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Pseudocode and bugs · Computer Basics

How do you write instructions so precise that a computer could follow them, before you know any programming language?

Course contents

Why you need it

Programmers often plan in plain words first and turn the plan into code afterwards. The plain-words version is called pseudocode.

It's built from just three kinds of step: do things in order, decide with if, and repeat. These are exactly what you'll write in the robot course next, and in every programming language after that. Here you can learn them with no rules about symbols to worry about.

The intuition

Pseudocode is instructions in everyday language, written as carefully as code. Almost every program is made of three kinds of step.

1. Sequence: one step after another.

put bread in the toaster
press the lever
wait until it pops up

2. Decision: if something is true do this, otherwise do that.

if it is raining
    take an umbrella
otherwise
    wear sunglasses

3. Repetition: do something again and again.

repeat 3 times
    clap your hands

or, when you don't know in advance how many times:

while the glass is not full
    pour a little more water

The indented lines (pushed in to the right) belong to the if or the repeat above them. Lines that are not indented happen once, after it.

You can also give a group of steps a name: "to make tea: boil water, put a tea bag in a cup, pour, wait 3 minutes". From then on, "make tea" is a new step you can use anywhere. That's called a function.

Bugs. When a step is wrong, missing or in the wrong place, the result is wrong. That's a bug, and finding and fixing it is debugging. Every programmer writes bugs every day; good programmers find them by tracing: following the steps one at a time, exactly as the computer would.

The machines on the right are the pseudocode above written in Python. Python reads almost like pseudocode, and you'll learn it soon.

Definition

Pseudocode

Pseudocode describes the steps of an algorithm in plain language, structured like code but not meant to be run by a computer.

Loop

A loop repeats a group of steps, either a set number of times or for as long as a condition is true.

Bug

A bug is a mistake in a program that makes it behave differently from what was intended.

Debugging is finding and fixing bugs.

Examples

Decision: umbrella or sunglasses

Try it yourself

The decision from above, in Python. True means yes and False means no. Change it and run again: only one of the two indented lines runs, but Go outside always runs because it isn't indented.

raining = True   # change me 改这里: True or False
if raining:
    print("Take an umbrella")
else:
    print("Wear sunglasses")
print("Go outside")
Step by step
  1. L1Is it raining? Yes.raining = True
  2. L2Check the condition: it's true.true
  3. L3So do the indented step under if.Take an umbrella
  4. L6The else part is skipped. This line isn't indented, so it always runs.Go outside

Repetition: countdown

Try it yourself

A while loop: repeat as long as the condition is true. Start from 5 or 10 instead.

count = 3   # change me 改这里
while count > 0:
    print(count)
    count = count - 1
print("Lift off!")
Step by step
  1. L1Start the count at 3.count = 3
  2. L2Is count more than 0? Yes, so do the indented steps.3 > 0
  3. L3Say the number.3
  4. L4Count down by one, then go back up to the check.count = 2
  5. L2After 3, 2 and 1, count is 0 and the check fails.0 > 0 is false
  6. L5The loop is over.Lift off!

Find the bug

Try it yourself

Every step is here, but two are in the wrong order: you can't squeeze out toothpaste with the cap still on. Swap the first two lines (cut one and paste it below the other) and run it again.

print("Put toothpaste on the brush")
print("Take the cap off the toothpaste")
print("Brush for two minutes")
print("Rinse your mouth")
Step by step
  1. L1The computer happily does this first, cap still on.Put toothpaste on the brush
  2. L2Too late: this should have come first.Take the cap off the toothpaste
  3. L3The rest is fine.Brush for two minutes

Common mistakes

Indentation decides what repeats

In repeat 3 times, only the indented lines repeat. Indent one line too many and it happens three times; one too few and it happens once. Python follows the same rule.

A computer can't fill in the obvious

"Brush your teeth" is enough for a person, who fills in the obvious steps. A computer needs every one: pick up the brush, take the cap off, squeeze… Missing steps are the most common bug of all.

Bugs are normal

Programs rarely work on the first try, for anyone. A bug is a puzzle, not a sign that you're bad at this. Trace the steps, find the one that's wrong, fix it, try again.

Quick quiz

1. What does this say?

temperature = 30
if temperature is above 25
    say "Shorts"
otherwise
    say "Jeans"
  1. AShorts
  2. BJeans
  3. CShorts, then Jeans
Show the answer

Answer: A

30 is above 25, so the condition is true: only the if part runs. The otherwise part is skipped.

2. What is said, in order?

repeat 2 times
    say "hip"
say "hooray"
  1. Ahip hip hooray
  2. Bhip hooray hip hooray
  3. Chip hooray
Show the answer

Answer: A

Only the indented line repeats: hip, hip. say "hooray" isn't indented, so it happens once, after the loop.

3. A recipe step says "add salt to taste". Why is that a problem for a computer?

  1. AIt isn't precise: a computer can't decide what "to taste" means
  2. BIt's too short
  3. CNothing is wrong with it
Show the answer

Answer: A

A person uses judgement; a computer needs an exact amount, such as "add 2 g of salt".

Key points

  • Programs are built from three kinds of step: sequence, decision (if / otherwise) and repetition (loops).
  • Indentation shows which steps belong to an if or a loop.
  • Bugs are normal. Trace the steps one at a time to find them.

You've finished Computer Basics. Next, Robot Quest turns these same building blocks into real Python commands for a little robot.

Practice

3 short exercises: think it through, change something and see what happens, then do one yourself. Answers are checked the moment you submit.

  1. ThinkTrace the stickers

    Trace this pseudocode one step at a time:

    money = 5
while money is more than 0
    buy a sticker for 2
    money goes down by 2

    How many stickers does it buy?

  2. ChangeClap, clap, clap, bow

    The goal is to clap 3 times and then bow once. But this machine bows after every clap. Run it and see.

    Fix the bug so it prints:

    clap
clap
clap
bow
  3. Your turnGuide the robot

    Write your own algorithm for a robot. The robot ^ is facing up. # is a wall and F is the flag:

    # # # #
# . F #
# . # #
# ^ # #
# # # #

    It understands three commands: M = move one square forward, L = turn left, R = turn right (turning doesn't move it).

    Write the shortest list of commands that gets the robot onto the flag, for example MLMR.

Practise this lesson

Open the interactive lesson to edit and run these examples and solve the exercises with instant feedback. Free to start, no sign-up.

Open the interactive lesson Course map

Interactive lessons open in order. If you have not done the earlier ones yet, you start at the course map.

Terms in this lesson

TermChineseDefinition
Pseudocode伪代码Steps written in plain language but structured like code.
Sequence顺序Steps carried out one after another, in order.
Condition条件A question with a yes-or-no answer that decides which steps run.
Loop循环A group of steps repeated a set number of times or while a condition is true.
Function函数A named group of steps you can use again and again by its name.
Bug程序错误A mistake in a program that makes it do something unintended.
Debugging调试Finding and fixing bugs, often by tracing the steps one at a time.