# Python 3: Loops

March 26, 2019

In this tutorial, you’ll learn about loops —a very important concept in programming. In plain English, loops correspond to “Do this action X times.” or “Keep doing that action as long as this condition is true.”

Let’s get started!

# Loops

Programmers don’t like to repeat themselves. Programming is all about automating things. That’s where loops come in handy.

people = ['Rachel', 'Monica', 'Phoebe', 'Ola', 'You']

We want to greet all of them by their name. But we don’t want to greet every person manually, right?

Let’s look at how we can do this:

# For loop

To go over the list of people we can use a for statement:

for name in people:

Here, name is a variable that temporarily stores the current value of the item in the people list as we go over all of them one by one. This is also referred to as iterating over a list.

The for statement behaves similarly to the if statement; code below both of these need to be indented four spaces.

Here is the full code to greet everyone:

people = ['Rachel', 'Monica', 'Phoebe', 'Ola', 'You']
for name in people:
print('Hi ' + name + '!')
print('Next person')

And when we run it:

Hi Rachel!
Next person
Hi Monica!
Next person
Hi Phoebe!
Next person
Hi Ola!
Next person
Hi You!
Next person

As you can see, everything you put inside a for statement with an indent will be repeated for every element of the list people.

Let’s look at another application of the for...in loop we just looked at, and apply it to something you may find yourself doing often. Suppose you find the sum of all the elements in a list. This is how we can go about it:

lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
sum_so_far = 0
for element in lst:
sum_so_far = sum_so_far + element
print(sum_so_far)

Output:

55

Awesome! Let’s fully understand what’s going on in the code above:

• On line 1, we define our list lst with the numbers we want to sum up.
• Next, on line 2, we initialize a variable sum_so_far with 0. This variable stores the sum after the current element in the list has been added.
• Line 3 starts the for ... in loop as usual. Here element is a variable that temporary stores the current value in the lst list as we iterate over it.
• Line 4 is by far the most important line. Here, we update sum_so_far to increase by the value of the current element in the list.

Here’s what the value of sum_so_far looks like over the 10 iterations:

End of iteration #1: element = 1, sum_so_far = 1
End of iteration #2: element = 2, sum_so_far = 3
End of iteration #3: element = 3, sum_so_far = 6
End of iteration #4: element = 4, sum_so_far = 10
End of iteration #5: element = 5, sum_so_far = 15
End of iteration #6: element = 6, sum_so_far = 21
End of iteration #7: element = 7, sum_so_far = 28
End of iteration #8: element = 8, sum_so_far = 36
End of iteration #9: element = 9, sum_so_far = 45
End of iteration #10: element = 10, sum_so_far = 55
• On line 5, once the for-loop completes and we have added all the elements, we print out the final value of sum_so_far.

Great, you can now sum up all the numbers in a list! Ready for a quiz?

## For loop with the range() function

You can also use for with the range function:

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

Which would print:

1
2
3
4
5

range is a function that creates a list of numbers. It works as follows:

• range(n) function returns the numbers 0, 1, … n-1
• range(a, b) returns a, a+1, … b-1 — up to but not including the last number.

Note that the second of these two numbers is not included in the list that is output by Python (meaning range(1, 6) counts from 1 to 5, but does not include the number 6). That is because “range” is half-open, and by that we mean it includes the first value, but not the last.

# While loop

Python also has another loop called the while loop. The while loop runs as long as the condition inside the while is True.

In that sense, a while loop is almost like an if-statement, except the code doesn’t run just once, but keeps executing again and again as long as the statement is True.

You may be wondering why we need another loop at all when we already have a for loop. For loops are great when we know the number of times we want to run the loop in advance — they are great for going over lists, for going over a range of numbers, and so on. On the other hand, a while loop is just perfect when we don’t know the number of executions in advance, but all we know is that we need to do something as long as something is True. It may have to run once, 10 times, 100 times, or maybe, not even once!

For example, suppose we want to write a program that prints all the power of 2 less than 100. We don’t know at the outset how many lines we’re going to print. This is a perfect opportunity to use a while loop:

current_value = 2
while current_value < 100:
print(current_value)
current_value *= 2

The output looks like this:

2
4
8
16
32
64

Wow! We just used a while loop effectively — we didn’t know how many times we’ll need to run the loop. All we knew was the termination condition.

Note that in a theoretical sense, for and while-loops are equivalent. What we mean by that is, every for-loop can be re-written as a while-loop, and every while loop can be re-written as a for-loop.

For example, let’s take our people list from above and write the greeting program using a while-loop:

people = ['Rachel', 'Monica', 'Phoebe', 'Ola', 'You']
i = 0
while i < len(people):
print('Hi ' + people[i] + '!')
print('Next person')
i = i + 1

Notice a number of important things that we had to take care of while writing the while loop:

1. On line 2, we had to declare a new variable i which stores the current index. This index, i, not only lets us access the elements one by one, but we also use its value to check if the while-loop should stop running.
2. On line 3, notice the condition for the while loop to keep running i < len(people). Note that we used < and not <= since the index of the last element in the list 'You' is 4 (indexes start with 0, remember?). 4 is one less than the number of elements in the list (which is 5). So, when i is 5, we should not enter the while-loop at all, which is why we used < and not <=.
3. On lines 4 and 5, we print what we want.
4. On line 6, we do something very important — we actually update the value of i and increase it by 1. It’s very important to get this right. If we forgot to write this statement, our while-condition will always stay True, and the loop will keep running forever! This is called an infinite loop — and is obviously something you don’t want in your program.

If it seems like your while-loop is not terminating, and is just repeating itself over and over, you should check your while-condition, and make sure you’re updating the variables inside the loop-body so that the condition actually becomes False when you’re done.

But most of the time, you will use a for loop when you have a list / range. On the other hand, you will use a while loop when you want to stop based on a condition.

# Break and continue

break and continue give us some more control over what code is executed inside a loop.

If a break is encountered, Python immediate stops iterating over the loop (it also ignores all the code after the break within the loop body, if any). Here’s an example:

a = [1, 5, 3, 2, 4, 6, ]
for element in a:
print(element)
if element == 2:
break              # quit the loop when element == 2
print(-element)        # when element == 2, this line too won't execute 

Here’s the code output:

1
-1
5
-5
3
-3
2

Note: 4 and 6 don’t get printed at all. 2 gets printed, but -2 does not, since break happens before that.

If a continue is encountered, Python ignores the code after the continue within the block, but “continues” iterating over the rest of the elements. Here’s an example:

a = [1, 5, 3, 2, 4, 6, ]
for element in a:
print(element)
if element <= 2:
continue           # don't execute the rest if element <= 2
print(-element)        # when element <= 2, this line too won't execute 

Here’s the code output:

1
5
-5
3
-3
2
4
-4
6
-6

Note: All numbers get printed. However, for 1 and 2, the negative values don’t get printed because of the continue.

Lastly, break and continue can be used inside while-loops as well.

# Summary

Awesome! In this tutorial, you learned about:

• for loops — help us iterate over our lists.
• the range() function — helps us specify a list of numbers.
• while loops — help us keep a code block running as long as a condition is True
• break and continue — give us more control over our for and while loops

Based on content from https://tutorial.djangogirls.org/en/python_introduction/