for Loops

This lesson introduces loops in Python, using for and while loops to iterate over sequences so you can automate repetitive tasks and apply the same code to many values.

Keywords

For loops, While loops, Iterate values

Approximate time: XX minutes

Learning Objectives

In this lesson, we will:

• Create for loops to iterate over sequences of values
• Learn how to use for loops with different data structures
• Understand the difference between for and while loops

Overview of lesson

One of the most common problems that programming solves is the tedium of performing the same task/calculation repeatedly. Instead of writing the same code over and over again, we can use loops to automate this process. This is essential in real analyses, where you might need to process hundreds of files or thousands of values consistently and without human error. In this lesson, we will learn about two types of loops: for loops and while loops.

for Loops

for loops are used to iterate over a collection of values, most commonly a list. An example of when this may be useful would be if we wanted to print out the names of all the species in a list. With the current skills we have learned, we could accomplish this task by writing the following code:

species = ["human", "mouse", "corn", "yeast"] 

print(species[0])
print(species[1])
print(species[2])
print(species[3])

However, this is not very efficient, especially if we had a long list of species or we were unsure how many elements were in the list. Instead, we can use a for loop to accomplish the same task in a less repetitive manner.

To create a for loop, we need to specify a sequence to loop over and the variable that will take on the value of each element. We will then iterate through the sequence, assigning the variable to each element in the sequence in turn, and execute the code block within the loop for each element. The general syntax of a for loop in Python is as follows:

for variable in sequence:
    # code to be executed

Creating a for Loop

And so if we wanted to print out the names of all the species in a list, we could use the following code:

species = ["human", "mouse", "corn", "yeast"] 

for s in species:
    print(s)

human
mouse
corn
yeast

We now have this value s. This is a variable that takes on the value of each element in the species list as we iterate through it. So, in the first iteration of the loop, s will be equal to "human", in the second iteration, s will be equal to "mouse", and so on.

We do not have to assign the sequence to a variable before we use them. For example, we could iterate over a list of numbers without doing any assignment first and run calculations on those numbers as follows:

for number in [2, 3, 5]:
  print("number", number)
  print("squared", number ** 2)

number 2
squared 4
number 3
squared 9
number 5
squared 25

Indentation Matters

As is the case with if statements, the code that we want to be executed within the for loop must be indented. If we forget to indent the code, we will get an error. For example, the following print() statement will throw an error because it is not indented properly.

for s in species:
print(s)

expected an indented block (<string>, line 2)

This is to once again emphasize that indentation is not just a matter of style in Python, but it is a fundamental part of the syntax. The indented code block is what gets executed as part of the loop, and if we forget to indent, Python does not know what to do.

Conditional Statements within for Loops

Taking what we learned in the lesson on conditional statements, we can also use if statements within our for loops to perform different actions based on certain conditions. This can be useful if we want to perform a specific action for certain elements in our list. For example, if we wanted to print out a different message for humans compared to the other species, we could use the following code:

for s in species:
    if s == "human":
        print("This is a human.")
    else:
        print("This is not a human.")

This is a human.
This is not a human.
This is not a human.
This is not a human.

Warning

Did not write exercises for this section yet. Will add in later.

Exercise 1

1. A question to evaluate Learning Objective 1
2. A followup question to question #1
3. …

range() Function

A common use of for loops is to iterate over a sequence of numbers. We can use the range() function to generate a sequence of numbers that we can iterate over. The range() function takes three arguments: start, stop, and step. The start argument is the number that the sequence will start from, the stop argument is the number that the sequence will stop at (but not include), and the step argument is the amount by which the sequence will increment.

For example, if we wanted to iterate over the indices of species, we could use the range() function as follows:

for i in range(0, len(species)):
    print(i)

0
1
2
3

Complex for Loop Example

We have been using the species list as an example to demonstrate how for loops work, but we can use for loops to perform more complex tasks as well. For example, we can use a for loop to perform calculations on a list of numbers. If we wanted to calculate the total of the doubled values of those numbers. We can accomplish this task using a for loop as follows:

numbers = [2, 5, 8, 10]

total = 0  # this will hold the sum of doubled numbers

for number in numbers:
    doubled = number * 2
    total = total + doubled   # add the doubled value to total

print("Total of doubled numbers:", total)

Total of doubled numbers: 50

We can follow along with the logic through each iteration fo the for loop to see how each of the variables are being updated at each step.

Table 1: Table of updated values of number, doubled, and total at each iteration of the loop.

Index	number	doubled	total
0	2	4	4
1	5	10	14
2	8	16	30
3	10	20	50

Exercise 2

1. A question to evaluate Learning Objective 2
2. A followup question to question #1
3. …

Warning

Add a section on list comprehensions here.

while Loops

while loops are similar to for loops in that they continuously execute a block of code as long as a certain condition is true. The general syntax of a while loop in Python is as follows:

while condition:
    # code to execute
    # update condition

The important distinction here is that we must updated our condition within the loop in order to keep iterating through our loop. We do not have a tangible end to the while unlike the for loop, where we stop iterating once we make it to the end of the sequence supplied in the statement.

Creating a while Loop

while loops are typically used when we want to repeat a block of code an unknown number of times, or when we want to repeat a block of code until a certain condition is met. For example, if we wanted to keep doubling a number until it is greater than 100, we could use a while loop as follows:

number = 1  # this will hold the current value of the number 
while number <= 100:
    print(number)
    number = number * 2  # update the value of number by doubling it

1
2
4
8
16
32
64

print("Last number:", number)

Last number: 128

Why is our last number 128 and not 64? This is because the condition number <= 100 is still true when number is equal to 64, so we execute the code block one more time, which doubles number to 128. After that, the condition number <= 100 is no longer true, so we exit the loop and print the last number.

Infinite Loop

The important thing to note about while loops is that they can potentially run indefinitely if the condition never becomes false. Therefore, it is important to make sure that the condition will eventually become false, otherwise we will have an infinite loop. For example, the following code will run indefinitely because the condition i < 5 will always be True:

i = 0
while i < 5:
    print(i)

Be sure to hit esc or ctrl+c to stop the loop if you accidentally run an infinite loop!!

Exercise 3

1. A question to evaluate Learning Objective 3
2. A followup question to question #1
3. …

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