If Then Else — two paths, one chosen

Learning Goals 3 min

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

1. Recognise the if-then-else C-block in the Control palette and explain why it has two body slots instead of one.
2. Predict which branch — the "then" or the "else" — will run for any given boolean question, and confirm that exactly one branch runs every time (never both, never neither).
3. Use if <> then ... else ... together with set [color v] effect to () to make a sprite that looks different depending on where it is or what it's touching.

Warm-Up — two scripts that should be one 7 min

Priya is writing a script. She wants the cat to be red when it's on the right half of the Stage and blue when it's on the left half. Last week she only knew about if <> then, so she wrote two of them:

when flag clicked
forever
  if <(x position) > (0)> then
    set [color v] effect to (0)
  end
  if <(x position) < (0)> then
    set [color v] effect to (100)
  end
end
        

This works, but Priya has a feeling there's a cleaner way. What's annoying about writing it as two if-thens?

Today we meet the block that fits that thought exactly: one question, two slots, exactly one runs.

New Concept — the if-then-else C-block 15 min

Last lesson you met if <> then — a C-block with one diamond and one body slot. The blocks inside run if the diamond reports true; otherwise nothing happens. The "otherwise" was silent.

Today's block makes the "otherwise" loud.

Where to find it

In the orange Control palette, right below the plain if-then, is a bigger C-block with the word else in the middle:

if <> then
else
end
        

It looks like two C-blocks fused together. That's almost what it is:

• The top slot (above the word "else") is the "yes" branch — runs only when the diamond reports true.
• The bottom slot (below the word "else") is the "no" branch — runs only when the diamond reports false.

Exactly one runs. Always.

This is the rule that makes if-then-else so useful. Every time Scratch reaches the block, it asks the diamond question, gets back true or false, and runs exactly one of the two slots:

• Never both. The "yes" branch and the "no" branch are mutually exclusive.
• Never neither. Even if the question is weird (like x = 0 from the warm-up), it still reports either true or false, so one slot always runs.

The branch picture

Imagine a path in the forest that splits in two. You walk up to the fork and ask "is the left path muddy?" If yes, you take the right path. If no, you take the left. You don't take both. You always take one. That's the mental model — your script branches.

if <(x position) > (0)> then
  set [color v] effect to (0)
else
  set [color v] effect to (100)
end
        

Why it's better than two if-thens

Three reasons:

• One question, one answer. Scratch only checks x > 0 once per loop tick, not twice. Faster, even if you can't feel it.
• The two outcomes are visibly linked. Reading the code, you see "yes does this, no does that" — they're in the same block, stacked together.
• No gap. With two if-thens, you could accidentally write conditions that both miss (like x > 0 and x < 0 both missing when x = 0). With if-then-else, one branch always runs.

Nesting inside a forever

Just like plain if-then, an if-then-else usually lives inside a forever loop so the question gets re-asked many times a second:

when flag clicked
forever
  if <(x position) > (0)> then
    set [color v] effect to (0)
  else
    set [color v] effect to (100)
  end
end
        

Worked Example — the seeker shouts Warmer or Colder 12 min

Back to our Hot or Cold project. The seeker already patrols. Today it shouts two ways. For now the treasure side is simply the right half — anywhere x > 0. Open your saved project and build it in seven steps.

Step 1 — Open the project

Open your saved Hot or Cold project. The seeker still patrols. Click the seeker sprite.

Step 2 — Start a fresh stack

From Events: when ⚑ clicked. From Control: forever under it.

Step 3 — Add the if-then-else

From Control: if <> then ... else .... Drop it inside the forever C. The loop expands to fit.

Step 4 — Ask which side it is on

From Operators: () > (). Drop it on the diamond slot. From Motion: drag x position into the left slot. Type 0 in the right slot.

Step 5 — Fill the "yes" branch

From Looks: say [Warmer!] for (1) seconds. Drop it in the top slot — the warmer-side reaction.

Step 6 — Fill the "else" branch

From Looks: say [Colder!] for (1) seconds. Drop it below the word "else" — the colder-side reaction.

Step 7 — Click the flag and drag the cat

Drag the seeker across the centre line with your mouse. On the right it shouts "Warmer!". On the left it shouts "Colder!". One side, then the other — never both, never silent.

The full assembled stack

when flag clicked
forever
  if <(x position) > (0)> then
    say [Warmer!] for (1) seconds
  else
    say [Colder!] for (1) seconds
  end
end
        

What you just built: the same branching pattern that decides "is the player alive? show the game, else show Game Over." Right now closeness is a rough left/right guess. In L02-08 the seeker measures real distance to the treasure — and "Warmer" gets honest.

Try It Yourself — three if-then-else drills 15 min

when flag clicked
forever
  if <key [space v] pressed?> then
    say [Yay!] for (1) seconds
  else
    say [Boo] for (1) seconds
  end
end
          

when flag clicked
forever
  if <touching [mouse-pointer v] ?> then
    set size to (150) %
  else
    set size to (100) %
  end
end
          

when flag clicked
set [signal v] to (1)
forever
  if <(signal) > (5)> then
    set [color v] effect to (0)
  else
    set [color v] effect to (70)
  end
  if <key [s v] pressed?> then
    set [signal v] to (10)
  end
end
          

Mini-Challenge — Aljay's broken switch 5 min

"The cat never stops being red"

Aljay wants a cat that's red while the space key is held down, and back to normal the moment you let go. He writes:

when flag clicked
forever
  if <key [space v] pressed?> then
    set [color v] effect to (0)
  end
  if <key [space v] pressed?> then
    set [color v] effect to (50)
  end
end
        

Click the flag mentally — what does the cat do, and what does Aljay want?

when flag clicked
forever
  if <key [space v] pressed?> then
    set [color v] effect to (50)
  else
    set [color v] effect to (0)
  end
end
          

Recap 3 min

The if <> then else block is a fork in your script. It asks one boolean question and runs exactly one of two body slots — the "yes" branch above the word "else", or the "no" branch below it. Never both, never neither. Compared to last lesson's plain if <> then (where "no" meant "do nothing"), if-then-else makes the "otherwise" path visible and guarantees one branch always runs. Whenever you find yourself writing two if-thens with opposite questions, you almost always want one if-then-else instead.

Branch

One of the two paths a script can take inside an if-then-else. Every run of the block picks exactly one branch.

Else

The keyword in the middle of the if-then-else block that separates the "yes" body from the "no" body. Means otherwise — not and also.

Mutually exclusive

Two outcomes that can't both happen at the same time. The yes and else branches of an if-then-else are mutually exclusive.

Boolean reporter

Same as last lesson — a hexagonal block that reports true or false. The diamond slot of if-then-else only accepts these.

Branchless

What you get if you put the same block in both branches: the if-then-else does nothing useful and should be removed. The block runs every tick regardless.

Homework 2 min

The Mood Cat. Build one cat that reads four different things about the world and changes its appearance accordingly. Use at least three if-then-else blocks.

1. If the cat is touching the right edge (touching [edge v]?), turn its colour effect to 100. Otherwise back to 0.
2. If the mouse pointer is touching the cat, grow to size 150. Otherwise size 100.
3. If the space key is pressed, change to costume costume2. Otherwise costume costume1. (Use switch costume to [costume2 v].)
4. Put all three if-then-elses inside one forever loop so they all run together every frame.
5. Bonus: add a fourth if-then-else of your own invention. Think of a question the cat could ask about the world (touching a sprite, key pressed, x-position) and a difference it should make in how the cat looks.

Save as HW-L2-07-Mood-Cat.sb3. Click the flag. Then try every combination — move the mouse, press space, drag the cat to the edge — and see all four moods in action at once.

Bring back next class:

• The .sb3 file.
• Your answer to: "In one of your if-then-else blocks, what would happen if you swapped the contents of the 'yes' slot and the 'else' slot? Try it — describe the change in one sentence."

Heads up for next class: SCR-L02-08 zooms in on the three comparison operators — () > (), () = (), () < () — that build the hexagonal questions you've been dropping into diamond slots.