Nested for loops and 2D lists

We can quite easily use nested for loops to work with 2D lists in a straightforward manner. Suppose we want to write some code that will print the sum of all integers in a 2D list. How might we go about it? First, let’s brush up on how we’d accomplish this with a 1D list.

This sort of code should look pretty familiar to you. If you need a refresher, take a look at week 5 in this course. We can use our i and j based, row-column system to traverse 2D lists in a similar way. The following code will print the sum of all integers in a 2D list. Keep in mind, i iterates over the rows of this 2D list, while j iterates over the columns. Of course, we can also think of this as i iterates over the lists inside the list of lists, while j iterates over the individual elements of each inner list. Notice that these nested loops will iterate over every possible combination of i and j. To understand why, you may want to copy the following code to a workspace, and add some print statements to take a look at how the values of i and j change over the course of the loops being run.

Of course, we can also use for loops to iterate over items of a list directly, though this may make some tasks more difficult or impossible. This works just fine for this particular task though, as you can see.

Nested for loops lend themselves perfectly to working with 2D arrays.