learn-co-students/dsc-while-loops-break-and-continue-houston-ds-082619

While Loops, Break and Continue

Introduction

Earlier in the course, we learned how to iterate over collections. But is there a way to have a loop without a collection to iterate over? Well, another way to create a loop is with while loops. We can use a while loop to perform the same action over and over until a condition is no longer True. We don't even need an iterable or collection to iterate over. We can just define a condition and perform the given code block until the condition is no longer True. Pretty cool, right?

Or what if we would like to have a loop that stops at a certain point? Let's say we only want to collect half of the elements of a list, or stop a list once we find the first matching element? To perform operations like these we'll need break and continue statements. These statements control the flow of our loops and will help us make our loops even more effective.

Objectives

You will be able to:

• Use a while loop
• Use break and continue to add control flow to a while loop

What is a while loop and how does it work?

A while loop is just that; a loop! Similar to a for loop, except there is no need for a collection to iterate over. Instead, a while loop uses a condition to know when to stop executing. When the condition is true, the block inside the while loop is executed. When that condition is false, we exit the while loop and move on to the next piece of our code.

Let's look at an example:

stop_number = 4
while stop_number > 0:
    print(stop_number)
    stop_number -=1
print("The stop_number reached", stop_number, "so the while loop's condition became False and stopped execution")

4
3
2
1
The stop_number reached 0 so the while loop's condition became False and stopped execution

Note the lack of a list or other collection, and that our second print statement only printed after our stop_number become 0.

Also, notice that the structure of a while loop is such that it could execute for an unknown amount of times. For example, if we didn't know the stop_number because it changed from time to time, our while loop could execute 100 times or 3.

For example, if we used a random number using the random library from numpy:

import numpy as np
random_num = np.random.randint(1,20)
while random_num > 0:
    random_num -= 1
    print(random_num)

8
7
6
5
4
3
2
1
0

However, we know that eventually that number will be less than 0 and the loop will eventually stop. This is of critical importance. A while loop must always have a condition that will stop the loop, otherwise we will have an infinite loop. Infinite loops can crash your browser or program if you don't have a way to end it. So, it is very important to make sure your loops have a fairly defined end case.

If you do ever accidentally create an infinite loop, don't worry. Your current Notebook might freeze, and then kill the page to stop the execution. You can then re-open the browser again normally.

When To Use While Loops

While loops are fairly straight forward. We use them in instances where we have a condition that serves as the point at which we want a process to stop. For example, if we think about our appetite, we should eat until we aren't hungry, right? Some days that might be two slices of pizza, some days that might be 5 slices of pizza (and that is assuming all pizza slices are of equal size, which is a generous assumption).

In keeping with our food theme, let's see how we can make sure we're drinking enough water during the day using a while loop:

hydration = 0
water = 1 # in gallons
while hydration < 100 and water > 0:
    print('----[sips water]-----')
    water -= .1
    print('ah, that was refreshing')
    hydration += 10
    print('hydration is now at', hydration, '%\n')

----[sips water]-----
ah, that was refreshing
hydration is now at 10 %

----[sips water]-----
ah, that was refreshing
hydration is now at 20 %

----[sips water]-----
ah, that was refreshing
hydration is now at 30 %

----[sips water]-----
ah, that was refreshing
hydration is now at 40 %

----[sips water]-----
ah, that was refreshing
hydration is now at 50 %

----[sips water]-----
ah, that was refreshing
hydration is now at 60 %

----[sips water]-----
ah, that was refreshing
hydration is now at 70 %

----[sips water]-----
ah, that was refreshing
hydration is now at 80 %

----[sips water]-----
ah, that was refreshing
hydration is now at 90 %

----[sips water]-----
ah, that was refreshing
hydration is now at 100 %

On To break And continue Statements

In the case of break and continue statements, it is almost best to not overthink. break and continue essentially do what they sound like. They are used in tandem with conditional statements (if, elif), inside loops, and they break out of a loop if a condition is met or continue a loop if a different condition is met. Before we dive too deeply into how these statements function, let's look at an example.

numbers = list(range(0, 30))
new_list = []
for num in numbers:
    if len(new_list) > 4:
        print(f'We have enough even numbers in new_list ({len(new_list)}). break will stop the for loop now')
        break
    elif num % 2 == 0:
        new_list.append(num)
    elif num % 2 != 0:
        continue
        print('i never get executed')
    print(num, 'is even.')
    print('this does not print for odd numbers\nbecause the continue statement skips\nthe code that follows in the for loop\nand goes straight back to the next element in the for loop')
    

0 is even.
this does not print for odd numbers
because the continue statement skips
the code that follows in the for loop
and goes straight back to the next element in the for loop
2 is even.
this does not print for odd numbers
because the continue statement skips
the code that follows in the for loop
and goes straight back to the next element in the for loop
4 is even.
this does not print for odd numbers
because the continue statement skips
the code that follows in the for loop
and goes straight back to the next element in the for loop
6 is even.
this does not print for odd numbers
because the continue statement skips
the code that follows in the for loop
and goes straight back to the next element in the for loop
8 is even.
this does not print for odd numbers
because the continue statement skips
the code that follows in the for loop
and goes straight back to the next element in the for loop
We have enough even numbers in new_list (5). break will stop the for loop now

So, let's unpack what's happening here.

First, we have a for loop that is iterating over our list, numbers, which has 30 numbers in it. Then we see that we have an if statement that only executes its code when our new_list has more than 4 numbers in it. Once this condition is met, we break out of our loop. We then have an elif statement that adds any even number to the new_list, and a final elif statement that tests to see if the number is odd, and if it is odd, then it continues to the next element in our for loop's iteration process.

So, first let's dig into the continue statement. continue is telling our loop to skip any code that comes after it and go straight to the next element in our loop. So, if we are dealing with number 1 in our list of numbers, we will hit our continue statement and immediately go to the next element in our for loop (i.e. 2). Any code that comes after the continue will not be executed, but our for loop does not end execution. This contrasts with the break statement. Our break statement is only executed when we have met our condition that the new_list has reached the number of elements we want it to have. Once the break statement is executed, it will end the execution of the for loop altogether. All code after the break statement will be ignored, similar to our continue, but there will be no next step to the iteration. It stops the loop and that's the end.

So, essentially, we can use continue to skip operation on elements and code below continue. We can then use break when we have a condition that tells us we want to stop the process altogether.

We can look at two examples to really reinforce this difference in execution.

for i in list(range(0, 5)):
    if True:
        print(i)
    print("Since we don't have a continue statement,\nI'll always get executed")

0
Since we don't have a continue statement,
I'll always get executed
1
Since we don't have a continue statement,
I'll always get executed
2
Since we don't have a continue statement,
I'll always get executed
3
Since we don't have a continue statement,
I'll always get executed
4
Since we don't have a continue statement,
I'll always get executed

for i in list(range(0, 5)):
    if True:
        print(i)
        continue
    print("I'll never get executed")

0
1
2
3
4

for i in list(range(0, 5)):
    if True:
        print(i)
        break
    print("I'll never get executed either")

0

These examples are a bit contrived but they very clearly show how continue and break work inside loops and conditional statements as well as the difference between their execution.

Identify Opportunities to Use Break and Continue

Let's say you have a collection of elements and that you want to filter out certain elements and create a new list with the elements you want. But you also want to perform a complex operation on each of the elements you do want. Well, you don't want to perform that operation on the elements that you don't want and you don't want to have to write multiple for loops on the collection. So, a continue statement would allow you to optimize your performance and avoid needing to perform those operations. And a break statement can be used when you want to put a limit on the number of iterations you perform, the number of elements you append to a new list, or even just stop your iteration when you have found the first element you want. Let's take a look:

names = ['aNNE', 'JaNe', 'willIAM', 'WanDA', 'WeSt', 'HELEN', 'tHoMaS', 'HENrY', 'John', 'Marshall', 'May']
formatted_names = []
check_count = 0

for name in names:
    if name.startswith('w') or name.startswith('W'):
        check_count += 1
        continue
    elif len(formatted_names) >= 4:
        check_count += 1
        break
    else:
        formatted_names.append(name.title())

print('before', names)
print('after', formatted_names)
print(check_count)

before ['aNNE', 'JaNe', 'willIAM', 'WanDA', 'WeSt', 'HELEN', 'tHoMaS', 'HENrY', 'John', 'Marshall', 'May']
after ['Anne', 'Jane', 'Helen', 'Thomas']
4

Okay, so, as we can tell from our code. We wanted to create a list of 4 names that are properly formatted and that don't start with the letter w. To optimize our code, we first check to see if the name starts with w. If it does, we skip all other operations that we need to do and go to the next name. Next we check to see if we have hit our quota of 4 names. If we have, then stop the iteration altogether. Lastly, if neither condition is met, format the given name and append it to our new list of formatted names. This way, we are optimizing our code by making sure that we eliminate performing any operations that aren't absolutely necessary at each step. You may have noticed the variable, check_count. If you are feeling unconvinced that the break and continue are not cutting down on the number of times our code is executing, run the cell below and compare the check_counts, which increment after each check of a conditional statement.

names = ['aNNE', 'JaNe', 'willIAM', 'WanDA', 'WeSt', 'HELEN', 'tHoMaS', 'HENrY', 'John', 'Marshall', 'May']
formatted_names = []
check_count = 0

for name in names:
    if len(formatted_names) < 4:
        check_count += 1
        if not (name.startswith('w') or name.startswith('W')):
            check_count += 1
            formatted_names.append(name.title())
            

print('before', names)
print('after', formatted_names)        
print(check_count)

before ['aNNE', 'JaNe', 'willIAM', 'WanDA', 'WeSt', 'HELEN', 'tHoMaS', 'HENrY', 'John', 'Marshall', 'May']
after ['Anne', 'Jane', 'Helen', 'Thomas']
11

See that?! We have cut down on the different checks we perform by more than half (50%)! And on top of that, to get the same result we have to have a nested if statement, which is, technically put, gross. All joking aside, nested if statements should be avoided if they can be since they make our code less readable and therefore harder to maintain on top of being half as efficient as our previous code.

It's important to note that these excess checks could represent much more expensive operations in our code. So, it is important to use break and continue when it makes sense.

Summary

Awesome! while loops are great, right? We can use them to perform operations based on the truthiness of a condition instead of needing to iterate over the elements in a collection. They provide a more dynamic way to perform operations, but we need to be careful when writing while loops because we need to have a condition that ends the loop. Otherwise we will have an infinite loop which will give us and our computer a real headache. In this lesson, we also introduced some control flow statements, break and continue, which allow us to make our conditional statements and the rest of our code more efficient and more readable. We call these statements control flow statements because they allow us to control the flow of our code's execution.