Thread: 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;
}

Originally Posted by zacs7

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.

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

Originally Posted by Salem

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!!

Originally Posted by maverix

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

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.

Originally Posted by tikelele

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

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!!