How to teach nested loops?

[Zeroth]

Right now, I am a TA for a couple of first year cosci courses, and some of my students are having a bit of difficulty grasping how nested loops run. I was just wondering if anyone has any advice on what kind of analogies, samples, have worked?

[jbking]

Looping over all the elements of a 2-dimensional array would be one example where you'd have a nested loop.

Alternatively, for an n-dimensional array it could be kind of scary for some I'd think.

Also along these lines, matrix multiplication would be a bit more concrete if they are familiar with matrix algebra.

[Vermette]

A 2d array would be ideal imo, but the concept of such a thing itself might be a little alien. You could use as a real world example a set of i numbered cardboard boxes with j books in each. Show them a nested array in psuedocode to read the title of all the books.

[wtd]

For concrete counting loops, try expanding the outer loop out as it would appear if the code were written linearly.

[gitoxa]

I think a good way to show how nested loops work would be to set up a multiplication table.
Simple, easy, fast. Chances are if they don't understand nested loops, they haven't touched 2d array yet.

[[Gandalf]]

Make sure they understand why the second for loop is executed for each iteration of the first, and that it has to be this way to work as expected.

[Zeroth]

Thanks for all the advice guys. Working with the teacher of the course, I developed a rather effective and graphic example of code that shows how Nested loops work. Outer loop iterated from 100-199, and the inner loop iterated through letters in an array. It would print out the number, and then each letter in the array is printed, all in one line. I had a lot of "OH!"'s . Always a good sign.

[syntax_error]

This maybe late; however, seeing how we learned it and it worked wonderfully [in our class], tell them to use a tracing table like actually do the table step by step, simple but clean.