Direction & Movement

🎯 Learning Objectives

What students will be able to do after Step 3:

• Use if / elif / else chains to handle mutually exclusive conditions
• Respond to arrow key events using pygame.KEYDOWN and pygame.K_UP etc.
• Update (x, y) coordinates by adding a direction vector
• Explain WHY a snake cannot reverse direction (the reverse guard)

💡 The Big Idea — Annotated Code

# Direction as dx, dy offset
dx, dy = 1, 0   # start moving right

# --- Inside the event loop ---
if event.type == pygame.KEYDOWN:
    if event.key == pygame.K_UP    and dy != 1:   # reverse guard
        dx, dy = 0, -1
    elif event.key == pygame.K_DOWN  and dy != -1:
        dx, dy = 0, 1
    elif event.key == pygame.K_LEFT  and dx != 1:
        dx, dy = -1, 0
    elif event.key == pygame.K_RIGHT and dx != -1:
        dx, dy = 1, 0

# --- Update step ---
head_x += dx
head_y += dy

Direction as a vector

(dx, dy) = (1, 0) means "move 1 cell right, 0 cells up/down." This is cleaner than storing a direction string like "right" — addition gives the new position directly without a lookup table.

pygame.KEYDOWN

Fired once when a key is first pressed (not held). Using pygame.KEYDOWN for direction changes is correct — pygame.key.get_pressed() returns True every frame while the key is held, which would fire too often.

Reverse guard — and dy != 1

Prevents UP being pressed when already going DOWN (which would move the head into the neck). Without this guard, the snake can immediately eat itself on the very next frame.

elif not if

We want only ONE direction change per frame. If all branches were if, pressing two keys simultaneously could cause the snake to process both and end up moving diagonally — which is not possible on a grid.

🔢 Activity Answers — step-3-activity.html

Tier 1 — Check Understanding

Tier 2 — Fix the Bug

Effect: You can ONLY press UP when already going down — the controls are inverted and broken.

Fix: Change == to !=: and dy != 1

Tier 3 — Extend: Add WASD controls

elif event.key == pygame.K_w and dy != 1:
    dx, dy = 0, -1
elif event.key == pygame.K_s and dy != -1:
    dx, dy = 0, 1
elif event.key == pygame.K_a and dx != 1:
    dx, dy = -1, 0
elif event.key == pygame.K_d and dx != -1:
    dx, dy = 1, 0

Tier 4 — New Task: Pause on spacebar

paused = False
# Inside event loop:
if event.type == pygame.KEYDOWN:
    if event.key == pygame.K_SPACE:
        paused = not paused   # toggle
# In update section:
if not paused:
    head_x += dx
    head_y += dy

Reflective Journal — Suggested Answers

⚠️ Mistakes Students Actually Make

🗣️ Teaching & Facilitation Notes

Opening hook

"What information does your brain use to move your hand? Direction and distance — the same two things as dx and dy."

Key pedagogical moment — the reverse guard

Draw a snake on the board. Mark the head and neck. Ask: "If the head is moving right and you press LEFT, where does the head go?" Students will see it moves into the neck. This makes the guard intuitive rather than a mysterious rule to memorise.

CT connection — Decomposition + Conditionals

The if/elif chain decomposes the key-handling problem into four mutually exclusive cases. Ask students: "What is the algorithm for deciding which direction to move?" This surfaces the idea that conditionals are a tool for expressing decisions.

Pacing

If students finish early, ask them to try making the snake start moving UP instead of RIGHT — this surfaces the guard logic because they need to adjust the initial dx, dy values and think through which guards change.