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Ignoring specific characters in a string

This is a discussion on Ignoring specific characters in a string within the C Programming forums, part of the General Programming Boards category; Hi, So I am writing a program that checks to see whether a string input by the user is a ...

  1. #1
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    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

  2. #2
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    So is this a palindrome to you:
    Code:
    rad5ar
    since you are going to ignore the '5'?

  3. #3
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    That's correct

  4. #4
    and the hat of int overfl Salem's Avatar
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    Well you could consider making a copy of your string, copying only those characters you wish to check with.
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    Quote Originally Posted by Salem View Post
    Well you could consider making a copy of your string, copying only those characters you wish to check with.
    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?

  6. #6
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    Quote Originally Posted by seanksg View Post
    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?
    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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  7. #7
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    Quote Originally Posted by bithub View Post
    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.
    Ah that makes more sense. Thanks! I'll try that out.

  8. #8
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    Yes, that's quite testable.

    Make a function that "RemovesNonLetters"

    Test it.

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

  9. #9
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    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.

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    Code:
    #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);
    }
    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
    Last edited by seanksg; 05-02-2011 at 06:48 PM. Reason: Forgot to add some information

  11. #11
    ATH0 quzah's Avatar
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    Code:
    while head < tail 
        if !isalpha head
            head++
        else
        if !isalpha tail
            tail--
        else
        if head == tail 
            head++
            tail--
        else
            fail
    success
    Quzah.
    Hope is the first step on the road to disappointment.

  12. #12
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    Code:
    while (TRUE && (exit = 0))
    You are doing an assignment here, not a comparison. You need to use the == operator.
    quzah likes this.
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