3.1. List Basics & Swap Block

This video introduces the basics of working with lists in BYOB:


SNAP (online BYOB) Users:

SNAP does not have a “Make a List” button. Instead, you make a normal variable:


Then you use the set block to set it to a list:




Checkpoint Exercises

List Shuffle:

Make a copy of the swap block shown in the video (the 14:07 mark of the video has the final version of the code).

Test your swap block by making a shuffle program. Make a list called mylist containing five values (A, B, C, D, E will work) and two variables: randomIndexA and randomIndexB.


Then make a script (block or just plain code) to mix up the values. Then implement this shuffle algorithm:

Set randomIndexA to (pick random (1) to (length of mylist)
Set randomIndexB to (pick random (1) to (length of mylist)
Swap (randomIndexA) and (randomIndexB) in (mylist)

Every time you run the script, two items in mylist should switch places. Once it is working, add a repeat block to make the process repeat 20 times. That should do a pretty good job of shuffling the list every time you run the algorithm.


This algorithm should be using the swap block you already wrote. Do not reimplement the logic of swap, that defeats the purpose of having a block!!


Middle Item:

Make a reporter block that reports the middle value of a list.


  1. Make a list scores that looks like the picture to the right.
  2. Make a block CP31b1. The inside should look like CP31b2.
  3. Setup a call to middle item of scores so you can test the block as you make it: CP31b4.


Although you are testing with scores we want this to work on any list. Do NOT use scores inside the block. Inside the block use CP31b5 anywhere you want to talk about “the list”.


(Step 4 is the final goal, steps 1-3 work towards the final solution in a way that allows you to test out what you are building)

  1. Have the block report the length of list … try changing the number of scores in the list and make sure your block always reports the correct length.
  2. Have the block report the length divided by 2. Test it.
  3. We don’t want values like 2.5 - we need to round those. So make the block report round(length / 2). Round is a green reporter block that is already built into BYOB. For a list of length 5, you should get 3. For a list of length 4, you should get 2. This is the middleLocation - you might want to make a variable to store it in.
  4. Now, instead of reporting that middleLocation, report the item at the (middleLocation) in (list). Once again, try changing the number of items in the list and make sure the block always reports the middle one.
No variable
Using variable
No variable
Using variable