Lists

Classroom resources

Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.

• Slides Download Copy address for Prepare Present from the browser, or export to PowerPoint or PDF.
• Worksheet Download Copy address for Prepare Print the student copy; the Teacher edition has the answers.

Students will be able to

• I can make a list and get an item by its place.
• I can say why the first item is at place 0.
• I can step through a list with a variable.

Timing

Predict and Run 7 min · Investigate 10 min · Modify 8 min · Mix it up 3 min · Make 9 min · Lunch menu 3 min

Before class

A list (MakeCode calls it an array, in Advanced › Arrays) holds many values in order under one name. Each item has a place (an index), counted from 0: in Yes, No, Maybe, Yes is at place 0. So a list of 3 has places 0, 1 and 2, and the last place is one less than the number of items. get value at takes out the item at a place. Picking at random is next lesson.

Running the lesson

1. Predict: most students say Yes. Ask: which place does the micro:bit start counting from? Or: where does a ruler start?
2. Run: No. Say it plainly: lists count from 0, like a ruler starts at 0.
3. Investigate: the last-place question is where counting from 0 bites.
4. Modify: adding Later and asking for place 3 makes students count.
5. Mix it up: a tilt meter from the Decide unit, read from the top.
6. Make: the chooser uses a variable as the place, and an if to go back to 0.

In class

On the slides at these points. Students can do the whole lesson on their own; these are for the room. In the deck, the Run, Modify and Make slides have the live playground in them (the tutorial link is the backup), and the notes above are in each slide's speaker notes.

• Line up from 0 (investigate, 3 min)

  Five students stand in a row holding cards 0 to 4. The class calls a place; that student steps forward.

  Ask for place 5. Nobody steps forward: five items stop at place 4.

• Round the list (end, 3 min)

  Swap mood choosers with the next pair. Press A until it goes round.

The hard part (on purpose)

Counting places from 0. Ask them to point at each item and count aloud from zero.

When students get stuck

Answers to the checks

1. Predict: You press A. What does the micro:bit show?
   No. The places are counted from 0: Yes is at 0, No is at 1, Maybe is at 2. Place 1 is No.
2. Places: The list is Yes, No, Maybe. What is at each place?
   Yes, No, Maybe
   The first item is at place 0. Three items use places 0, 1 and 2.
3. The last place: A list has 5 items. What is the place number of the last one?
   4
   Places start at 0, so 5 items use places 0, 1, 2, 3 and 4. The last place is one less than the number of items.
4. Jo's first word: Jo wants A to show the first word, Yes. She uses get value at 1. What does A show?
   No
   Place 1 is the second item, No. The first item is at place 0.
5. Mix it up: A tilt meter: what shows for each value of acceleration x?
   right arrow, small diamond, small square
   600 is more than 500, so the first part runs. 200 is not, but it is more than 0. -50 is neither, so else runs.

Solutions

One possible solution for each task. Other correct programs are fine.

Modify › One more word

let answers = ["Yes", "No", "Maybe", "Later"]
input.onButtonPressed(Button.A, function () {
    basic.showString(answers[1])
})
input.onButtonPressed(Button.B, function () {
    basic.showString(answers[3])
})

Modify › The first word

let answers = ["Yes", "No", "Maybe", "Later"]
input.onButtonPressed(Button.A, function () {
    basic.showString(answers[1])
})
input.onButtonPressed(Button.B, function () {
    basic.showString(answers[3])
})
input.onButtonPressed(Button.AB, function () {
    basic.showString(answers[0])
})

Make

let moods = ["Happy", "Tired", "Hungry"]
let choice = 0
input.onButtonPressed(Button.A, function () {
    choice += 1
    if (choice == 3) {
        choice = 0
    }
    basic.showString(moods[choice])
})

Change a lunch menu

Open task: any program that does what the step asks.

Stretch and support

Stretch: Make B step backwards through the moods, going round to the last one after the first.

Support: Give students the A event with change and show already in, and ask them only to add the if that goes back to 0.

Exit ticket

A list has Red, Green, Blue. What is at place 2, and what is at place 3?

Blue is at place 2. There is no place 3: three items use places 0, 1 and 2.

What each step is for

• Investigate: A list keeps items in order, counted from 0, so the first item is at place 0 and the last place is one less than the number of items.
• Modify: Every item has a place, and get value at takes out any one of them.
• Mix it up: The first true question wins, so the bigger tilt is asked about first.

Where it leads

Next lesson: pick an item at random, however long the list is. Class idea: a list of the week's homework, one item per day.