Loop patterns

You’ve learned and practiced some of the core actions of working with lists. You can create a list, add and remove items, and perform actions for all of the items. Now it’s time to learn to write code that uses lists and loops to solve problems.

Using lists and loops together, you can answer questions about data.

• What’s the smallest (or largest) item in this list?
• What’s the sum of all the items in the list? What’s the average?
• What are the items in the list that fit this condition?

When solving more complicated problems with lists, it’s also helpful to have more powerful debugging techniques. In this lesson, you’ll learn problem solving with loops, including common patterns and strategies for figuring things out.

Video: Lists and Loops

Looping through a list with for

When you first saw the for loop, you learned that it could loop through a list, or through a range. You’ve been using this the whole time, but as a quick refresher, here’s a loop that will print the numbers from 5 down to 1:

for number in [5,4,3,2,1]:
	print(number)

And here’s another version, using range:

for number in range(5,0,-1): # start at 5, go down to (but not including) 0, by -1 each time
	print(number)

We’ll be using for loops for almost all of the loop and list problems here. A while loop can work in some circumstances, but it’s usually a little more work.

Sum of numbers in a list

Here’s a python program to add up all the numbers between 1 and 10:

total = 0
for number in [1,2,3,4,5,6,7,8,9,10]:
	total += number

print(total) # 55

Here’s what this program does:

• Step 1: Creates a variable called total, which starts at 0
• Step 2: Loop through the numbers 1 through 10. Increase total by the number each time.
• Step 3: After the loop, print out the total

Solving problems using loops

Many loop problems can be solved using the pattern in this example: update the variable every time you go through the loop. In this lesson, you’ll see lots of different problems you can solve with this pattern.

🔑 Common pattern

Step 1: Create a variable before the loop starts and set the initial value.

Step 2: Perform some computation on each item in the loop body, which may change the value of the variable.

Step 3: Use the value of the variable when the loop completes.

Let’s try it out.

Practice

total = 0
# We want to include 100, so we stop at 101
# We want even numbers, so step by 2
for i in range(10,101,2):
  total+=i
print(total)