Programming Lab

Variables are labelled boxes

A variable is a named box that holds one value; an assignment works out the right-hand side and puts the answer in the box, replacing what was there, and the program uses the latest value

Can we play outside? AND, OR and NOT

NOT turns true into false; AND is true only when both parts are true; OR is true when at least one part is true, so a long condition can be worked out one part at a time

If it rains, take an umbrella

An if / else statement checks a condition: when it is true the program runs the then part, otherwise the else part, never both

Is it a leap year?

A year is a leap year when it divides by 4 and not by 100, or when it divides by 400; the remainder % tests divisibility and AND / OR join the rules into one condition

Three tries for a PIN

A loop condition joined with AND stops as soon as either part is false: here when the PIN is right or when three tries are used up

Adding the digits of a number

n % 10 is the last digit of n and floor(n / 10) is n without it, so a while loop that repeats both until n is 0 visits every digit once

FizzBuzz: loops, % and decisions

A loop can make a decision on every repeat: i % 3 == 0 is true when 3 divides i exactly, and in a chain of ELSE IFs the first true check wins, so the most special case must come first

Repeat with a loop

A loop repeats the same lines: a for loop runs once for each value of its counter, an accumulator adds something on every repeat, and a while loop repeats until its condition becomes false

Saving up with a while loop

A while loop repeats its body as long as its condition is true, so the body must change something that makes the condition false in the end, or the loop never stops

Lists and their positions

A list (array) keeps items in order; each item has an index, counted from 0, so a list of n items has indexes 0 to n − 1

Find it one by one

Linear search compares the target with each item in order; it needs 1 comparison at best and as many as there are items at worst, or when the target is missing

A function that converts temperatures

A function keeps a rule in one place: to_fahrenheit(c) returns c × 9 ÷ 5 + 32, and every call gives it a new argument for its parameter c

A traffic light is a state machine

A state machine is always in one of a few states and moves to the next state by a rule; a traffic light keeps its state in a variable and goes round red, green, yellow with (state + 1) % 3

Help the robot reach the battery

An algorithm is a list of exact steps done in order; changing the order or one step changes the result, and debugging means finding the step where the plan breaks