Quiz Master I

Review — the Vending Machine arc 5 min

• ([floor v] of ()) and (() mod ()) break an amount into coins.
• (pick random (1) to (6)) makes a machine different every run.
• A variable is a box with a name, and it holds exactly one thing.

Quick-fire

Your machine sold three snacks. Where did it keep the three prices while it added them up?

Today’s Topic 3 min

• A list — a variable that holds many things, in order
• add [thing] to [questions v]
• (item (1) of [questions v]) — and why rows start at 1
• (length of [questions v]) — how many are in there

Learning outcome

By the end of this lesson you will be able to:

1. Make a list and put things into it.
2. Read any row back out by its number.
3. Write one loop that works on a list of any length.

Learnt — the box that holds a quiz 28 min

1 · Why a variable is not enough

You want a quiz with five questions. With variables that is q1, q2, q3, q4, q5 — five boxes, five names, and five copies of nearly the same blocks.

Then somebody asks for a sixth question, and you have to edit the code.

A list is one box with many rows in it. Five questions, one name, and a sixth costs you nothing.

2 · Making one, and filling it

1. Go to Variables and click Make a List.
2. Call it questions, for all sprites.
3. A tall empty box appears on the Stage. That is the list.
4. Drag out add [thing] to [questions v] and type a question into it.
5. Click the block. Watch a row appear in the box.

when flag clicked
delete all of [questions v]
add [What colour is a banana?] to [questions v]
add [How many legs has a spider?] to [questions v]
add [What do bees make?] to [questions v]
add [Which planet do we live on?] to [questions v]
add [What is frozen water called?] to [questions v]
            

3 · Reading one row back

Every row has a number. In Scratch, the first row is number 1 — not 0.

(item (1) of [questions v])
              

(item (3) of [questions v])
              

4 · How many are in there?

(length of [questions v])
            

Put the two together and you can reach the last row of a list without knowing how long it is:

say (item (length of [questions v]) of [questions v])
            

5 · Walking the whole list

A counter and a repeat is all it takes. The counter goes 1, 2, 3… and (item (n) of [questions v]) follows it down the rows:

when flag clicked
set [n v] to (0)
repeat (length of [questions v])
  change [n v] by (1)
  say (item (n) of [questions v]) for (1.6) seconds
end
            

Basic — a quiz that walks its own list 24 min

Start a new project. Pick a host sprite and a backdrop you like.

Step 1 — the list

1. Make a list called questions.
2. Under the green flag, put delete all of [questions v].
3. Add five questions of your own, one block each.
4. Press the flag twice and check you still have five rows, not ten.

Step 2 — read one out

1. Add say (item (1) of [questions v]) for (2) seconds.
2. Change the 1 to a 4 and run it again.

Step 3 — the counter

1. Make a variable called n and set it to 0 under the flag.
2. Add repeat (length of [questions v]).
3. Inside it: change [n v] by (1), then a say using (item (n) of [questions v]).

Step 4 — number them

1. Say (join [Q] (n)) before each question.
2. Check the numbers on the Stage match the rows in the box.

Step 5 — prove it scales

1. Add a sixth question with one more add block.
2. Run it. Do not touch the loop.
3. Now delete two questions from the top of the script and run it again.

when flag clicked
delete all of [questions v]
add [What colour is a banana?] to [questions v]
add [How many legs has a spider?] to [questions v]
add [What do bees make?] to [questions v]
set [n v] to (0)
repeat (length of [questions v])
  change [n v] by (1)
  say (join [Q] (n)) for (0.5) seconds
  say (item (n) of [questions v]) for (1.6) seconds
end
say [That is the lot.] for (2) seconds
            

A sixth question, free

Prove your quiz grows on its own.

• Add a sixth question with one add block.
• Change nothing else at all.
• The quiz asks six, and says so at the end.

How many to go

Tell the player where they are up to.

• Before each question: “Q3 of 5”.
• After it: how many are still to come.
• Both numbers come from the list, not from anything typed.

say (join [Q] (join (n) (join [ of ] (length of [questions v])))) for (1) seconds
set [to go v] to ((length of [questions v]) - (n))
            

Name the last one

Make a fuss of the final question.

• Before the last question only, the host says “Last one!”.
• It works whether the list has three questions or thirty.
• At the end, the host repeats what the last question was.

if <(n) = (length of [questions v])> then
  say [Last one!] for (1) seconds
end
say (item (n) of [questions v]) for (1.6) seconds
            

A score out of the list

Add a second list of answers, mark each question against it, and give a score at the end — out of however many questions there are.

• A second list, answers, with the answers in the same order.
• Row 3 of the answers is the answer to row 3 of the questions.
• The score card reads “2 out of 5”, worked out from the list.

say (item (n) of [questions v]) for (1) seconds
if <(answer) = (item (n) of [answers v])> then
  change [score v] by (1)
  say [Right!] for (1) seconds
else
  say (join [No - it is ] (item (n) of [answers v])) for (1) seconds
end
            

Summary 5 min

• A list is one box that holds many things, in rows, in order.
• add [thing] to [questions v] puts a row on the end.
• (item (3) of [questions v]) reads one row back. Rows start at 1.
• (length of [questions v]) counts them — so a loop can handle a list of any size.
• A list survives the green flag. Start with delete all of [questions v] or it grows every run.

List

A variable that holds many values, numbered from 1.

Item

One row of a list.

Index

A row’s number.

Length

How many rows a list has right now.

Hand in

Save your quiz. Next lesson the list stops being something you fill in at the start: questions get deleted as they are answered, and new ones dropped into the middle.