Card Battle I

Review — the Quiz Master arc 5 min

• (item (3) of [questions v]) reads row 3.
• (length of [questions v]) drives a loop of any size.
• <[accepted v] contains (answer)?> searches the whole list at once.

Quick-fire

<[names v] contains [Bat]?> says true. Which row is the Bat in?

Today’s Topic 3 min

• Three lists kept in step — one card spread across three rows
• (item # of [Bat] in [names v]) — a position, not a value
• 0 as the answer for “not in the list”
• Walking a deck to find the biggest, and to add it all up

Learning outcome

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

1. Hold several facts about one thing across lists that move together.
2. Find which row something is in, and cope with it not being there.
3. Walk a list keeping a “best so far” and a running total.

Learnt — a deck, and how to reach into it 28 min

1 · One card, three facts

A card in this game has a name, a power and a picture. Scratch has no way to put three things in one list row, so we use three lists and keep them in step:

Row 4 of each list is one card. You met this in Lesson 3-7 with questions and answers; here it is three lists instead of two, and it matters more.

when flag clicked
delete all of [names v]
delete all of [powers v]
delete all of [pics v]
add [Dragon] to [names v]
add (9) to [powers v]
add (1) to [pics v]
add [Shark] to [names v]
add (7) to [powers v]
add (2) to [pics v]
            

2 · Dealing is one row number, used three times

To put a card on the table you need all three facts, and they all come from the same n:

switch costume to (item (n) of [pics v])
set [power v] to (item (n) of [powers v])
say (join (item (n) of [names v]) (join [ - power ] (power))) for (1.3) seconds
            

3 · Where is it? — item # of

Every list block so far has answered “what is at row 4?”. This one answers the opposite question:

(item (4) of [names v])
              

(item # of [Bat] in [names v])
              

And if the thing is not in the list at all, it reports 0 — which is a useful answer, because there is no row 0.

Basic — build the deck, find a card 24 min

Start a new project. Paint or pick a table backdrop, and add one card sprite.

Step 1 — six costumes

1. Add six creature sprites from the library as costumes of ONE sprite.
2. Note the order they are in — that is what the pics list will hold.
3. Set the sprite’s size to about 55 so it fits a slot.

Step 2 — three lists

1. Make names, powers and pics.
2. Empty all three under the green flag.
3. Add your six cards, three blocks each, always in the same order.
4. Show the names and powers boxes and check they line up.

Step 3 — deal one

1. Make a variable n and set it to 1.
2. switch costume to (item (n) of [pics v]).
3. Say the name and the power from the same row.
4. Change n to 4 and run it again.

Step 4 — deal several

1. Wrap steps 3 in repeat (4) with change [n v] by (1) at the top.
2. Add a glide from the deck to your slot so each card arrives.

Step 5 — find one

1. Add ask [Which card?] and wait.
2. set [at v] to (item # of (answer) in [names v]).
3. If at is 0, say it is not in the deck. Otherwise show it.

ask [Which card?] and wait
set [at v] to (item # of (answer) in [names v])
if <(at) = (0)> then
  say (join (answer) [ is not in the deck.]) for (2) seconds
else
  switch costume to (item (at) of [pics v])
  say (join [Row ] (join (at) (join [, power ] (item (at) of [powers v])))) for (2) seconds
end
            

The strongest card

Walk the whole deck and find the card with the highest power.

• You may not look at the deck and type the answer in.
• It must still work after you change a power, or add a card.
• The winner’s name, power and picture all come out right.

set [best v] to (0)
set [best row v] to (0)
repeat (length of [powers v])
  change [n v] by (1)
  if <(item (n) of [powers v]) > (best)> then
    set [best v] to (item (n) of [powers v])
    set [best row v] to (n)
  end
end
            

Deal a hand

Take three cards out of the deck and into a hand.

• Each dealt card is added to a hand list.
• It is deleted from all three deck lists.
• At the end, the hand has 3 and the deck has 3.

What is the deck worth?

Add up every power in the deck.

• A running total that starts at 0.
• Each row is added as you pass it.
• It is right for a deck of any size.

set [total v] to (0)
repeat (length of [powers v])
  change [n v] by (1)
  set [total v] to ((total) + (item (n) of [powers v]))
end
            

Whose hand is better?

Two hands, two totals, one winner.

• Two lists of powers: yours and theirs.
• One loop totals both at the same time.
• The bigger total wins, and the machine says the score.

repeat (length of [mine v])
  change [n v] by (1)
  set [my total v] to ((my total) + (item (n) of [mine v]))
  set [their total v] to ((their total) + (item (n) of [theirs v]))
end
if <(my total) > (their total)> then
  say (join [I win, ] (join (my total) (join [ to ] (their total)))) for (3) seconds
end
            

Summary 5 min

• Several facts about one thing go in several lists, kept in step by their row number.
• (item (4) of [names v]) is “what is at row 4”.
• (item # of [Bat] in [names v]) is “which row is the Bat in” — and 0 when it is nowhere.
• A row number found in one list reads the others.
• Best-so-far and running-total are both one walk down a list.

In step

Lists whose row numbers line up, one row per thing.

Position

Which row something is in.

Best so far

A variable holding the best you have seen while you keep looking.

Running total

A sum built up one row at a time.

Hand in

Save your deck. Next lesson you write item # of yourself — and find out how much work it has been doing for you.