Looping over Lists

We have a list of names. We want to greet each one of them. We could do

names = ["Tommy", "Lance", "Cortez"]
print(f"Hello, {names[0]}!")
print(f"Hello, {names[1]}!")
print(f"Hello, {names[2]}!")

Once again, we repeat code. What if the list has 100 names? We won't write 100 prints. We use a for loop. Technically you could also use a while loop, but a for is more suitable here

names = ["Tommy", "Lance", "Cortez"]
for i in names:
    print(f"Hello, {i}!")

Output

Hello, Tommy!
Hello, Lance!
Hello, Cortez!

What does this mean? We tell Python like this: we use i to go through the list names. The first time, i will be Tommy, then i will be Lance and so on. The behavior is similar to for i in range(), except this time we go through a list

We can also use range() with len() if we want the index as well as the value

names = ["Tommy", "Lance", "Cortez"]
for i in range(0, len(names)):
    print(f"Hero number {i + 1}: {names[i]}")

Output

Hero number 1: Tommy
Hero number 2: Lance
Hero number 3: Cortez

Why i + 1? Because we count from 0 in code, but humans usually start from 1 when they say "first, second, third". So we add 1 just for display

Both styles are useful. for n in names is cleaner when we just want the value. for i in range(...) is necessary when we need the position too

A classic use case: summing numbers in a list

prices = [10, 20, 30, 40]
total = 0
for i in prices:
    total = total + i
print(total)

Output 100. We started with total = 0, then for each price i we added it to total. This is one of the most common patterns in programming, and you will write it many, many times. Read the code line by line and make sure you understand why it works

Mission: The Ship's Crew

On the right you have the list of the 20 crew members, named crew. Ground control has selected three people for a spacewalk (EVA): the ones at indexes 5, 10 and 12 in the list

Loop over the list with a for loop and print each member numbered (1. Mary, 2. Andrew and so on; use the index with i + 1). For the members at indexes 5, 10 and 12 add -> selected for EVA at the end

Careful: we count from 0, so index 5 is the 6th member printed

Output

1. Mary
2. Andrew
3. Helen
4. Michael
5. Anna
6. Victor -> selected for EVA
7. Joanna
8. George
9. Diana
10. Robert
11. Christine -> selected for EVA
12. Alex
13. Gabrielle -> selected for EVA
14. Steven
15. Laura
16. Brian
17. Andrea
18. Paul
19. Rose
20. Daniel

Change the selected indexes or add a new member and run again, watch the EVA tag move