Pseudocode and flowcharts

Learn

What you’ll learn

• How to write pseudocode
• How to draw a flowchart
• How to turn either one into real code

Pseudocode

Pseudocode is code-like plain English. It isn’t a real language, so there’s no syntax to get wrong; it’s for thinking.

forever:
    read distance
    if distance < 20 cm:
        turn left
    else:
        drive forward

Flowcharts

Shape	Meaning
Rounded box (oval)	Start / end
Rectangle	An action: “drive forward”
Diamond	A decision with yes/no arrows: “distance < 20?”
Parallelogram	Input or output: “read sensor”, “print”
Arrows	The order of steps

  (Start)
     │
 [Read distance] ◄──────────┐
     │                      │
  <dist < 20?> ─yes─► [Turn left] ─┤
     │ no                          │
 [Drive forward] ──────────────────┘

Why plan first?

Bugs are much cheaper to fix on paper. Walking through a plan with a few example values (“what if the distance is 15? 50?”) catches mistakes before you write a line of code.

Examples

Example: pseudocode to code

Pseudocode:

forever:
    if both line sensors see white: go straight
    else if left sees black: turn left
    else if right sees black: turn right

Robot simulator code:

function loop() {
  if (!lineLeft() && !lineRight()) motors(50, 50);
  else if (lineLeft()) motors(10, 60);
  else if (lineRight()) motors(60, 10);
}

Example: tracing with values

distance	Decision	Action
80	80 < 20? no	drive forward
25	25 < 20? no	drive forward
15	15 < 20? yes	turn left

Practice

Try it

Turn this pseudocode into working code: forever: if distance < 30 turn right, otherwise drive forward slowly.

Pseudocode to code

▶ Run↺ Reset

Your code (JavaScript)function loop() { // write your code here }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Write the codeEasy

Implement the pseudocode above in the simulator. if (distance() < 30) motors(50, -50); else motors(35, 35);

Draw the flowchartMedium

Draw a flowchart for a night light: read the light level; if it’s dark, LED on; otherwise LED off; repeat.

Plan a parking robotHard

Write pseudocode for a robot that drives forward, slows down below 50 cm, stops at 10 cm, beeps three times and then waits.

Show an answer

forever:
    d = read distance
    if d > 50: drive fast
    else if d > 10: drive slow
    else:
        stop
        repeat 3 times: beep
        wait forever

Quick check

1. In a flowchart, a decision is drawn as a…

RectangleDiamondOvalCircle

2. Why write pseudocode?

Computers run it directlyTo plan the logic without worrying about syntaxIt runs fasterIt replaces testing

3. Tracing a plan with example values helps you…

Find bugs before codingCharge the robotMake it run fasterChoose colours

Check my answers