Why am I printing garbage with putchar(); in this for loop?

Question

I'm trying to teach myself C, as you've probably heard so many times, and I cannot figure out what I am doing wrong. The point of the exercise I'm doing is to replace tabs with a specified number of spaces. It's supposed to be a mimic of the unix command expand. Here's what I have so far:

#include <stdio.h>

int main (int argc, char *argv[]) 
{
    int spacecount = atoi(argv[2]);
    int i, d, b, g, toput;
    char string[1000];

    for (d = 0, b = 0; (string[d + b] = getchar()) != EOF; d++) {
        if (string[d + b] = '\t') {
            toput = d % spacecount;
            for (i = 0; i < toput; i++) {
                if (string[d + b] = '\n')
                    i = toput;
                string[d + b] = ' ';
                b++;
            }
        }
    }
    g = d;

    putchar('\n');

    for (d = 0; d < g; d++)
        putchar(string[d]);
    putchar('\n');
}

I run it and will start typing, finish by hitting ctrl + d, (because I'm using GNU/Linux) and let it print garbage characters. I have tried the same second for loop with another string, one that has been assigned getchar() in a for loop without an if statement, and it prints non-garbage characters. I guess I am really confused. What am I doing wrong?

Answer 1

Enable warnings in your compiler, it really should give you a warning for this code.

(string[d + b] = '\t') should be (string[d + b] == '\t').

Compile with gcc -std=c99 -pedantic-errors -Wall.

Answer 2

The problem is you are comparing the using = operator instead of ==. = is a asign operator you can not use it to compare two elements.

so use

if (string[d + b] == '\t')

instead of

if (string[d + b] = '\t')

Also use

if (string[d + b] == '\n')

instead of

if (string[d + b] = '\n')