Loops (continued)#

Goals of this lecture#

  • Loops, revisited.

  • while loops in Python.

Loops, revisited#

A loop is a way to repeat the same piece of code multiple times.

Last lecture, we discussed for loops, in which the same code is run for each element of a sequence.

  • for element in list or str…

  • …do “X” (where “X” = the code you want to run).

for loops in action#

A for loop can be run with any sequence.

Here, we see a for loop used with a list.

simple_list = [1, 2, 3]
for item in simple_list:
    print(item)

1
2
3

Here, we see a for loop used with a str.

simple_str = 'css'
for ch in simple_str:
    print(ch)

c
s
s

Check-in#

Write a for loop that iterates through a range(10), printing each element along the way, but stops once it reaches 5.

### Your code here

Other important features#

  • for loops can be combined with conditional statements.

  • for loops can be nested.

  • Keywords like continue and break can give further control over for loops.

while loops#

A while loop is a procedure to repeat the same piece of code while some condition is met.

For example:

  • Add numbers to a shopping_bill variable while shopping_bill < 50.

  • Increase a temperature variable while temperature < 85.

  • while some condition is met, continue running a simulation.

Check-in#

How are for and while loops similar? How are they different?

while loops in action#

A while loop is created using the while keyword, following by a condition. As long as this condition is met, the while loop will continue!

In the code below:

  • The start variable begins at 0.

  • We then declare a while loop, which will run as long as start < 2.

  • Then, the start variable is incremented by 1 with each iteration, guaranteeing that eventually we’ll reach the condition where start >= 2 (thus “breaking” the loop).

start = 0
while start < 2: ### Conditional statement
    print(start)
    start += 1

0
1

Iterating through a list#

while loops are often used to iterate through a list.

To do this, we use an index variable, which simply keeps track of “where” in the list we are.

  • Recall that we can index into a list using the syntax list_name[0].

  • We can also retrieve the length of that list using len(list_name).

numbers = [1, 2, 3] ## List to iterate through
index = 0 ## Start index at 0
while index < len(numbers):
    print("Index: {i}. Number: {n}.".format(i = index, n = numbers[index]))
    index += 1

Index: 0. Number: 1.
Index: 1. Number: 2.
Index: 2. Number: 3.

Check-in#

You want to keep your grocery costs low, so you decide not to buy anything above 9. How would you write a while loop that:

  • Iterates through costs.

  • Tracks a final_bill variable.

  • Only adds items to final_bill if they’re below 9$?

Hints:

  • You can retrieve the length of a list using len(list_name).

  • If you’re using an index, remember to increment it so you don’t get stuck in a loop.

costs = [5, 8, 4, 10, 15]
#### YOUR CODE HERE

Solution#

index = 0
final_bill = 0
while index < len(costs):
    if costs[index] < 9:
        final_bill += costs[index]
    index += 1
print(final_bill)

17

Stuck in a loop?#

A common issue that programmers encounter is getting “stuck” in an infinite while loop. This happens because they haven’t ensured that the condition will eventually evaluate as False.

  • This is surprisingly easy to do, even as an experienced programmer.

  • For this reason, I typically prefer to use a for loop rather than a while loop, unless I absolutely have to.

If you do find yourself stuck, you can “cancel” the loop manually:

  • Pressing the Stop button in the Jupyter toolbar.

  • Pressing Command + C in the Terminal.

Check-in#

What will the final value of room_temperature be if the following while loop is run? What about the final value of body_temperature?

room_temperature = 40
body_temperature = 92
while room_temperature < 70:
    room_temperature += 1
    body_temperature += .2

Check-in#

Write a while loop to count the number of vowels in a string. The code block below starts with a list of vowels alreay, which you can use to cross-reference when iterating through a string.

Hint: If you’re feeling extra ambitious, you might think about how to handle upper-case vowels.

vowels = ['a', 'e', 'i', 'o', 'u']
example_string = "CSS is great"
### Your code here

Solution#

### Practice 1
vowels = ['a', 'e', 'i', 'o', 'u']
example_string = "CSS is great"
num_vowels = 0
index = 0
while index < len(example_string):
    if example_string[index].lower() in vowels:
        num_vowels += 1
    index += 1
print(str(num_vowels) + " vowels in '{x}'".format(x = example_string))

3 vowels in 'CSS is great'

Conclusion#

This concludes our unit on loops! We’ll continue using both for and while loops throughout the course.

Key takeaways:

  • A loop can be used to repeat the same piece of code many times.

  • A for loop iterates through a sequence and does the same thing for each item in that sequence.

  • A while loop runs the code code as long as some condition is met.

    • while loops sometimes get “stuck”, if you’re not careful about ensuring this condition will eventually evaluate to False.