3.12. For Loops

for loops can be used to repeat code.

The structure of a for loop is:

for variable in iterable:
    # code you want to execute

Note

An iterable is some sequence of values you can go through. In general, our iterables will be lists, but they can also be strings!

Here is an example:

for number in [1, 5, 8, 2]:
    print(number)

1
5
8
2

What’s happening here?

We can see our iterable is the list [1, 5, 8, 2], and number is our variable name.

At the first iteration, number will store the first value in our list i.e. number = 1. Then we will execute the code on line 2 and print 1.

At the second iteration, number will store the second value in our list i.e. number = 5. Then we will execute the code on line 2 and print 5.

At the third iteration, number will store the third value in our list i.e. number = 8. Then we will execute the code on line 2 and print 8.

At the fourth iteration, number will store the fourth value in our list i.e. number = 2. Then we will execute the code on line 2 and print 2.

After the number 2 there are no more numbers left in the list so the loop terminates.

This is how we can represent this code diagrammatically.

Here is another example, this time with a larger code block:

for number in [1, 5, 8, 2]:
    print(number * 2)
    print('I have doubled that number for you!')
print('All done here!')

2
I have doubled that number for you!
10
I have doubled that number for you!
16
I have doubled that number for you!
4
I have doubled that number for you!
All done here!

Take note of the following:

• for is a keyword

• in is a keyword

• : is placed at the end of the for loop definition

• The code you want repeated must be indented

Question 1

What do you think the output of the following code will be?

for value in [3, 1, 4, -2, 3]:
    print(value + 1)

Solution

In this example the variable value will take on the values 3, 1, 4, -2, 3. The line print(value + 1) is indented, which means that this line will get repeated each time so the program will print 3 + 1, 1 + 1, 4 + 1, -2 + 1, 3 + 1 , which is 4, 2, 5, -1, 4.

Question 2

What do you think the output of the following code will be?

for letter in ['A', 'B', 'C']:
    print(letter)
print("Now I know my ABC's")

Solution

Solution is locked

Question 3

What is wrong with the following program?

For i in [1, 2, 3]:
    print(i)

1. This will result in a NameError because i is not defined.

2. This will result in a SyntaxError because for should be spelt with a lower case ‘f’.

3. This will result in an IndentationError because print(i) should not be indented.

4. This will result in an infinite loop because i is not incremented at the end of the loop.

Solution

Solution is locked

Question 4

Write a program that sums the numbers 1 to 10 and prints the result.

Solution

Solution is locked

Question 5

Implement the algorithm illustrated in the diagram below in Python.

The output of your program should be:

Start your engines...
3
2
1
Go!

Solution

Solution is locked

Code challenge: Cubes

You have been provided with a list of numbers.

numbers = [4, 7, 9, 12, 30, 12, 86, 341, 19, 3, 8, 6529, 7, 0, 345]

Write a program that loops through the list and for each number \(n\), prints out \(n^3\) on a new line.

The output of your program should look like this:

64
343
729
...
343
0
41063625

Hint

The ** operator can be interpreted as ‘to the power of’. For example, 2**3 is \(2^3\).

Solution

Solution is locked

Code challenge: Multiply All

You have been provided with a list of numbers.

numbers = [4, 17, 9, 5, 7, 12, 3, 14, 3, 23, 45, 8, 103, 21, 6, 9, 3, 3, 13]

Write a program that loops through numbers and multiplies all the numbers together and prints out the final result.

Example: This example is with a smaller list of numbers.

numbers = [2, 3, 6]

The above list would produce the output:

36

Hint

It might be helpful to create the variable result = 1, and then multiply result by each number.

Solution

Solution is locked