have to initialize any number say 1 to any character say chk, and skip the remaining code of the loop using break statement.
After exiting from the loop whether using break statement or after running the whole loop. We have to check whether the variable chk holds its original value, that is 0 or not. If it holds its original value, then none of the opposite character mismatched (means it is palindrome), and if it doesn't holds its original value, then any of the opposite character mismatched (means it is not palindrome).
Here mismatched occurs, means the string is not a palindrome, otherwise the string is a palindrome.

Here is another sample run for, a palindrome string:

And here is one more sample run for, not a palindrome string:

Check Palindrome String in C without using String Function

Here is another program that will not use any string function. As you can see from the previous program, string function strlen() is used to find length of the entered string. But here, this program will calculate length of string without using any library function.

#include<stdio.h>
#include<conio.h>
int main()
{
    char str[50];
    int i, j, len, chk=0;
    printf("Enter any String (word): ");
    scanf("%s", str);
    len = 0;
    while(str[len]!='\0')
        len++;
    for(i=0, j=(len-1); i<=(len-1); i++, j--)
    {
        if(str[i] != str[j])
        {
            chk=1;
            break;
        }
    }
    if(chk==1)
        printf("\nIt's not a Palindrome String");
    else
        printf("\nIt's a Palindrome String");
    getch();
    return 0;
}

The code:

len=0;
while(str[len]!='\0')
   len++;

is used to find length of string. For example, if user enters codes as input, then initially 0 gets initialized to len before going to the while loop. The '\0' is known as null terminated character. At last of every string, a null terminated character gets automatically initialized. So the dry run of above code works like:

For first run, str[len] or str[0] or c is not equal to '\0'. Therefore len++ gets executed, so value of len gets incremented and becomes 1
For second run, str[len] or str[1] or o is not equal to '\0'. Therefore len++ gets executed, so value of len gets incremented and becomes 2
For second run, str[len] or str[2] or d is not equal to '\0'. Therefore len++ gets executed, so value of len gets incremented and becomes 3
For sixth run, str[len] or str[5] or '\0' is equal to '\0'. Therefore the condition evaluates to be false, so the loop gets terminated, and we have a variable, len that holds the length value of string.

Check Palindrome String in C using Function

This is the last program to check whether given string is a palindrome or not using user-defined function.

#include<stdio.h>
#include<conio.h>
int checkPalindromeStr(char []);
int main()
{
    char str[50];
    int chk;
    printf("Enter any String (word): ");
    scanf("%s", str);
    chk = checkPalindromeStr(str);
    if(chk==1)
        printf("\nIt's not a Palindrome String");
    else
        printf("\nIt's a Palindrome String");
    getch();
    return 0;
}
int checkPalindromeStr(char str[])
{
    int i, j, len;
    len = 0;
    while(str[len]!='\0')
        len++;
    for(i=0, j=(len-1); i<=(len-1); i++, j--)
    {
        if(str[i] != str[j])
            return 1;
    }
    return 0;
}

This program will produce the same output as produced by first program given, earlier of this article. To learn more about function, refer to Function in C tutorial.

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