Thread: Array question

Hi.. Working on a function.. which should reverse a simple array...

Code:

void reverser(char input[], char output[])
{
	int i, c;
	for(i=0; input[i] != '\0'; ++i);
	for (c=0;i !=0; ++c){
		output[c] = input[i];
		--i;
	}
	output[c+1] = input[i]

	/*while (i != 0) {
		if (i==0)
			output[c+1] = input[i];
		output[c] = input[i];
		--i;
		++c;
		}
	*/

	/*output[0] = input[4];
	output[1] = input[3];
	output[2] = input[2];
	output[3] = input[1];
	output[4] = input[0]; */

	printf("C = %d I = %d \nWhich makes : %s\n", c, i, output);
}

This is the code .. As you can see I tried it before with a while statement which is marked out now .. same result .. the printf of output is empty ..

As you can see in the end is a marked out code of manually reversing it .. which does strangly work...

As far as I can see c and i in the for statement represent the same numbers as when I manually fill the array of output.

what is going wrong here??

Basically you don't need two loops to do this...

You need your loop to start at the beginning of the array.
On each iteration of the loop, swap two values ... array[i] for array[end - i]
when you get to the array size / 2 you're done.

I see your point...

Though I must add I;m into my 3th day of C programming so that's why my codes are somewhat bloated...

But aside from that (which I'm interested in also ofcourse...). This could should be working as far as I can see. why isn't it.?

Thanks for the reply ...

I do not really understand what is not right about swapping the bytes in an array ...
This code is just for learning purposes so I'm not asking for a code re-write, I want to understand what is going on here..

The way I see it .. When I reverse the array manually :

Code:

output[0] = input[4];
	output[1] = input[3];
	output[2] = input[2];
	output[3] = input[1];
	output[4] = input[0];

It does work .. (but I don't have to eleborate why this isn;t ideal :P)
But when I automate it, it does not output anything.

the c and i var represent the same numbers (even tested this with a printf of it after bumping my head 3 times ) as the ones in the manually reverse.

Thanks.

Originally Posted by viper2k

I see your point...

Though I must add I;m into my 3th day of C programming so that's why my codes are somewhat bloated...

But aside from that (which I'm interested in also ofcourse...). This could should be working as far as I can see. why isn't it.?

Well, here's a little exercise for you that will help you develop troubleshooting skills...
Take your 2 loops, on paper and track the values in the arrays through the loops. Don't look at your code, look at the result of your code...

*shrug*

You either got lucky, or you managed a very specific test case.

Either way, I'll give you a hint.

After you follow up with CommonTater's suggestion.

Soma

Hmm...

I've been following the values .. and the numbers check out...
I've edited the code so it outputs the value changes.

Code:

void reverser(char input[], char output[])
{
	int i, c;						//initialize vars
		
	for(i=0; input[i] != '\0'; ++i);// counts length of input
	printf("starting value of i :%d\n", i);	// tests the result of i
	
	for (c=0;i !=0; ++c){			// for as long as i is not 0 loop & increase c by 1
		printf("i=%d c=%d\n", i, c);// tests results of i & c while looping
		output[c] = input[i];		// copy input into output
		--i;						// decrease i by 1
	}
	output[c+1] = input[i];			//copy final character on array

	printf("Final : c = %d i = %d Which makes : %s\n", c, i, output); // test final output
}

When executed it results :
(hello & ^Z are my input)

input or EOF ?
hello



input or EOF ?
^Z
starting value of i :5
i=5 c=0
i=4 c=1
i=3 c=2
i=2 c=3
i=1 c=4
Final : c = 5 i = 0 Which makes :

My analyses of this is:
The values itself are correct so the loops (although it might be done more efficient) do the job right.

The copying of input to output goes wrong though

Code:

output[c] = input[i];		// copy input into output

There... Though the strange thing is .. when I replace the c in output to i ... It DOES copy array content.. Though ofcourse it's not the right output.


Am I analyzing this wrong??

The original does not work because i is at the end of the array - pointing to the null terminator. You are indexing input[i] one element beyond where it should be.
And because the loop stops early, the assignment after it should be output[c] = input[i];

Sorry, I just posted this before seeing your above analysis. I think you've covered it.

Hint: (As I promised.) "C Strings" are null terminated.

Soma

Ah .. that is it ...

thats fun about coding ... this is something I should have known.. I just was unable to see it. ..


I placed a

Code:

--i;

After the strlen

and realized that

Code:

output[c] = input[i];			//copy final character on array

Was useless ..

**edit ... incorrect ofcourse... this IS useful.. but the c+1 I originally had was wrong.

Thanks a bunch guys ... I'm learning

btw minor side question... I know the mark // is actually a C++ syntax .. but it's easier for marking a single or partial line ..

I'm trying to learn ANSI C no vague interpertation .. should I drop this habbit or is it ok to use in the world of C ??

It isn't standard C89, but it is standard C99.

Soma