Can C presume which array I want to store my characters? - c

Question

I'm writing code which checks if an array is palindrome or not:
Write a program that reads a message, then checks whether it's a palindrome
(the letters in the message are the same from left to right as from right to left):
Enter a message: He lived as a devil, eh?
Palindrome
Enter a message: Madam, I am Adam.
Not a palindrome
When I have entered He lived as a devil, eh?,
it gives me the output Not a palindrome,
but the real output should be palindrome.
Below code is what I have tried so far.
#include <stdio.h>
#include <ctype.h>
#define MAX_LEN 100
int main(void) {
char message[MAX_LEN];
char c, *p = message, *q;
printf("Enter a message: ");
while ((c = toupper(getchar())) != '\n' & p < message + MAX_LEN) {
if (isalpha(c))
*p++ = c;
}
p--;
for (q = message; q < p; q++, p--) {
if (*p != *q) {
printf("Not a palindrome\n");
return 0;
}
}
printf("Palindrome\n");
return 0;
}

Answer 1

For starters you should declare the variable c as having the type int. The user can interrupt the input process in which case the function getchar returns integer value EOF and you should check whether this occurred.
char *p = message, *q;
int c;
There is a bug in the condition of the while statement
while ((c = toupper(getchar())) != '\n' & p < message + MAX_LEN) {
Instead of the bitwise operator & you have to use the logical AND operator &&.
As I already have said you should check in the condition of the while statement whether the user interrupted the input. For example
while ( p < message + MAX_LEN && ( c = toupper(getchar())) != EOF && c != '\n') {
if (isalpha(c))
*p++ = c;
}
The argument of a call of toupper or isalpha should be converted to the type unsigned char. Otherwise in general without the casting such a call can invoke undefined behavior.
It is desirable not to exclude from an entered string numbers. SO it is better at least to call the function isalnum instead of the function isalpha.
The user can enter an empty string in this case this decrement of the pointer
p--;
also can invoke undefined behavior.
And it is better when a program has one point to exit.
The program can look the following way
#include <stdio.h>
#include <ctype.h>
#define MAX_LEN 100
int main(void)
{
char message[MAX_LEN];
printf( "Enter a message: " );
char *p = message;
for ( int c; p < message + MAX_LEN && ( c = getchar() ) != EOF && c != '\n'; )
{
if( isalnum( ( unsigned char )c ) )
{
*p++ = toupper( ( unsigned char )c );
}
}
int palindrome = 1;
if ( p != message )
{
for ( char *q = message; palindrome && q < --p; ++q )
{
palindrome = *q == *p;
}
}
printf( "The entered message is %spalindrome\n",
palindrome ? "" : "not " );
return 0;
}
Its output might look for example like
Enter a message: He lived as a devil, eh?
The entered message is palindrome
or like
Enter a message: Madam, I am Adam
The entered message is not palindrome
Pay attention to that instead of using a loop with numerous calls of the function getchar you could use only one call of the function fgets
fgets( message, sizeof( message ), stdin );
or
if ( fgets( message, sizeof( message ), stdin ) != NULL )
{
// check whether the entered string is a palindrome
}

Answer 2

Before check the palindrome you have to remove white spaces and punctuation marks. For an example if you use civic?, it is not palindrome because of ?. In the other hand if you use civ ic, it is not palindrome because of white space. There for
Convert all letters to uppercase or lowercase.
Remove white spaces.
remove punctuation marks.
Check palindrome or not.
You can do it by using # include <string.h>
First thing is you have to use scanf() which accept string with white space.
printf("Enter a string = ");
scanf("%[^\n]%*c", word);
Then you have to convert that string to Uppercase or Lowercase because a != A. We know civic is a palindrome but Civic is not a palindrome('Civic != civiC) because Uppercase letters have different ASCII values and Lowercase letters have different ASCII values.
(a - z) -: 97 - 122
(A - Z) -: 65 - 90
In my case I have converted lowercase to uppercase.
while(strlen(word) >= i)
{
if(word[i] >= 97 && word[i] <= 122)
{
word[i] = word[i] - 32;
}
i++;
}
Another case is your if you enter civ ic with white space, it's palindrome word is ci vic. You can see civ ic != ci vic. There for you have to remove white spaces in your program. And also you have to remove punctuation marks because if you use civic, it's reversed word is ,civic'. You can seecivic, != ,civic`.
int len = strlen(word);
while(a < len)
{
for(i = 0; i < len; i++)
{
if(word[i] == ' ' || !(word[i] >= 'A' && word[i] <= 'Z'))
{
for(j = i; j < len; j++)
{
word[j] = word[j+1];
}
len--;
}
}
a++;
}
Final thing is we have to revers our string and need to check if our reversed string is equal to our original string. If it is true our String is palindrome. If it is false our String is not a palindrome.
for(i = 0; i < len; i++)
{
if(word[i] == word[len - 1])
{
len--;
}
else
{
printf("%s is not a palindrome\n", word);
return 0;
}
}
printf("%s is a palindroeme\n", word);
This the full code after you merging above parts
# include <stdio.h>
# include <string.h>
int main (void)
{
char word[100];
int i = 0;
int j, x = 0;
int a = 0;
printf("Enter a string = ");
scanf("%[^\n]%*c", word);
while(strlen(word) >= i)
{
if(word[i] >= 97 && word[i] <= 122)
{
word[i] = word[i] - 32;
}
i++;
}
printf("After converting it to uppercase = %s\n", word);
int len = strlen(word);
while(a < len)
{
for(i = 0; i < len; i++)
{
if(word[i] == ' ' || !(word[i] >= 'A' && word[i] <= 'Z'))
{
for(j = i; j < len; j++)
{
word[j] = word[j+1];
}
len--;
}
}
a++;
}
printf("After removing spaces = %s\n", word);
for(i = 0; i < len; i++)
{
if(word[i] == word[len - 1])
{
len--;
}
else
{
printf("%s is not a palindrome\n", word);
return 0;
}
}
printf("%s is a palindroeme\n", word);
return 0;
}
First test Output -:
Enter a string = He lived as a devil, eh?
After converting it to uppercase = HE LIVED AS A DEVIL, EH?
After removing spaces = HELIVEDASADEVILEH
HELIVEDASADEVILEH is a palindroeme
Second test Output -:
Enter a string = Madam I am Adam.
After converting it to uppercase = MADAM I AM ADAM.
After removing spaces = MADAMIAMADAM
MADAMIAMADAM is not a palindrome