Session 2: Loops & Variables - The Engine of Continuous Motion

🔁 Lesson 2: Loops and Variables

🎯 Learning objectives

After this lesson, you will be able to:

• Tell the difference between the repeat and forever loops.
• Use loops to make programs shorter.
• Create variables to store counts, scores or speed.
• Combine loops and variables to control a sprite.

1. What is a loop?

Many things in everyday life happen again and again:

• A heart beats continuously.
• A wheel turns many times.
• A warning light blinks on and off.
• A robot checks its sensors over and over.

In programming, a loop runs a group of blocks again without you having to write the same block many times.

For example, instead of using the block move 10 steps ten times, you can write:

when green flag clicked
repeat 10
    move 10 steps

The sprite moves a total of: 10 times × 10 steps = 100 steps

2. Two main kinds of loop

The repeat [10] block runs the blocks inside it exactly the chosen number of times, then stops.

when green flag clicked
repeat 4
    move 80 steps
    turn right 90 degrees

This program moves the sprite around a square.

Use it when:

• You know in advance how many times to repeat.
• You want a light to blink exactly 5 times.
• You want the robot to drive exactly 3 laps.
• You want to draw a shape with a fixed number of sides.

The forever block keeps running until you click the red Stop button 🛑.

when green flag clicked
forever
    move 5 steps
    if on edge, bounce

Use it when:

• The robot must keep checking its sensors.
• A game must keep checking the keyboard.
• A traffic light runs in a cycle.
• The robot must keep moving until it meets an obstacle.

3. Variables — boxes that store information

A variable is like a labelled box that stores information in your program.

For example, the variable Speed starts at 5:

set [Speed] to [5]

To increase the speed by 1:

change [Speed] by [1]

The value of the variable changes:

5 → 6 → 7 → 8

Some common variables:

• Speed: how fast the sprite moves.
• Score: the player's score.
• Laps: the number of laps completed.
• Distance: a reading from a sensor.
• Time: the time remaining.

4. Combining loops and variables

Program the sprite to speed up after each move:

when green flag clicked
set [Speed] to [2]

repeat 8
    move [Speed] steps
    change [Speed] by [1]
    wait 0.2 seconds

Each time round the loop, the sprite moves further:

2 → 3 → 4 → 5 → 6 → 7 → 8 → 9

To stop the speed from growing too high, we need the if ... then condition block, which you will learn in Lesson 3. For now, limit the speed by choosing a suitable number of repeats.

5. Practice: the polygon-drawing robot

Use variables so the robot can draw different shapes:

1. 1Create the variables Sides and Shapes.
2. 2The robot draws every side of the shape using a loop.
3. 3After each side, the robot turns by 360 ÷ Sides degrees.
4. 4After each shape, increase Shapes by 1 and Sides by 1.

when green flag clicked
erase all
pen down
set [Sides] to [5]
set [Shapes] to [0]

repeat 3
    repeat [Sides]
        move 60 steps
        turn right (360 ÷ Sides) degrees
    change [Shapes] by [1]
    change [Sides] by [1]
    wait 0.5 seconds

The robot draws a 5-sided, a 6-sided and a 7-sided shape in turn. The variable Shapes shows how many shapes have been drawn.

6. Common mistakes

Reset your variables every time the program starts:

when green flag clicked
set [Laps] to [0]

If the change variable block sits outside the loop, the variable only changes once.

The program may run too fast. Add:

wait 0.1 seconds

🧩 Challenge

Build a countdown from 5 to 1:

when green flag clicked
set [Countdown] to [5]

repeat 5
    say [Countdown] for 1 seconds
    change [Countdown] by [-1]

say "Go!" for 2 seconds

Can you change the program to count down from 10?

✅ Remember

• repeat [ ] is for when you know how many times to repeat.
• forever is for programs that must run non-stop.
• Variables store information that can change.
• set variable gives it a new value.
• change variable increases or decreases the current value.
• Loops combined with variables make programs shorter and smarter.

1. 1How do loops help a program?
2. 2When should you use the forever loop?
3. 3If Speed = 5 and you increase it by 2, what is the new value?
4. 4Why should you limit the speed when controlling a robot?