{"post":"

Palindrome in C Program

What is a palindrome?

A palindrome is nothing but a text or string which when read forward or backward is read the same. It can contain any type of characters.
There are even palindrome numbers which are numbers which are the same when read forward and backward.
For e.g: MADAM, NAN, 12321, ANNA etc.
Note that, the palindrome can have even or odd number of characters.

Approach 1: Using the inbuilt C functions from string.h:

We first scan the string.
We then copy it to another string as well.
We reverse the copied string here it is string 2 using strrev
We compare the characters of both strings using strcmp; to check if the strung read forward and backward reads same or not.
This is not recommended approach as strrev is from Microsoft C library and does not work in standard C library. If you are using a Turbo C++ IDE, only then will the below approach work. The GNU library has no such function defined and will throw you an error.
Here to scan the string we are using fgets method in order to understand how it is used, although it is not recommended.

Code:

#include<conio.h>\n#include <string.h>    //include the header file which contains definition of string functions
\nint main()
\n{
\nclrscr();
\nchar string1[50], string2[50] ;
\nprintf(\"Enter a string : \\n\");
\nfgets(string1, 100, stdin);     //read string input from standard input screen.
\nstrcpy(string2,string1);     //copy string 1 into string 2
\nstrrev(string2);                  //reverse string 2 in order to compare both string in next step
\nif( strcmp(string1, string2) == 0 )
\nprintf(\"Entered string is a palindrome.\\n\");
\nelse
\nprintf(\"Entered string is not a palindrome.\\n\");
\ngetch();
\nreturn  0;
\n}

Output:

Case 1: String is a palindrome – odd numbered.

Enter a string: MADAM\nEntered string is a palindrome.

Case 2: String is a palindrome – even numbered.

Enter a string: 12ANNA21

Entered string is a palindrome.

Case 3: String is not a palindrome.

Enter a string: STUDYMITE

Entered string is not a palindrome.

Approach 2: Without using pre-defined functions of C:

In this approach we will first accept the input string.
Next, we calculate the length of the string.
We know that palindrome should be read same forward and backward.
Hence, we divide the array into two and traverse the elements while always comparing the elements of the first section and the second section to match with each other proceeding sequential.
It shall work for both odd and even palindromes; as for even palindromes the string will be equally divided and each element compared; whereas for odd palindromes we will compare all elements except the middle element.

Code:

#include <stdio.h>\nint main()
\n{
\nchar string[100];
\nint start, mid, end, size = 100, bytes_read, length, i, j;
\nprintf(\"Enter a string: \");
\nstring = (char *) malloc (size);
\nbytes_read = getline (&string, &size, stdin);
\nlength = bytes_read - 1;   //since getline() stores '\\n' at last block. hence bytesread - 1 = length of string.
\nend = length - 1;
\nmid = length/2;
\nfor (start = 0; start < mid; start++, end--)
\n{
\nif (string[start] != string[end])
\n{
\nprintf(\"Entered string is not a Palindrome.\\n\");
\nbreak;
\n}
\n}
\nif (start == mid)
\nprintf(\"Entered string is a Palindrome.\\n\");
\nreturn 0;
\n}

Output:

Case 1: String is a palindrome – odd numbered.

Enter a string: POP\nEntered string is a Palindrome.

Case 2: String is a palindrome – even numbered.

Enter a string: MADAM

Entered string is a Palindrome.

Case 3: String is not a palindrome.

Enter a string: STUDYMITE

Entered string is a Palindrome.

Approach 3: Program for number palindrome:

Here we scan an integer as input.
We then manually calculate the reverse of the number.
Next, both the original numbers and its reverse are compared; on returning true it means it is a palindrome ; if false then it is not.

Code:

#include <stdio.h>\nint main()
\n{
\nint num, rem = 0, rev, n ;
\nprintf(\"Enter an integer to check if it is palindrome or not\\n\");
\nscanf(\"%d\", &num);
\nn = num;
\nwhile (n != 0)              //reversing the number
\n{
\nrem = n % 10;
\nrev = rem + ( rev* 10);
\nn = n/10;
\n}
\nif (num == rev)                 //comparing to check palindrome or not
\nprintf(\"%d number is a palindrome.\\n\", num);
\nelse
\nprintf(\"%d number isn't a palindrome.\\n\", num);
\nreturn 0;
\n}

Output:

Enter an integer to check if it is palindrome or not\n12321 \n

12321 number is a palindrome.

Enter an integer to check if it is palindrome or not\n12312 \n

12312 number isn't a palindrome.

Approach 4: Program using pointers for palindrome:

We have used a combination of functions and pointers to address the concern.
We initially call a function check_palindrome; whose function is to return a flag to main() based on the comparison output of the string and its reverse.
We have created separate functions for copying the original string, then reversing it and then comparing them.

Code:

#include <stdio.h>\nint check_palindrome(char*, int);
\nvoid copy_string(char*, char*);
\nvoid reverse_string(char*, int);
\nint compare_string(char*, char*);
\nint main()
\n{
\nchar *string;
\nint result, size = 100, bytes_read, l;
\nprintf(\"Enter a string: \");
\nstring = (char *) malloc (size);
\nbytes_read = getline (&string, &size, stdin);
\nl = bytes_read - 1;
\nresult = check_palindrome(string, l);
\nif (result == 1)
\nprintf(\"Entered string is a palindrome.\\n\");
\nelse
\nprintf(\"Entered string is not a palindrome.\\n\");
\nreturn 0;
\n}
\nint check_palindrome(char *string, int length)
\n{
\nint check;
\nchar *reverse;
\nreverse = (char*)malloc(length+1);
\ncopy_string(reverse, string);
\nreverse_string(reverse, length);
\ncheck = compare_string(string, reverse);
\nfree(reverse);
\nif (check == 0)
\nreturn 1;
\nelse
\nreturn 0;
\n}
\nvoid copy_string(char *dest, char *source)
\n{
\nwhile(*source)
\n{
\n*dest = *source;
\nsource++;
\ndest++;
\n}
\n*dest = '\\0';
\n}
\nvoid reverse_string(char *string, int length)
\n{
\nint c;
\nchar *start, *end, temp;
\nstart = string;
\nend = string;
\nfor (c = 0; c < (length - 1); c++)
\nend++;
\nfor (c = 0; c < length/2; c++)
\n{
\ntemp = *end;
\n*end = *start;
\n*start = temp;
\nstart++;
\nend--;
\n}
\n}
\nint compare_string(char *string, char *reverse)
\n{
\nwhile(*string==*reverse)
\n{
\nif (*string == '\\0' || *reverse == '\\0')
\nbreak;
\nstring++;
\nreverse++;
\n}
\nif (*string == '\\0' && *reverse == '\\0')
\nreturn 0;
\nelse
\nreturn -1;
\n}

Output:

Enter a string: madam\nEntered string is a palindrome.\n

Enter a string: Neon\nEntered string is not a palindrome.

Thus, we have seen various methods to perform string reverse operation in C.

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