Lesson 7 of 26 · about 14 min

for loops and range

A for loop does something once for each item in a sequence. range() makes a sequence of numbers, and an accumulator builds up a result.

🧭 What you need first: Repeating with while

The story

A teacher hands back 30 tests. For each test in the pile, they write the grade in the gradebook. They don’t need to check “are there still tests?” before each one; they just go through the pile, one by one, until it’s empty.

That’s a for loop: “for each thing in this collection, do this.” Use while when you’re waiting for something to happen. Use for when you have a known collection to go through.

Another way: dealing cards

Dealing a deck: for each card in the deck, give it to the next player. The deck decides how many rounds there are. You never need a separate counter.

Another way: a checklist

Packing for a trip with a list: for each item on the list, put it in the bag and tick it. When the list ends, you’re done.

Play with it

Watch an accumulator grow

This adds up 1 + 2 + 3 + 4 + 5. Step through and watch i take each value while total builds up.

Watch the code run, one step at a time

1total = 0
2for i in range(1, 6):
3 total = total + i
4print("Sum:", total)

➜ yellow = the line about to run · ✓ = the line that just ran

Name it

The real words

A for loop goes through a sequence one item at a time. The loop variable (i above) names the current item.
You can loop over many things: for letter in "hello": gives one letter at a time.
range() makes number sequences:

Code	Gives
`range(5)`	0, 1, 2, 3, 4
`range(1, 6)`	1, 2, 3, 4, 5
`range(0, 10, 2)`	0, 2, 4, 6, 8
`range(5, 0, -1)`	5, 4, 3, 2, 1

The stop number is never included. It feels odd at first, but it means range(5) has exactly 5 numbers.

The accumulator pattern: create a variable before the loop, update it inside, use it after. It works for sums, counts, products, longest word, and more.
Nested loops: a loop inside a loop. The inner loop runs completely for every single round of the outer one. 3 outer × 4 inner = 12 total runs.
break stops the loop early. continue skips to the next item.

Build it

Multiples of 3 or 5

Add up all the numbers below 100 that are multiples of 3 or 5 (like 3, 5, 6, 9, 10, 12…). Store the answer in total.

or Ctrl/⌘ + Enter

Challenge

Draw a triangle with nested loops

Print this triangle, using a loop inside a loop (the inner loop prints one * at a time with end=''):

or Ctrl/⌘ + Enter

🧩 Order these lines to count how many letters in a word are vowels.

Use the arrows to put the lines in the right order, then check.

for letter in word:
count = 0
print(count)
word = "banana"
count += 1
if letter in "aeiou":

Remember it

What numbers does range(2, 8, 2) give?

Got it?

Finish the lesson to color it in on your map and collect its concept cards.

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