Flowchart for programming: four worked examples

1. Start
2. ↓ Input the first number (a)
3. ↓ Input the second number (b)
4. ↓ sum = a + b
5. ↓ Display the sum
6. ↓ End

The simplest kind of program runs straight down: each step once, in order. This is called a sequence.

It teaches the three parts nearly every program has: take input, work something out, show output. Notice that sum has to be worked out before it can be shown: the order of the steps matters.

The same flowchart in Python

1a = float(input("What is the first number? "))2b = float(input("What is the second number? "))3sum = a + b4print("The sum is " + str(sum))

1. Start
2. ↓ Input age
3. ↓ Is age >= 18?
4. ↓ Yes: display "You are an adult" · No: display "You are a minor"
5. ↓ End

A decision (the diamond) asks a question whose answer is yes or no, and the program takes a different path for each. In code, this is an if … else.

Try it with 17, 18 and 19: testing the values on either side of the condition is how you check that >= is right and > isn't.

The same flowchart in Python

1age = int(input("What is your age? "))2if age >= 18:3    print("You are an adult")4else:5    print("You are a minor")

1. Start
2. ↓ Input a number
3. ↓ Is number % 2 == 0?
4. ↓ Yes: display "It is an Even number" · No: display "It is an Odd number"
5. ↓ End

% gives the remainder after dividing. Every even number divides by 2 with nothing left over, so number % 2 == 0 is true exactly when it's even.

It's a small example of turning a rule from maths into a condition a computer can check, which is most of what programming logic is.

The same flowchart in Python

1number = int(input("Enter a number "))2if number % 2 == 0:3    print("It is an Even number")4else:5    print("It is an Odd number")

1. Start
2. ↓ count = 1
3. ↓ Repeat 10 times:
4. ↓ display "Count: " + count
5. ↓ count = count + 1
6. ↓ End

A loop runs the same steps again and again. Here a counter starts at 1, and each time round the loop shows it and adds 1.

It teaches the two things every loop needs: something that changes each time (count), and a way to stop (after 10 times). Forget either and the loop never ends, or never starts.

The same flowchart in Python

1count = 12for _ in range(10):3    print("Count: " + str(count))4    count = count + 1