Loops in C++

[mike_w]

One of the classes I use in a project of mine is a combinatorial class - it's essentially a list of digits, and each time it is incremented it represents another unique set of digits. For instance, say you're choosing three digits from five, the increments would be:

[0,1,2]
[0,1,3]
[0,1,4]
[0,2,3]
and so on.

The problem I have is with looping. It has a method .isAtEnd() which simply returns a Boolean true if the combinatorial is the final one generated by the class. Unfortunately, this doesn't lend itself especially well to looping. To iterate throw each value, a loop could look like this:

Combination combination(args...);

while (true) {
    doStuff(combination);
    if (combination.isAtEnd()) {
        break;
    } else {
        ++combination;
    }
}

Or alternatively:

Combination combination(args...);

doStuff(combination);
while (!combination.isAtEnd()) {
    ++combination;
    doStuff(combination);
}

Of course, neither is ideal. The only way I can see around this is to start each instance of the class in an invalid, and set it to the first valid start when increment is called for the first time, so the code would become:

Combination combination(args...);

while (!combination.isAtEnd()) {
    ++combination;
    doStuff(combination);
}

Is that the sort of solution I should be looking at? The downside is that the object starts in an invalid state. Of course, I might be missing something obvious here (hence the post...), so any thoughts are appreciated.

Thanks

Mike

[TheAnimus]

ok, first off i'm drunk, second off i'm not sure i've understood what your asking, but do you want something like

do
{
++combination;
doStuff(combination);
} while (!combination.isAtEnd());

[mike_w]

That would only work if the class started off in an invalid state, and the first call of ++combination put it into the first valid state, as described in my final example. Otherwise, it doesn't call doStuff on the first state.

[Fraz]

I think you need to explain what this "class started off in an invalid state" business is all about.

I don't see a real problem with any of your alternatives. Particularly your last one, or what TheAnimus wrote.

[aidanjt]

Behold, the awesome power of forloops!

for(doStuff(); !combination.isAtEnd(); combination++) {
    doStuff();
}

*disclaimer: I've been awake for 14 hours and my head feels like it's going to melt, so if I've I completely misread what you're trying to do then by all means, ignore what I just said

[yamangman]

Are you thinking in terms of optimization? If so does isAtEnd just return a boolean property for said class, or does it perform a calculation and return?

[mike_w]

Originally Posted by Fraz

I think you need to explain what this "class started off in an invalid state" business is all about.

I don't see a real problem with any of your alternatives. Particularly your last one, or what TheAnimus wrote.

Well, in the first couple of solutions, doStuff() is repeated - hardly ideal, especially when doStuff() may not really be that simple i.e. parameters!

Secondly, if you use an iterator-like approach, which is more or less what I suggested at the end / TheAnimus suggested, this means you have an object which can be passed around but potentially might have unusable values e.g.

If the the first (valid) value that this class might take is [0, 1, 2] (after the first call of ++), what value does it take beforehand? One solution is to rewrite it as an iterator, and have next() and hasNext() methods, but this breaks existing code (or, at least, moreso than other solutions).

The question isn't so much "What is a solution to this problem," rather "What is the best solution to the problem?" - so probably what makes for the clearest code?

isAtEnd just performs a simple integer comparison.