3.2. Lists & Looping

One of the main reasons to put data into lists is so that we can process the entire list in the same way. Instead of saying:

Do something with variableA
Do something with variableB
Do something with variableC

We can say:

Do something with item 1 from myList
Do something with item 2 from myList
Do something with item 3 from myList

Why is that important? Well, everything in those lines is the same except for the item number. We can use a counter to represent the item number and replace that list of instructions with a loop and one instruction. This video shows how:

Make sure you remember the basic recipe for working through a list:


Inside the repeat, if you want to talk about “the current item number” you use the counter index:


Inside the repeat, if you want to talk about “the current value” you use the item (index) of (list):


Checkpoint Exercises

List Averager:

Make a block that averages the numbers in a list. You can start with the total numbers in list code from the 13:12 mark in the video. It already finds the total, you just need to divide that by the length of the list before reporting the answer.

By modifying the total algorithm
Or (even better), just use the existing total algorithm (assuming you have made the block) to get the total. Then do work to modify that answer.

Reverse Order:

Try making a block that says numbers in reverse order. Use the basic loop through list strategy, but. Start your index = to the length of the list. Change it by -1 inside the repeat to count down instead of up.