C: odd behaviour with nested loop and array

Question

I have an array of int that is terminated with a '\0' created elsewhere in my code. I know it's null terminated because I've tested it.

Say, for this example, the array is [7, 8, 9, 11, 12, '\0']

When I feed it to my function:

int edgesInCluster(Graph *g, int *nodes) {
    int count = 0;
    int i = 0;
    int j = 0;
    while(nodes[i] != '\0') {
        while(nodes[j] != '\0') {
            if(i<j) {
                printf("%i %i\n", nodes[i], nodes[j]);
                count += isNeighbour(g, nodes[i], nodes[j]);
            }
            j++;
        }
        i++;
    }
    return count;
}

The printf is outputting:

When it should be outputting:

Which means that for some reason either 'i' isn't being incremented (but we can see that it is) or nodes[0] == '\0' which we know isn't true since the loop with 'j' works fine.

So, any ideas what's going on?

P.S. when I change the whiles to for loops, it works but only if I know the length of 'nodes'

Kerrek SB · Accepted Answer · 2011-07-10 18:00:53Z

4

You don't reset your j after the inner loop.

But why such a horribly verbose code? Try this:

size_t count = 0;

for (size_t i = 0; nodes[i]; ++i)
{
  for (size_t j = i + 1;  nodes[j]; ++j)
  {
    printf("%i %i\n", nodes[i], nodes[j]);
    count += isNeighbour(g, nodes[i], nodes[j]);
  }
}

return count;

(If you want strict ANSI C89, you have to pull the declarations of i and j out of the loops of course.)

Also note that '\0' is a char-literal of value 0, so you might just as well, and more correctly, say 0. And instead of if (x != 0) you can just say if (x), which is what I did in the for loops.

answered Jul 10, 2011 at 18:00

Kerrek SB

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6 Comments

Griffin Over a year ago

Ah, silly me! I write verbose until I know it all works then I can reduce it if necessary. For what it's worth I compile as c99 with -Wall and -pedantic and I get no warnings - seems more important than less verbosity. Thanks for your help.

Kerrek SB Over a year ago

@Griffin: But your verbosity was actually obscuring the logic. The for-loops are ideal for expressing the semantics of a ranged loop, so by "doing it manually" all you did was confuse yourself... Also note that my loop is more efficient by starting at the correct value and not having to do the if (i < j) check every time.

Griffin Over a year ago

Point taken, thanks. Just out of interest, why size_t instead of int?

Kerrek SB Over a year ago

@Griffin: The natural data type for array indices and counts is size_t. It's the return type of the sizeof operator, and usually typedef'd to unsigned int. Since your values are unsigned integers, best to use the most appropriate data type.

Griffin Over a year ago

@Kerrek SB (sorry didn't realise that was the etiquette). Noted about the unsigned ints, thank you again.

|

Femaref · Accepted Answer · 2011-07-10 18:01:45Z

1

You aren't resetting the value for j at the beginning of the j loop. That way, after the first loop, nodes[j] is '\0' for all following loops, aborting the j loop immediately.

answered Jul 10, 2011 at 18:01

Femaref

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2 Comments

Griffin Over a year ago

Cheers, Kerrek SB just pipped you to the post!

Femaref Over a year ago

yeah, his proposal is better anyway.

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