Let’s Think Python: COMSC020 PreTeXt Interactive Edition

Here’s the problem: Let’s look at the list and the index number after each iteration through the loop:

Now index moves up to 2, and that skips over the -47. That is definitely a problem. Let’s continue...

But our loop goes from 0 up to and including 5, and there is no longer an element at that index, and that is where the program crashes.

That means we need to do two things:

At this point, we do not want a for loop, because it can’t do either of the things we need to do:

This is really a job for a while loop, where we are in control of the index value, and the condition is checked every time we go through the loop. Here is the new, revised plan in pseudo-code:

def remove_negative(a_list):
    set index to 0.
    while index < length of a_list:
        if a_list[index] < 0:
            delete item at the current index
        else:
            move forward to next index

It’s your turn. Implement the new plan and see if it does what we want.

Sometimes your first plan simply doesn’t work. The plans for your program are not engraved in stone; think of them as engraved in jello¹.

When a program doesn’t work, don’t go right into a “change code and see if that fixes it” cycle. Instead, stop and diagnose the problem. Find out where the problem is occurring and figure out why. It’s really hard to fix a problem if you don’t know what it is.

Change the program or the plan after you know what the problem is.