Detab Function Loop Hang

**neu_greg** · 01-26-2009

Hello, I'm a scheme programmer teaching myself C with the K and R book (what can I say, I'm bored with my Java OOP class). The end of chapter one asks you to write a function to replace all tabs with a set number of spaces. So I wrote the function below. Anyhow, the program works fine if I enter no tabs, but if I enter a tab it goes into a never ending loop.

Two questions:
1) Why
2) A *tip* on what a better way to solve this is

I know there are answers that can be found online, but they are not very helpful to a C-novice like myself because they do not explain precisely what they or are doing, or more importantly, why.

Code:

void detab(char s[], char x[], int space)
{
	int i;

	i = 0;
	while(s[i] != '\0')
	{
		if (s[i] == '\t') {
			for(int y = space; y == 0; y--)
				x[i+y] = ' ';

		} else {
			x[i] = s[i];
			i++;
		}
	}
}

/** note: to compile this use: gcc -std=gnu99

**MK27** · 01-26-2009

A big problem is how the tab got there -- while there is a character '\t', if the string was supplied from keyboard input or from a text file, for example, it will be a number of spaces and cannot possibly be an actual '\t'.

**Dino** · 01-26-2009

A couple things.

1) You need two index variables. You can't do it with 1. One for s[] and one for x[]. Right now, you are losing the position in x[] since i is used for s[].
2) Your tab replacement FOR loop is bad. If there are 4 spaces to add, the largest offset you should use is 3, not 4.

**brewbuck** · 01-26-2009

In my opinion, transducer-like functions such as this are more easily understood using pointers rather than indexes.

Code:

void detab(char s[], char x[], int space)
{
    char *in = s;
    char *out = x;
    for(; *in; ++in)
    {
        if(*in == '\t')
            for(int y = 0; y < space; ++y)
                *out++ = ' ';
        else
            *out++ = *in;
    }
    *out = '\0';
}

**itCbitC** · 01-27-2009

Your code doesn't show the portion that reads input; and how many spaces make up a <TAB>? Post the entire code here.
Reason it's stuck in an infinite loop: the if statement doesn't increment i after you have replaced the <TAB> with spaces.

**Dino** · 01-27-2009

Variable in is updated 1 byte each time through the FOR loop. See ++in, so that's not the problem.

[EDIT] Sorry itCbitC - I was looking at brewbuck's code. I think the OP is long gone.

**matsp** · 01-27-2009

Code:

for(int y = space; y == 0; y--)

This for loop is all wrong - unless space is 0, it wont loop even once. If space is 0, it will loop once, then break the loop, with y = -1.

Look at brewbucks code for a correct loop.

--
Mats

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