Iteration

Objectives

• Define iteration in programming and its importance.
• Recognize the concept of loops as a fundamental element of iteration.
• Differentiate between for and while loops in Python.
• Explain when to use a for loop and when to use a while loop.
• Describe the purpose of the range() function in for loops.
• Illustrate the use of for loops to iterate over sequences like lists, tuples, and strings.
• Explain the concept of nested for loops, and demonstrate the use of nested for loops with examples.
• Understand the structure of a while loop in Python.
• Compare and contrast for loops and while loops.

Iteration is a fundamental concept in programming that allows you to execute a block of code repeatedly. Think of "iteration" is a word that simply means "loop", doing something over and over again.

Iteration allows certain sections of our program to be repeated. We just have to be careful not repeat these statements for ever!

Consider, if wanted to print a list of all values between 1 and 10 we could do the following:

print(1)
print(2)
print(3)
print(4)
print(5)
# etc..

Clearly this gets tedious and the loop constructs in Python makes this so much easier.

Example 7

# Example program with for and while loops
print("Counting using for loop:")
for i in range(1, 6):
    print(i)

print("\nCounting using while loop:")
count = 1
while count < 6:
    print(count)
    count += 1

Predict what the output of the program will be for the following:

• What will be printed by the for loop?
• What will be printed by the while loop?

Run the program in a Python environment and compare the actual output with your predictions.

Python provides two main types of loops: for and while loops.

The for loop

A for loop is sometimes call a counted loop because, simply, it repeats a set number of times effectively counting as it goes. For this we need to provide a starting value and a stopping value:

for i in range(1,11):
    print(i)            # print values from 1 to 10

The range() function generates a sequence of numbers. In this example, the range() function has two arguments, the first is the start value, \(1\), the second is the stop value, \(11\). Why \(11\)? This may seem confusing at first but it means that when the control variable, i, has the value \(11\) the loop will end and the following print() statement will not be executed.

There are two variations on the range() function:

• for i in range(10): only the stop value has been specified. The start value defaults to \(0\).
• for i in range(1,20,2): an additional parameter, a skip value. Here i will start with \(1\) and when the loop is repeated the skip value is added making \(3\) and the \(5\) and so on.

A for loop can also used to iterate over a sequence (such as a list, tuple, string) and execute a block of code for each item in the sequence.

Note

We've not yet met lists but they are useful when we have a collection of items that share a common name e.g. colours = ["red", "brown", "white", "black]. A for loop is excellent for moving over each of the items in the list.

colours = ["red", "brown", "white", "black"]
for item in colours:
    print(item)

A string is actually a list of characters. Try the following code in your IDE:

phrase = "Hello, World!"
for ch in phrase:
    print(ch)

Nested for Loops

A loop can be placed inside another loop:

for i in range(0,2):
    for j in range(1,3):
        print(i, j)

This will output:

0 1
0 2
1 1
1 2

Check that you understand why this output is as it is.

The while Loop

Python offers an alternative construct for iteration, the conditional loop. This uses the keyboard while and the code in this loop will execute as long as a specified condition is True. It can be a little more tricky to handle than the for loop as we, the programmer, have to control how many times the loop will execute.

i = 1
while i <= 10:
    print(i)
    i = i + 1

• We need to give the control variable an initial value, i = 1.
• The condition is checked using a relational operator against our desired stop value.
• The control variable needs to be updated in the body of the loop

The for loop and the while loop are interchangeable, it matters little which you use as long you keep the logic correct. Sometimes it is just a matter of personal preference.

Note

The while loop is also known as a top-tested loop as the condition is placed at the top of the loop and if that condition is not satisfied it may never be executed. An alternative form is the do ... while or repeat ... until loops. These are bottom-tested loop constructs. the condition comes at the bottom of the loop so the code in the loop will be executed at least once. Python does not have a bottom-tested loop but you will encounter it in other programming languages.

Examples

Example 1: for Loop

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

Example 2: while Loop

count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1

Activity

Using the following program, amend the code to:

• change the range of the multiplication table
• iterate over a list of numbers rather than a string
• use a while loop to count from 1 through to 5
• amend the while loop to print all the even numbers between 1 and 100

# Modified example with nested loops and sequence iteration
print("Multiplication table using nested for loops:")
for i in range(1, 4):
    for j in range(1, 4):
        print(i * j, end=' ')
    print()

print("\nIterating over a string with for loop:")
text = "Python"
for char in text:
    print(char)

print("\nCounting down using while loop:")
count = 5
while count > 0:
    print(count)
    count -= 1

Climate Quest Project

Throughout this topic we'll be working on a large scale project: Climate Quest. In this project a player embarks on a journey to combat the effects of climate change by making decisions that impact the environment. Each choice affects the outcome of the game, emphasizing the importance of individual actions in addressing climate change.

Go to task 7 - Iteration

Summary

Cheat sheet for iteration (and selection)¹

Questions

1. What is the primary purpose of iteration in programming?

To execute a block of code once

To execute a block of code repeatedly

To skip a block of code

To define variables

Iteration allows a section of a program to be executed multiple times, enabling repeated actions or calculations.

Submit

2. Which of the following is a key difference between a for loop and a while loop?

A for loop repeats indefinitely, while a while loop does not.

A while loop is used for a fixed number of iterations, while a for loop depends on a condition.

A for loop runs for a set number of times, while a while loop runs as long as a condition is true.

A while loop does not require a condition to run.

A for loop is typically used when the number of iterations is known, while a while loop continues to execute as long as a specified condition remains true.

Submit

3. What will be the output of the following for loop?

for i in range(1, 6):
    print(i)

1 2 3 4 5

1 2 3 4 5 6

0 1 2 3 4 5

0 1 2 3 4

The for loop iterates from 1 to 5, printing each number on a new line. The range function stops before the second argument.

Submit

4. In Python, what does the range() function do when used in a for loop?

It generates a sequence of floating-point numbers.

It generates a sequence of integers.

It creates a list of strings.

It defines the condition for the loop.

The range() function generates a sequence of integers, which is often used to control the number of iterations in a 'for' loop.

Submit

5. What will the following nested for loops output?

for i in range(2):
    for j in range(1, 3):
        print(i, j)

0 1, 0 2, 1 1, 1 2

0 0, 1 1, 2 2

1 2, 1 3, 2 3

1 1, 2 2

The outer loop runs twice (i = 0 and i = 1), and for each iteration of the outer loop, the inner loop runs twice (j = 1 and j = 2).

Submit

6. When is a while loop preferred over a for loop?

When the number of iterations is known.

When the loop should execute at least once.

When the loop's continuation depends on a condition that is not tied to a fixed number of iterations.

When you want to iterate over a sequence like a list or string.

A while loop is preferred when the number of iterations is not known in advance and depends on a condition that may change during the loop's execution.

Submit

7. What is the output of the following code?

count = 1
while count <= 3:
    print(count)
    count += 1

1 2 3

1 2 3 4

2 3 4

0 1 2 3

The while loop increments the count variable from 1 to 3, printing each value before exiting the loop when count exceeds 3.

Submit

8. Which statement about the for loop in Python is correct?

It can only be used with lists.

It cannot iterate over strings.

It always requires a range() function.

It can iterate over any sequence, such as lists, tuples, and strings.

The for loop in Python is versatile and can be used to iterate over any sequence, including lists, tuples, and strings.

Submit

9. Why is it important to update the control variable in a while loop?

To ensure the loop runs indefinitely.

To prevent the loop from running indefinitely.

To increase the speed of the loop.

To make the loop execute fewer times.

Updating the control variable ensures that the condition of the while loop will eventually become false, preventing an infinite loop.

Submit

10. What does the following for loop do?

for ch in 'Python':
    print(ch)

Prints the word 'Python' six times

Prints each character of the string 'Python' on a new line

Prints 'ch' six times

Prints the ASCII values of the characters in 'Python'

The for loop iterates over each character in the string Python, printing each character on a new line.

Submit

Programming Tasks

For each of the following tasks write a program using Python.

Multiplication Table

The text demonstrated how to program a multiplication table using two `for` loops. Rewrite that program, replacing the `for` loops with `while` loops

Read more ...

Factorial Calculator

Write a Python program that uses a for loop to calculate and print the factorial of a given number. Remember, 5! = 5 x 4 x 3 x 2 x 1

Read more ...

Number Guessing Game

Write a Python program that generates a random number between 1 and 100 and asks the user to guess it using a `while` loop. Provide hints (higher or lower) and count the number of guesses until the user guesses correctly. Your output should be:

Read more ...

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1. [https://ehmatthes.github.io/pcc_3e/cheat_sheets/(https://ehmatthes.github.io/pcc_3e/cheat_sheets/)] ↩