Pick from a list

Pick an item from a list at random, so it works however long the list grows.

A magic 8-ball, a game that deals a random card, a playlist on shuffle: each picks a place at random, then takes the item at that place.

Last lesson you took an item out of a list by its place. Now the micro:bit chooses the place. pick random (in Math) gives a whole number between two numbers, a different one each time. length of list (in Arrays) counts the items in a list.

Predict: Read the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.
Run: Run it and compare the result with your prediction.
Investigate: Trace how each line of code connects to what you see.
Modify: Change the code on purpose and check the result.
Make: Build something new using what you have learned. Expect a few tries; that is normal.

Explain it to me

Pick a random place, then take the item there · Sound on for the voiceover

pick random 0 to 2 gives 0, 1 or 2, a new one each time.

To pick an item at random, pick a place, then take the item at that place: get value at (pick random 0 to length − 1).

length of list counts the items. The last place is one less, because places start at 0. Using the length means the list can grow without changing anything else.

Show as code

let answers = ["Yes", "No", "Maybe"]
input.onGesture(Gesture.Shake, function () {
    basic.showString(answers[randint(0, answers.length - 1)])
})

The example, as blocks

What it makes

PredictRead the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.

Predict

Show as code

lesson.ts

let answers = ["Yes", "No", "Maybe"]
input.onGesture(Gesture.Shake, function () {
    basic.showString(answers[randint(0, answers.length - 1)])
})

pick random 0 to length of answers − 1 chooses a place. get value at takes the item at that place.

You shake this magic 8-ball twice. Can it give the same answer both times?It always says YesNo: it never repeats until every answer has been usedYes: each shake picks a new random place, and it can be the same oneNo: it goes in order, Yes, then No, then Maybe

RunRun it and compare the result with your prediction.

Run

Click SHAKE several times. Does an answer ever come up twice in a row?

Show as code

lesson.ts

let answers = ["Yes", "No", "Maybe"]
input.onGesture(Gesture.Shake, function () {
    basic.showString(answers[randint(0, answers.length - 1)])
})

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It takes a few seconds to load. Your work in this step is kept.

I shook it several times and compared the answers with my prediction.

InvestigateTrace how each line of code connects to what you see.

Investigate

Show as code

lesson.ts

let answers = ["Yes", "No", "Maybe"]
input.onGesture(Gesture.Shake, function () {
    basic.showString(answers[randint(0, answers.length - 1)])
})

Kai's program:

Show as code

let answers = ["Yes", "No", "Maybe"]
input.onGesture(Gesture.Shake, function () {
    basic.showString(answers[randint(1, 3)])
})

Ana's program is in the next step's editor.

The list has 3 answers. Which places can pick random 0 to length − 1 choose?Only 00, 1 or 21, 2 or 30, 1, 2 or 3

Kai's magic 8-ball never says Yes, and sometimes shows nothing at all. Why?pick random is brokenThe list needs four itemsPlaces start at 0, but Kai picks from 1 to 3: he skips Yes and asks for a place that is not thereYes is too short to show

Ana added two answers, so the list has five. Her code still picks from 0 to 2. What happens?The two new answers never come upOnly the new answers come upAll five come upIt shows an error

ModifyChange the code on purpose and check the result.

Modify

Fix Ana's 8-ball with length of list, so all five answers can come up. Shake to check.

Need a hint?

length of array is in Arrays. Drag it into the second number of pick random.

Another hint

Then take 1 away from it with the minus block from Math.

Make A show how many answers are in the list.

Need a hint?

Drag in on button A pressed, with show number length of array answers.

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Fix Ana's 8-ball so every answer can come up, using the length of the list.

This ticks by itself when your blocks do what is asked.

Make A show how many answers there are.

This ticks by itself when your blocks do what is asked.

InvestigateTrace how each line of code connects to what you see.

Mix it up

Show as code

lesson.ts

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

This is last lesson's mood chooser. choice starts at 0. You press A four times. Which mood shows after the 4th press?HappyHungryNothingTired

MakeBuild something new using what you have learned. Expect a few tries; that is normal.

Make

Make rock, paper, scissors. Each shake shows Rock, Paper or Scissors, picked at random from a list. Play against a friend with their own micro:bit (or the simulator).

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Need a hint?

Make a list with Rock, Paper and Scissors.

Another hint

In on shake: show string get value at pick random 0 to length of list − 1.

Rock, paper, scissors: shaking picks one of three choices from a list.

This ticks by itself when your blocks do what is asked.

ModifyChange the code on purpose and check the result.

Change a team picker

This picks a name at random from a class list. Put in names from your class, then make B show how many names there are.

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For teachers

Students pick an item from a list at random with pick random 0 to length − 1, so every item can come up however long the list grows. About 40 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can pick a random item from a list.
I can say why the range is 0 to length − 1.
I can add items to a list without changing the random part.

Before class

pick random (Math) gives a whole number between two numbers. length of array (Arrays) counts the items. A random item is get value at (pick random 0 to length − 1): the length is one more than the last place, because places start at 0. Using the length means new items are included without changing anything else.

Running the lesson

Predict: some students think it cycles through or never repeats. Random has no memory.
Run: shake several times. Repeats happen.
Investigate: Kai counts from 1; Ana's range stays fixed when her list grows. Both are fixed by 0 to length − 1.
Modify: students fix Ana's 8-ball with length, then show the length.
Mix it up: last lesson's chooser, traced by hand.
Make: rock, paper, scissors is the same pattern with new items.

The hard part (on purpose)

Why it is length − 1. Ask them to count the places of a 3-item list aloud from 0.

When students get stuck

One item never comes up. Ask: Which places can pick random choose from? Fix: Pick from 0 to length − 1, not from 1 and not a fixed number.
The minus block will not fit. Ask: Is it the round number minus block from Math? Fix: Drag Math's 0 − 0 block into the slot, then put length of array on its left and 1 on its right.
A number shows instead of the word. Ask: Is the random number inside get value at? Fix: pick random gives the place; get value at turns the place into the item.

Stretch and support

Stretch: Make a dice: a list of six show leds pictures is hard to make, so use show number pick random 1 to 6 instead, and say why this one starts at 1.

Support: Give students the list and the on shake block with show string already in, and ask them only to build get value at with pick random.

Where it leads

Next: the final challenge, a pocket pet that uses everything in this course.

What each step is for

Investigate: pick random 0 to length − 1 gives any place in the list, so every item can come up, and new items are included without changing anything else.
Modify: Using the list's length means the program still works when the list grows.
Mix it up: A variable can be the place: change it, and get value at takes out a different item.

Now you can

Pick a random item from a list.
Say why the range is 0 to length − 1.
Add items to a list without changing the random part.

Next lesson11. Pocket petBuild a pet that lives on your micro:bit, using everything in this course. It gets hungry as time passes, you feed it, and its face shows how it feels.

Source: content/code/microbit/microbit-pick/lesson.md