Repeating actions with loops

Repeating actions with loops#

Objectives

  • Explain what a for loop does.

  • Correctly write for loops to repeat simple calculations.

  • Trace changes to a loop variable as the loop runs.

  • Trace changes to other variables as they are updated by a for loop.

Questions

  • How can I do the same operations on many different values?

Repeating Actions with Loops#

So far, we’ve learned to access and manipulate individual elements of a list. But what if we want to perform the same operation on many different elements in the list (e.g. print them all out)? Or aggregate them somehow (e.g. calculate the total sum of all traffic)? This is where loops come in.

Let’s say we have the following list of morning traffic counts, from 7am to 11am:

morning_traffic = [500, 600, 550, 450, 350]

As we saw in the previous episode, we can access an element of a list using its index. For example, we can get the first number in the list morning_traffic, by using morning_traffic[0]. So one way to print each number is to use 5 print statements:

morning_traffic = [500, 600, 550, 450, 350]
print(morning_traffic[0])
print(morning_traffic[1])
print(morning_traffic[2])
print(morning_traffic[3])
print(morning_traffic[4])

500
600
550
450
350

This is a bad approach for three reasons:

  1. Not scalable. Imagine you need to print a list that has hundreds of elements. It might be easier to type them in manually, but it’s tedious and error-prone.

  2. Difficult to maintain. If we want to annotate each printed element somehow (e.g. add a unit), we would have to change multiple lines of code. While this might not be a problem for very small lists, it would definitely be a problem for longer ones.

  3. Fragile. If we use it with a list that has more elements than what we initially envisioned, it will only display part of the list’s elements. A shorter list, on the other hand, will cause an error because it will be trying to display elements of the list that do not exist.

Here’s a better approach: a for loop

for cars in morning_traffic:
    print(cars)

500
600
550
450
350

This is shorter, and more robust as well. For example, if we change the list by adding a new number at the end, we don’t need to change the loop at all:

morning_traffic.append(200)

for cars in morning_traffic:
    print(cars)

500
600
550
450
350
200

The general form of a loop is:

for variable in collection:
    # do something using variable (e.g. print it)

The first line of the loop (the one that starts with for) must end with a colon. If we forget this, we’ll get an error:

for cars in morning_traffic
    print(cars)

  Cell In[4], line 1
    for cars in morning_traffic
                               ^
SyntaxError: expected ':'

The things we want to do inside the loop are called the “loop body”, and must be indented. If we don’t indent, we’ll also get an error:

for cars in morning_traffic:
print(cars)

  Cell In[4], line 2
    print(cars)
    ^
IndentationError: expected an indented block after 'for' statement on line 1

We can call the loop variable anything we like.

In the example above, the loop variable was given the name cars, since the value it holds is the number of cars counted in a given hour. We can choose any name we want for variables. We might just as easily have chosen the name number for the loop variable, as long as we use the same name when we invoke the variable inside the loop:

morning_traffic = [500, 600, 550, 450, 350]
for number in morning_traffic:
    print(number)

500
600
550
450
350

It is a good idea to choose variable names that are meaningful, otherwise it would be more difficult to understand what the loop is doing.

Challenge: Printing with labels

Given the list:

morning_traffic = [500, 600, 550, 450, 350]

Write a loop that prints each number with “traffic: “ before it. The output should be:

traffic: 500
traffic: 600
traffic: 550
traffic: 450
traffic: 350
Solution
for cars in morning_traffic:
    print('traffic: ', cars)

More complicated operations in loops#

The operation inside the loop can be whatever we want. So far, we’ve only printed the values in the list, but loops can be used to perform more sophisticated calculations.

For example, what if we want to know how many hours of traffic we have recorded? Each entry in the list corresponds to one hour, so we to do this we can count the number of entries in the list.

morning_traffic = [500, 600, 550, 450, 350]
entries = 0

for cars in morning_traffic:
    entries = entries + 1
print('Morning traffic hours:', entries)

Morning traffic hours: 5

It’s worth tracing the execution of this little program step by step:

  • Since there are 5 items in morning_traffic, the loop will be executed 5 times.

  • The first time around, entries is zero (the value assigned to it at the start) and cars is 500. The statement entries = entries + 1 adds 1 to the old value of entries, producing 1, and updates entries to refer to that new value.

  • The next time around, cars is 600 and entries is 1, so entries is updated to be 2.

  • The next time, entries is 2 and cars is 550, so entries is updated to be 3.

  • This process repeats for each entry in the list, until the loop has gone through all 5 items.

  • After the loop has finished, entries is 5, so the program prints “Morning traffic hours: 5”.

Note that finding the length of an object is such a common operation that Python actually has a built-in function to do it called len:

print(len(morning_traffic))

5

Using len is faster and much more convenient than writing our own loop.

Looping over other things#

We can loop over any collection of several values, not just a list. For example, a string is a collection of it’s individual characters. So we can loop over the letters in a string:

time_of_day = 'morning'
for letter in time_of_day:
    print(letter)

m
o
r
n
i
n
g

Challenge: Summing a List

Given the list:

morning_traffic = [500, 600, 550, 450, 350]

Write a loop that adds up all the entries in morning_traffic to calculate the total traffic, and then prints the result. The output should be:

Total traffic: 2450
Solution
total = 0
for cars in morning_traffic:
    total = total + cars
print('Total traffic:', total)

Just like there’s a built-in len function to count the number of elements, Python also has a built-in sum function to quickly calculate the sum of a list:

print('Total traffic:', sum(morning_traffic))

Challenge: From 1 to N

Python has a built-in function called range that generates a sequence of numbers. range can accept 1, 2, or 3 parameters.

  • If one parameter is given, range generates a sequence of that length, starting at zero and incrementing by 1. For example, range(3) produces the numbers 0, 1, 2.

  • If two parameters are given, range starts at the first and ends just before the second, incrementing by one. For example, range(2, 5) produces 2, 3, 4.

Using range, write a loop that prints numbers from 4 to 7. The output should be:

4
5
6
7
Solution
for number in range(4, 8):
    print(number)

Now write a loop that prints the indices of the list morning_traffic. The output should be:

0
1
2
3
4

Hint: Remember you can use len(morning_traffic) to get the length of the list.

Solution
for i in range(len(morning_traffic)):
    print(i)

Challenge: Dynamic Labels

Given the list:

morning_traffic = [500, 600, 550, 450, 350]

This list contains traffic counts for 7am, 8am, 9am, 10am, and 11am (in that order). Write a loop that prints each traffic count with a label showing which hour it’s for. The output should be:

traffic at 7 am: 500
traffic at 8 am: 600
traffic at 9 am: 550
traffic at 10 am: 450
traffic at 11 am: 350

Hint: You’ll need to use range(len(morning_traffic)) to get the index, then use that index to both access the traffic count and calculate which hour it represents (remember: we started at 7am, so index 0 is 7am, index 1 is 8am, etc.).

Solution
morning_traffic = [500, 600, 550, 450, 350]
for i in range(len(morning_traffic)):
    hour = 7 + i
    print('traffic at', hour, 'am:', morning_traffic[i])
Contents