Ignoring specific characters in a string

Hi,

So I am writing a program that checks to see whether a string input by the user is a palindrome or not. Part of the problem involves ignoring any character that isn't a letter when verifying the string.

I know that in C, strings are just arrays of characters. So my initial thought was to progress through the array and use something like isalpha() to determine whether or not the character is a letter. The issue is that I don't know how to ignore those characters when I compare the strings. Can anyone give me any insight?

Thanks.

-Sean

So is this a palindrome to you:

Code:

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rad5ar

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since you are going to ignore the '5'?

That's correct

Well you could consider making a copy of your string, copying only those characters you wish to check with.

Quote:

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Originally Posted by Salem

Well you could consider making a copy of your string, copying only those characters you wish to check with.

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My roommate just suggested that to me. It seems to make the most sense. I'm assuming I'd use something like strcpy(). But how would i keep from copying the unwanted characters?

Quote:

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Originally Posted by seanksg

My roommate just suggested that to me. It seems to make the most sense. I'm assuming I'd use something like strcpy(). But how would i keep from copying the unwanted characters?

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Don't use strcpy. Instead use a loop to go over the string character by character. For each character that is a letter, copy it into the new string.

Quote:

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Originally Posted by bithub

Don't use strcpy. Instead use a loop to go over the string character by character. For each character that is a letter, copy it into the new string.

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Ah that makes more sense. Thanks! I'll try that out.

Yes, that's quite testable.

Make a function that "RemovesNonLetters"

Test it.

Then make a function that tests for palindromism. ;-)

Why copy anything. Test first character against last, as long as both are alpha. 2nd against 2nd last, etc. Keeping 2 indexes which will be moved towards each other.

Code:

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#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <ctype.h>
#include <string.h>

#define TRUE 1
#define FALSE 0

int pal(char *input);

int main( )
{
        char *stop = "done";
        char input[81];

        printf("\nEnter a string of characters for palindrome verification: ");
        scanf("%80s", input);
               
        while (strcmp(input,stop) != 0)
        {
                printf("\n%s\n\n", input);
                if (pal(input))
                {
                        printf("Input is a palindrome\n\n");
                }
                else
                {
                        printf("Input is not a palindrome");
                }
                printf("\n\nEnter a string of characters for palindrome verification: ");
                scanf("%80s", input);
        }

        printf("\n******Program Terminated******\n\n");
               
          return (EXIT_SUCCESS);
}
int pal(char *input)
{
        int length = strlen(input) - 1;
        int i = 0;
        int j = length;
        char start = input[i];
        char end = input[j];
        int exit = 0;

        while (TRUE && (exit = 0))
        {
                while (isalpha(start) != 0)
                {
                        ++i;
                }
                while (isalpha(end) != 0)
                {
                        --j;
                }
                if (i > j)
                {
                        return (1);
                }
                else
                {
                        while (isalpha(end) == 0 && isalpha(start) == 0)               
                        {
                                if (start == end)
                                {
                                        ++i;
                                        --j;
                                }
                                else
                                {
                                        exit = 1;
                                }
                        }
                }
        }

        return (0);
}

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This is the code I've developed. I decided to try comparing characters individually, because I couldn't figure out how to reverse the string. Nevertheless, it doesn't work. Does anyone have any insights?

Thanks.

-Sean

Code:

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while head < tail
    if !isalpha head
        head++
    else
    if !isalpha tail
        tail--
    else
    if head == tail
        head++
        tail--
    else
        fail
success

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Quzah.

Code:

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while (TRUE && (exit = 0))

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You are doing an assignment here, not a comparison. You need to use the == operator.