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Lesson 3 of 6
  • Year 9

Iterating through data structures

I can use iteration to repeatedly add items to lists and check the contents of a string.

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View unit
Lesson 3 of 6
New
New
  • Year 9

Iterating through data structures

I can use iteration to repeatedly add items to lists and check the contents of a string.

Download all

These resources were made for remote use during the pandemic, not classroom teaching.

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Lesson details

Key learning points

  1. Condition-controlled iteration is implemented using while loops in Python.
  2. A string is like a list in structure but is immutable, so elements cannot be changed, added or removed.
  3. Some operations that can be performed on lists can also be performed on strings.

Keywords

  • Iteration - the process of repeating a sequence of instructions within a program loop

  • String - a sequence of characters, e.g. letters, numbers, spaces, and other symbols

  • Mutable - able to be changed

  • Immutable - not able to be changed

Common misconception

Strings are mutable meaning that any character within the string can be changed.

Strings are immutable. The elements of a string cannot be changed. If changes to the string are required, the whole string must be rewritten.


To help you plan your year 9 computing lesson on: Iterating through data structures, download all teaching resources for free and adapt to suit your pupils' needs...

A string can be referenced using an index value. Break apart on a whiteboard how the characters of a string are referenced starting with the first character at index 0 and then every subsequent character with the next index value.
Teacher tip

Equipment

Pupils will need access to a device that can edit and run Python programs. Examples in this lesson use the RPF Code Editor https://oak.link/python-editor

Licence

This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Prior knowledge starter quiz

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6 Questions

Q1.
What is the correct syntax in Python to initialise a variable called age and set its value to the numerical value 13?

age (13)
Correct answer: `age = 13`
age = "13"
set age to 13

Q2.
What is the correct syntax in Python to initialise a variable called name and set its value to Laura?

name (Laura)
name ("Laura")
Correct answer: `name = "Laura"`
name = Laura

Q3.
Which programming construct is used to repeatedly perform actions in a program?  

sequence
selection
Correct answer: iteration
decomposition

Q4.
are used to tell a program whether a condition has been met or not.

Correct Answer: Flags

Q5.
A while loop will run ...

a set number of times.
Correct answer: until a condition is no longer met.

Q6.
12
colours = ["red", "blue", "yellow", "green"] print (colours[1])
 What colour would be printed by the program?
Code colour

When programmers write code, they use a special tool called an IDE (Integrated Development Environment). In an IDE, different colours are used to help programmers understand the code:


  • • Blue - numbers and boolean values
  • • Green - strings
  • • Purple - keywords

red
Correct answer: blue
yellow
green

Assessment exit quiz

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6 Questions

Q1.
A is a data type that is a collection of characters.

Correct Answer: string

Q2.
Lists are , which means that their contents can be modified, without having to create a new list.

Correct Answer: mutable

Q3.
Which code would check the first character of name1 does not appear anywhere in name2?

name1[0] == name2[0]
len(name1) != len(name2)
Correct answer: `name1[0] not in name2`

Q4.
Which of the following statements about strings is true?

Strings are mutable.
Correct answer: Strings are immutable.
Strings can only hold letters.

Q5.
What code would you use if you wanted to check the value of the first letter held in the string: message = "Hello"?

Correct answer: `message[0]`
message[1]
message["H"]
message[first]

Q6.
If you update the value held by a string, the previous value is .

Correct Answer: overwritten