swaping characters

This is a discussion on swaping characters within the C Programming forums, part of the General Programming Boards category; Hi I wrote this to swap characters in a string. as long as the string is even in legnth, it ...

  1. #1
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    swaping characters

    Hi
    I wrote this to swap characters in a string. as long as the string is even in legnth, it works fine. But when it is odd it prints the last character on a new line and a ascii character. I know it is because the last character does not have anything to swap with. My goal is to just print the last character at the end if it is odd. I know I need modulus but van;t figure out where.

    Code:
    #include "string.h"
    #define MaxStringLen 200
    
    int main(int argc, char* argv)
    {
    	char userString[MaxStringLen + 1];
    	char *copyString;
    	int size;
    	int i;
    
    	printf( "Please enter some text: ");
    	fgets(userString, sizeof(userString), stdin);
    	
    	size = strlen(userString);
    
    	copyString = malloc(size + 1 * sizeof(char));	
    	
    	for (i = 0; i < size + 1; i += 2)
    	{
    		copyString[i] = userString[i + 1];
    		copyString[i + 1] = userString[i];
    	}
    
    	printf( "Here is the string you entered and its byte-swapped counterpart:\n");
    	printf( "  %s", userString);
    	printf( "  %s\n", copyString);
    
    	free(copyString);
    	copyString = NULL;
    
    	return 0;
    }

  2. #2
    Woof, woof! zacs7's Avatar
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    Something like,

    Code:
    /* if it's odd */
    if(size &#37; 2 != 0)
        lastChar = userString[size - 1];
    Or whatever

  3. #3
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    Quote Originally Posted by zacs7 View Post
    Something like,

    Code:
    /* if it's odd */
    if(size % 2 != 0)
        lastChar = userString[size - 1];
    Or whatever
    hmmm I get it. But I really want to stop the loop at the right time don't I? I get Heap overruns if I don't.

  4. #4
    Woof, woof! zacs7's Avatar
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    I would check if it's odd before the loop maybe? And act accordingly (ie trim the last chars off 'size' so it is even).

    You should also be checking if the malloc() failed or succeeded.

  5. #5
    and the hat of wrongness Salem's Avatar
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    > for (i = 0; i < size + 1; i += 2)
    Since there is also a +1 inside the loop, the +1 here as well combine for a total of size+2 - aka overrun.

    You might want to check the precedence of + and * in your malloc calculation.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  6. #6
    abyss - deep C
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    You could use the code below:

    Code:
    for (i = 0; i < size/2; i++)
    {
      copyString[2*i] = userString[2*i+1];
      copyString[2*i+1] = userString[2*i];
    }
    copyString[2*i] = '\0';
    The for loop takes care of the last single character.
    Also, we shouldn't forget to end the copyString with the '\0' character.

    Code:
    copyString[2*i] = '\0';
    cheers
    maverix

  7. #7
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    Quote Originally Posted by Salem View Post
    You might want to check the precedence of + and * in your malloc calculation.
    Thanks Salem. I did not see the precedence issue, I fixed it. I still get the odd character issue. I 'll keep at it!!

  8. #8
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    Quote Originally Posted by maverix View Post
    You could use the code below:

    Code:
    for (i = 0; i < size/2; i++)
    {
      copyString[2*i] = userString[2*i+1];
      copyString[2*i+1] = userString[2*i];
    }
    copyString[2*i] = '\0';
    The for loop takes care of the last single character.
    Also, we shouldn't forget to end the copyString with the '\0' character.

    Code:
    copyString[2*i] = '\0';
    cheers
    maverix
    Thanks. This works but it still prints the last char on a newline

    input:
    01234
    output:
    1032
    4

  9. #9
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    Here's what I have with changes (Thanks to all thus far)

    Code:
    #include "string.h"
    #define MaxStringLen 200
    
    int main(int argc, char* argv)
    {
    	char userString[MaxStringLen + 1];
    	char *copyString;
    	int size;
    	int i;
    
    	printf( "Please enter some text: ");
    	fgets(userString, sizeof(userString), stdin);
    	
    	size = strlen(userString);
    
    	copyString = malloc((size + 1) * sizeof(char));
    
    	for (i = 0; i < size; i += 2)// removed + 1 here per Salem
    	{
    		copyString[i] = userString[i + 1];		
    		copyString[i + 1] = userString[i];
    	}
    	//for (i = 0; i < size/2; i++)
    	//{
    	//	copyString[2*i] = userString[2*i+1];
    	//	copyString[2*i+1] = userString[2*i];
    	//}
    	if (size % 2 != 1)
    	{
    	copyString[i] = userString[i];
    	//copyString[i] = '\0';
    	}
    
    	printf( "Here is the string you entered and its byte-swapped counterpart:\n");
    	printf( "  %s", userString);
    	printf( "  %s\n", copyString);
    
    	free(copyString);
    	copyString = NULL;
    
    	return 0;
    }
    It produces desired results when an even number of char are input, and comes real close when an odd number of char are input.

    input:
    01234
    output:
    1032
    4

    I know (hope) it is something simple or I am not seeing. I'll keep pluggin'
    Thanks all.

  10. #10
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    fgets() reads all text up to and including the newline that is given to it. So if you enter "01234", it gets "01234\n" as the string. You need to remove the newline on the end. To be perfectly safe, you should do something like this:
    Code:
       int len;
       ....
       len  = strlen(input);
       if (input[len-1] == '\n') input[len-1] = 0;
       ...
    This avoids removing the last entry if the string is too long to fit in the buffer given.
    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  11. #11
    abyss - deep C
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    Quote Originally Posted by tikelele View Post
    Thanks. This works but it still prints the last char on a newline

    input:
    01234
    output:
    1032
    4

    Hi,

    As pointed out by matsp, your input has a '\n' at the end of the input, hence the behavior.
    Using the code excerpts given by matsp should do the trick.

    cheers
    maverix

  12. #12
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    thanks folks. that's an aspect of fgets that I did not even think of. I suppose I should look fgets over to get more comfortable with it!!

  13. #13
    Woof, woof! zacs7's Avatar
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    Remember to update the length of the string if you remove the newline character, I would suggest calculating it's size after you've removed it. Ie,

    Code:
    size_t len = 0;
    char * nl = NULL;
    
    if((nl = strchr(input, '\n')) != NULL)
        *nl = '\0';
    
    len = strlen(input);

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