Quiz Engine I

Review — two quizzes you have built 5 min

• Level 1’s Quiz Show asked each question with its own ask block. A new question meant new blocks.
• Level 3’s Quiz Master kept questions in one list and answers in another, in step. A new question meant typing into two lists, in the same row.
• Kart Racer IV ended the last arc: a rival that drove waypoints from lists, start lights and a finish.

Quick-fire

In Quiz Master, somebody adds a question to the questions list but forgets the answers list. What goes wrong?

Today’s Topic 3 min

• Text is made of letters: (letter (i) of (line)) and (length of (line))
• A split My Block that cuts a line at every | into a parts list
• A bank of lines that plays as a quiz

Learning outcome

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

1. Walk through text one letter at a time.
2. Turn one line of text into a list of parts.
3. Build a quiz where a new question is one new line, not new blocks.

Learnt — cutting a line apart 28 min

1 · Text is a row of letters

Scratch can read text one letter at a time. (length of (line)) says how many letters there are. (letter (4) of (line)) gives you the fourth.

when flag clicked
set [line v] to [bee|hive]
say (join [length of (line) = ] (length of (line))) for (1.2) seconds
set [i v] to (1)
repeat (length of (line))
  say (join [letter ] (join (i) (join [ is ] (letter (i) of (line))))) for (0.5) seconds
  change [i v] by (1)
end
            

2 · The split My Block

Walk the line. Every letter that is not a | joins onto the end of word. A | means the word is finished: add it to parts and start a new one.

define split (text)
delete all of [parts v]
set [word v] to []
set [i v] to (1)
repeat (length of (text))
  set [letter v] to (letter (i) of (text))
  if <(letter) = [|]> then
    add (word) to [parts v]
    set [word v] to []
  else
    set [word v] to (join (word) (letter))
  end
  change [i v] by (1)
end
add (word) to [parts v]
            

3 · Playing one line

After the split, the question is part 1 and the answers are parts 2, 3 and 4. The player types a number. Answer 1 is part 2, so the answer they chose is part (their number + 1).

split [How many legs does a spider have?|6|10|8|8] :: custom
ask (join (item (1) of [parts v]) (join [  1) ] (join (item (2) of [parts v]) (join [  2) ] (join (item (3) of [parts v]) (join [  3) ] (item (4) of [parts v]))))))) and wait
set [pick v] to (answer)
if <(item ((pick) + (1)) of [parts v]) = (item (5) of [parts v])> then
  change [score v] by (1)
  say [Right!] for (0.8) seconds
else
  say [No!] for (0.8) seconds
end
            

4 · A bank of lines

Put five lines in a list called bank. One loop splits line 1 and asks it, then line 2, and so on.

when flag clicked
set [score v] to (0)
set [q v] to (1)
repeat (length of [bank v])
  split (item (q) of [bank v]) :: custom
  ask (join (item (1) of [parts v]) (join [  1) ] (join (item (2) of [parts v]) (join [  2) ] (join (item (3) of [parts v]) (join [  3) ] (item (4) of [parts v]))))))) and wait
  set [pick v] to (answer)
  if <(item ((pick) + (1)) of [parts v]) = (item (5) of [parts v])> then
    change [score v] by (1)
    say [Right!] for (0.8) seconds
  else
    say [No!] for (0.8) seconds
  end
  change [q v] by (1)
end
            

Now add a sixth line to bank — a question about the largest animal. Change nothing else.

Basic — a five-question quiz 25 min

Start a new project. Choose the Kiran sprite and our studio backdrop.

Step 1 — the stage and the lists

1. Save our backdrop, quiz-studio.svg, and use Upload Backdrop.
2. Make two lists, For all sprites: bank and parts. Make the variables score, q, pick, word, i and letter.
3. Type five lines into bank, each like What do bees make?|honey|milk|silk|honey. Write your own questions, or use ours.

Step 2 — split

1. Make a Block called split, with a text input text. Tick Run without screen refresh.
2. Build its blocks from Learnt section 2.

Step 3 — the quiz

1. Build the loop from Learnt section 4 on Kiran.
2. Before it, add a welcome: say [Welcome to the quiz! Type 1, 2 or 3.] for (1.5) seconds.
3. After it, say the score: say (join [You scored ] (join (score) (join [ out of ] (length of [bank v])))).

Only 1, 2 or 3

Close the hole where typing 4 always wins.

• After the ask, check the number is 1, 2 or 3.
• If it is not, ask “Type 1, 2 or 3.” — and keep asking until it is.
• Count how many times you had to ask again.

repeat until <<(pick) > (0)> and <(pick) < (4)>>
  change [asked again v] by (1)
  ask [Type 1, 2 or 3.] and wait
  set [pick v] to (answer)
end
            

Tell them the answer

A quiz should teach, not just mark.

• A wrong answer says “No - it was …” with the right answer.
• The right answer comes from the line, not from a block you typed.

say (join [No - it was ] (item (5) of [parts v])) for (1.2) seconds
            

Four answers

Give every question a fourth answer — without writing “4” anywhere.

• Lines now have six parts: the question, four answers, the right one.
• Use (length of [parts v]) to find the right answer: it is always the last part.
• Build the question text with a loop, so a line could have any number of answers.

split (item (q) of [bank v]) :: custom
set [last v] to (length of [parts v])
set [question v] to (item (1) of [parts v])
set [k v] to (1)
repeat ((last) - (2))
  set [question v] to (join (question) (join [  ] (join (k) (join [) ] (item ((k) + (1)) of [parts v])))))
  change [k v] by (1)
end
ask (question) and wait
set [pick v] to (answer)
if <(item ((pick) + (1)) of [parts v]) = (item (last) of [parts v])> then
  change [score v] by (1)
end
            

A bank checker

A line with a missing bar breaks a question halfway through a game. Find it before anyone plays.

• Split every line in the bank, one after another.
• Any line that does not make exactly five parts is reported, by its number.
• If they are all fine, say so.

set [bad v] to (0)
set [q v] to (1)
repeat (length of [bank v])
  split (item (q) of [bank v]) :: custom
  if <<not <(length of [parts v]) = (5)>> and <(bad) = (0)>> then
    set [bad v] to (q)
    set [bad parts v] to (length of [parts v])
  end
  change [q v] by (1)
end
            

Summary 5 min

• Text is a row of letters: length of counts them, letter (i) of reads one.
• split cuts a line at every | into a list of parts. The last part is added after the loop.
• A quiz made of lines grows by one line, not by new blocks.
• Check what a player types before you use it.

Separator

A character that marks where one part ends and the next begins — here, the bar |.

Split

Cutting text into parts at every separator.

Bank

The list of questions a quiz is played from.

Hand in

Save your quiz. Next lesson it stops asking questions in the same order every time — and the right answer stops sitting in the same place.