Lesson 3: Iteration¶

Video link

Sequence¶

So far, each line of our code runs one after the other. This is called sequence. It's the normal way programs run: the program starts at the top and works its way down, one line at a time.

This movement through the code is called the flow of the program, like water flowing through a pipe.

Introduction to flowcharts¶

A flowchart is a diagram that shows each step in a program and how the program moves from one step to the next. Different shapes represent different parts of the program:

• rectangles show a process (something the program does)
• parallelograms show input or output (getting or showing information)
• arrows show the flow (the direction the program moves)

If we wanted a program to say hello to six people, the flowchart would look like this:

Create a new file, type in the code below, and save it as iteration.py.

1
2
3
4
5
6
7
8

# Our iteration program

print("Hello Hunter")
print("Hello Jordi")
print("Hello Adam")
print("Hello Jesse")
print("Hello Bryce")
print("Hello Ben")

The Shell shows:

Hello Hunter
Hello Jordi
Hello Adam
Hello Jesse
Hello Bryce
Hello Ben

Because the program runs in sequence, the order of the lines matters. Change the order of the lines so our code matches the code below.

1
2
3
4
5
6
7
8

# Our iteration program

print("Hello Jesse")
print("Hello Bryce")
print("Hello Ben")
print("Hello Hunter")
print("Hello Jordi")
print("Hello Adam")

Sequence is fine for small programs, but it becomes a problem with bigger ones. Imagine saying hello to 500 or even 1,000 people. That would take a long time to type. And if we wanted to change Hello to Good morning, we would have to change every single line.

In Digital Technologies, we say this code is not scalable: it doesn't work well as the program gets bigger.

Iteration¶

Look at lines 3 to 8 again. They're almost the same. The only thing that changes is the name.

This breaks the DRY principle. DRY stands for Don't Repeat Yourself. It means we shouldn't write the same code over and over again.

One way to avoid repeating ourselves is iteration (also called loops). A loop repeats the same code again and again, with a small change each time. This is perfect for our program, because we want to repeat print("Hello", name) with a different name each time.

for loops¶

The first loop we will use is the for loop. It's the first control structure we've used. A control structure changes how the program flows, instead of just running from top to bottom.

Change our code so it matches the code below.

1
2
3
4
5
6

# Our iteration program

names = ["Hunter", "Jordi", "Adam", "Jesse", "Bryce", "Ben"]

for name in names:
    print("Hello", name)

Line 3 creates a list. A list works like a real-life list:

• it has a number of items, called elements
• the elements are in a set order
• [ and ] show where the list starts and ends
• a comma (,) separates each element
• we give the list a name, just like our turtle or window

Line 5 is how we write a for loop in Python:

• for is a special word that starts the loop
• in names tells Python to go through each element in the names list
• name stores the element the loop is currently using
• : tells Python that the indented lines below belong to the loop

Line 6 is indented. The indentation shows which code repeats in the loop. Indents should be four spaces. In Thonny, pressing Tab adds four spaces for us.

for loop flowchart¶

To draw a for loop as a flowchart, we need two more symbols:

• terminators show the start and end of the program
• decisions are questions the program asks. The answer changes the path the program takes.

Here is the flowchart for our for loop. The shapes inside the dotted box repeat once for each element in the list.

Tracing with the debugger¶

Another way to understand a for loop is to use Thonny's debugger. A debugger lets us run our code one step at a time and see what happens.

Start the debugger by clicking the bug icon next to the Run button.

Keep pressing F7 and Thonny will move through the code one step at a time. Watch the values in the Variables panel as you do this.

Code blocks¶

Indented lines that belong together are called a code block. Let's see how that works. Add line 7 so our code matches the code below.

1
2
3
4
5
6
7

# Our iteration program

names = ["Hunter", "Jordi", "Adam", "Jesse", "Bryce", "Ben"]

for name in names:
    print("Hello", name)
    print("How are you?")

The Shell shows:

Hello Hunter
How are you?
Hello Jordi
How are you?
Hello Adam
How are you?
Hello Jesse
How are you?
Hello Bryce
How are you?
Hello Ben
How are you?

All of the code in the code block repeats. Every line in a code block must be indented by the same number of spaces.

What happens if a line isn't indented? Add line 9 so our code matches the code below. Make sure line 9 is not indented.

1
2
3
4
5
6
7
8
9

# Our iteration program

names = ["Hunter", "Jordi", "Adam", "Jesse", "Bryce", "Ben"]

for name in names:
    print("Hello", name)
    print("How are you?")

print("Come in and sit down")

The Shell shows:

Hello Hunter
How are you?
Hello Jordi
How are you?
Hello Adam
How are you?
Hello Jesse
How are you?
Hello Bryce
How are you?
Hello Ben
How are you?
Come in and sit down

Because line 9 isn't indented, it isn't part of the for loop, so it runs after the loop has finished. The flowchart for our updated code looks like this:

Exercises¶

In this course, the exercises are the make part of PRIMM. Work through them to make your own code.

Starter files are in the lesson_03 folder of the tutorial files. Solutions are on the Exercise Solutions page.

Exercise 1¶

Starter: lesson_03/ex1_greet_friends

Can you change the program so it says hello to your friends instead? Use a list called friends and a loop variable called friend.

Exercise 2¶

Starter: lesson_03/ex2_school_greeting

Can you change the program so it greets the class at the start of a lesson? It should:

• print Good morning and the name of each person
• ask each person to get their laptop out
• print Let's start the lesson once, after everyone has been greeted

Exercise 3¶

Starter: lesson_03/ex3_indent

What happens if we indent the last line, print("Come in and sit down"), so it lines up with the two print lines inside the loop? Why do you think this happens?