3 Ways to Loop Without do/do-while/for/foreach/etc...

There are some crazy questions out there to test your knowledge of CS theory that can leave you stumped. It could come from an interviewer who is trying to weed out hobbyists or a student that you’re mentoring who has a tough professor.

One I recently had to think about was this:

Write a program that accepts input for a number from the user then prints a message that number of times WITHOUT using a loop construct, i.e do/do-while/for/foreach explicitly.

There’s one strategy that is likely the one that the questioner is looking for. I’ll give you three so you can stun them…

Strategy #1: Recursion

If an interviewer is asking you, they’re likely looking for you to have a good grasp of how recursion can do this.

public class LoopsWithoutLoops {
    private readonly ITestOutputHelper _output;

    public LoopsWithoutLoops(ITestOutputHelper output) {
        _output = output;
    }

    [Fact]
    public void looping_without_loops_recursion() {
        _output.WriteLine("Recursive strategy...");
        CountdownRecursive(10);
        //Recursive strategy...
        //10
        //9
        //8
        //7
        //6
        //5
        //4
        //3
        //2
        //1
        //0
    }

    public void CountdownRecursive(int i) {
        if (i < 1) {
            _output.WriteLine("0");
            return;
        }
        _output.WriteLine(i.ToString());
        CountdownRecursive(i - 1);
    }
}

Strategy #2: `goto`

Here’s another strategy - goto. I’m not saying it’s ideal or preferred or anything, it’s just available…

public class LoopsWithoutLoops {
    private readonly ITestOutputHelper _output;

    public LoopsWithoutLoops(ITestOutputHelper output) {
        _output = output;
    }

    [Fact]
    public void looping_without_loops_goto() {
        _output.WriteLine("goto strategy...");
        CountdownGoto(10);
        //goto strategy...
        //10
        //9
        //8
        //7
        //6
        //5
        //4
        //3
        //2
        //1
        //0
    }

    public void CountdownGoto(int i) {
    Label:
        if (i >= 0) {
            _output.WriteLine(i.ToString());
            i--;
            goto Label;
        }
    }
}

Strategy #3: `Enumerable.Repeat`

Finally, there’s LINQ’s Enumerable.Repeat. Since there’s no explicit call to any do/do-while/for/foreach, I’d say we’ve met the requirements:

public class LoopsWithoutLoops {
    private readonly ITestOutputHelper _output;

    public LoopsWithoutLoops(ITestOutputHelper output) {
        _output = output;
    }

    [Fact]
    public void looping_without_loops_linq() {
        _output.WriteLine("LINQ strategy...");
        CountdownLINQ(10);
        //goto strategy...
        //10
        //9
        //8
        //7
        //6
        //5
        //4
        //3
        //2
        //1
        //0
    }

    public void CountdownLINQ(int i) {
        var a = i;
        Enumerable.Repeat(i, i).Where(j => {
            _output.WriteLine(a.ToString());
            a--;
            return true;
        }).ToList();
        _output.WriteLine(a.ToString());
    }
}

Summary

So there you have it. The question is probably being asked to make sure you’re aware of the recursive strategy so if you’re interviewing, you probably want to use that solution. I’d be pretty impressed if someone listed three ways without hesitation, so feel free to keep them in your pocket.

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Related Posts:

3 Ways to Loop Without do/do-while/for/foreach/etc...

Strategy #1: Recursion

Strategy #2: goto

Strategy #3: Enumerable.Repeat

Summary

Strategy #2: `goto`

Strategy #3: `Enumerable.Repeat`