lesson 3.1.5 loops in python

Round and round again.

Imagine if you had to write "I will not talk in class" 100 times on a whiteboard. You’d get bored very quickly! Luckily, computers love doing boring, repetitive tasks. Today, you are going to learn the superpower of iteration. You will master the specific controls of the for loop to generate exact sequences of numbers, process text, and lists, before learning the while loop to build a game that can outsmart you! You are stepping into the shoes of a Software Developer or Automation Engineer where efficiency is everything.

Learning Outcomes

The Building Blocks (Factual Knowledge)

Define the terms "definite iteration" and "indefinite iteration" in the context of Python programming.

Recall the three parameters of the range() function: start, stop, and step.

State the syntax for iterating over strings and lists.

The Connections and Theories (Conceptual Knowledge)

Distinguish between the outputs of range(5), range(1, 5), and range(1, 10, 2).

Explain how a loop variable changes value during each iteration of a cycle.

Predict the output of a loop that processes a list of items.

The Skills and Methods (Procedural Outcomes)

Apply the range() function to generate precise sequences, including countdowns.

Construct for loops to iterate through strings and lists to process data.

Create a while loop to build a robust "Number Guessing Game".

Digital Skill Focus: C.1.1 Core Skills - Develop touch-typing skills to improve speed and accuracy of syntax entry within an IDE

Why repeat yourself?

In programming, we follow a rule called DRY: Don't Repeat Yourself. If you copy and paste the same line of code five times, you are wasting time. Instead, we use loopsI have no idea what this means (also called iterationI have no idea what this means). There are two types of iteration:

The

for

Loop

Use a

for

loop when you want to repeat a block of code a specific number of times. We often call this type of iteration definite because we know exactly how many times the loop will run. The main ways we can definitely iterate are:

Using the

range()

function to construct an iterable:

range(stop)

range(start,stop)

range(start,stop,step)

- step can be negative as well...

Using a string as the iterable.

Using a list as the iterable.

Task 1 PRIMM: The Range Rover and other iterables

As we've seen, there are a number of ways in which we can specify the iterable in a

for

loop. In this task, you are going to use the PRIMM methodology to learn the basics. You will use a read-only inline IDE and Thonny as well to give you some practice managing your workspace and coding in a bonefide IDE.

Get organised

Open up Thonny on your PC. If you haven't got Thonny available, ask your teacher to suggest a suitable IDE to use instead.

Organise your workspace!

The

range()

iterable

Arguably the most straightforward iterator is the

range()

function. It actually generates a list of values based on the parameters that you give it and then you use a

for

loop to iterate through them one at a time. The simplest option is to specify only the

stop

value instructing the loop to count from 0 up to (but not including) the number you give it. Let's get PRIMMing!

⭐ Easing you in:

range(stop)

🤔 Code Explainer

Line 1: Define the type of interaction (definite), declare the loop variable (

) and define the range of values assigned to it during the loop.
Line 2: Print a string "

This is loop number

" concatenated with a current value of the loop variable. The comma (

) automatically typecasts the integer in

to a string.

Predict the first number and the last number this code will print.

Run the script. Were you right?

Investigate the values printed on each line. What does stop value represent?

Here comes Thonny... Now copy the code and paste it into a new Thonny script. Make sure it's saved.

Modify the code so it prints the numbers 0 to 9. Change the message to say "Chapter " followed by each number.

Make a script containing a loop that prints the first 5 multiples of 3. Hint: calculate

i * 3

inside the print statement.

Challenge
Ask the user to input a number

, then loop

times printing "Python is cool" each time.

Additionally, we can specify the

stop

parameter as well so we can truely define the range of numbers that the loop should start counting at and end counting at. Let's get PRIMMing...

⭐⭐ Moving on:

range(start,stop)

🤔 Code Explainer

Line 1: Define the type of interaction (definite), declare the loop variable (

) and define the range of values assigned to it during the loop.
Line 2: Print a string "

i has the value:

" concatenated with a current value of the loop variable. The comma (

) automatically typecasts the integer in

to a string.

Predict what will happen when you run the script. Will it print

Run the script - was your prediction correct?

Investigate the values printed. What stop value would you specify if you wanted the last number to be

Here comes Thonny... Now copy the code and paste it into a new Thonny script. Make sure it's saved.

Modify the loop to count from

Make a new script which prints the 6 times table but it MUST start with 1*6 = 6.

Challenge
Ask the user for a start number and a stop number, then run a loop between those two numbers.

The ultimate

range()

function has all three of it's parameters set -

start

stop

and

step

. Now it's getting interesting so let's get PRIMMing!

⭐⭐⭐ Ultimate control:

range(start,stop,step)

🤔 Code Explainer

Line 1: Define the type of interaction (definite), declare the loop variable (

) and define the range of values assigned to it during the loop.
Line 2: Print a string "

i has the value:

" concatenated with a current value of the loop variable. The comma (

) automatically typecasts the integer in

to a string.

Predict what numbers will appear when you run the script.

,... what else?

Run the script - was your prediction correct?

Investigate the values printed. Can you see which number prints last and why?

Here comes Thonny... Now copy the code and paste it into a new Thonny script. Make sure it's saved.

Modify the step to

so the script counts up in 5s.

Make a new script which asks the user for the

start

stop

and

step

values and then prints out the correct numbers.

Challenge
Make a countdown timer by using a negative step value. Yes, that's possible! Try using this

range()

function:

range(10,0,-1)

in place of the one in your script and see what happens.

A string iterable

OK, so iterating through a list of numbers generated using the

range()

function is all well and good but there are some other cool things we can iterate through as well, including strings...

🤔 Code Explainer

Line 1: Declare the variable

word

and store the string "

Hello

" in it.
Line 2: Define the type of iteration (definite), declare the loop variable (

letter

) and assign it the values it will take during the loop, in this case the string contained in the variable

word

.
Line 3: Print out the current value of the loop variable,

letter

Predict how the word

Hello

will print when you run the script.

Run the script - was your prediction correct?

Investigate the output. Are you happy that you know what has happened?

Here comes Thonny... Now copy the code and paste it into a new Thonny script. Make sure it's saved.

Modify the script so that it prints a different word.

Make a new script which asks the user for their name using the

input()

function and then

print()

s out each letter in it.

Challenge
Can you make it print your name in UPPER CASE a letter at a time using the

.upper()

function? Hint:

print(letter.upper())

A list iterable

Lastly in the world of definite iteration we meet the list iterable. We'll learn more about lists in another module but for now, just recognise that they are a container for any values we want; integers, floats, characters, strings, Booleans or a combination thereof.

Firstly, a list of integers...

🤔 Code Explainer

Line 1: Define the type of loop, specify the loop variable (

item

) and the iterable.
Line 2: Print out the current value of the loop variable.

Predict what will happen when you run the script.

Run the script - was your prediction correct?

Investigate the values printed. Do they make sense?

Here comes Thonny... Now copy the code and paste it into a new Thonny script. Make sure it's saved.

Modify the script to add a list of the number of days in each month of the year (not a leap year).

Make a new script which prints out the double of each number in the list. Hint: You will have to include

item*2

inside the

print()

statement.

Challenge
Type the following code into a new Thonny script and run it...

for item in [77,97,103,105,99]:
    print(chr(item),end="")

Can you figure out what's going on here?

...and finally, a list of strings...

🤔 Code Explainer

Line 1: Define the type of loop, specify the loop variable (

name

) and the iterable.
Line 2: Print out the string

Hello

concatenated with the current value of the loop variable.

Predict what will happen when you run the script.

Run the script - was your prediction correct?

Investigate the values printed - do they make sense?

Here comes Thonny... Now copy the code and paste it into a new Thonny script. Make sure it's saved.

Modify the script by changing the list to one of your friends names.

Make a new script which contains a list of your favourite foods and prints out

I love [food]!

for each one.

Challenge
Write a script which takes this list...

words = ["Welcome","to","Python","for","beginners!"]

...and prints it out as a sentence like this

Welcome to Python for beginners!

Hint: You will have to use the the

end=" "

parameter in the

print()

statement.

Outcome: I have predicted, run and modified Python code in a

for

loop (definite iteration).

The

while

loop

Sometimes we simply don't know how many times to loop! We want to keep going an indefinite number of times while a condition is

true

. Try this (very easy) guessing game out by pressing the ▶️ button.

🤔 Code Explainer

Line 1: Make sure the

answer

variable is empty.
Line 2: Define the loop as indefinite (

while

) and state the condition -

answer != "Paris"

. The loop will continue to run while this condition is

true

.
Line 3: Ask the user for their guess and store it in

answer

.
Line 4: Only if the condition is false will this line run and print

Correct!

Task 2 The Number Guessing Game

We will combine variablesI have no idea what this means, loopsI have no idea what this means (

while

), and selectionI have no idea what this means (

if

else

) to make a simple guessing game. Run the game by pressing ▶️ and try to win.

🤔 Code Explainer

Line 1: Import a special set of functions to create random numbers.
Line 2: Choose a random number between 1 and 20 and store it in the variable

secret

.
Line 3: Make sure

guess

.
Line 4: Print an introductory message.
Line 5: Define the loop as indefinite (

while

) and state the condition,

guess != secret

. The loop will continue to run if this condition stays

true

.
Line 6: Ask the user for their guess and convert it to an integer.
Line 7: If the guess is less than the secret number then run the next code block only.
Line 8: Print

Too low!

Line 9: Else if the guess is greater than the secret number, run the next code block only.
Line 10: Print

Too high!

Line 11: The guess must be correct - it's the only option left!
Line 12: Print

You got it!

. At this point, the loop will end because the condition is false.

Try these:

What is a MINIMUM number of guesses you would need to guess the number?

For the quiz that is written, what is the MAXIMUM number of guesses you would EVER need to guess the correct number?

Can you make the quiz harder by increasing the range of numbers that it choosen?

Outcome: I have combined iteration, selection, and variables to create a fully functioning interactive game.

Today you have learnt how to control the flow of a program using loops. You can use for loops when you know the count, and while loops when you are waiting for a specific condition. You have applied this to create a game that responds intelligently to the user.