for Loops, range() & the in Operator

Same job, less work

count = 1                # start
while count <= 5:        # condition
    print(count)
    count += 1           # update

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

Where did the three parts go?

Both loops print the same thing. The for version is shorter, and it is impossible to forget the update, which is the most common cause of an infinite loop (a loop that never stops).

Anatomy of a for loop

What Python does, step by step, for for letter in "Hi":

1. Python takes the first item, "H", and stores it in letter. It runs the body: prints H.
2. Back to the top. The next item, "i", goes into letter. It runs the body again: prints i.
3. Back to the top. No items are left, so the loop ends and Python continues after the body.

You never write letter = ... yourself. The for line does the assigning for you, once per item.

Just repeating, not using the number? Some programmers name the variable _ (an underscore) to say "I don't need this value": for _ in range(3):.

range(stop): count from 0

for i in range(3):
    print("Hip hip hooray!")

Why start at 0?

Python counts positions starting at 0. The first letter of a string is at position 0, and the first item of a list is at position 0. Because range(5) gives 0 to 4, it lines up perfectly with the 5 positions of a 5-item list. You will see why that is handy when you study lists.

Seeing what a range holds

Printing range(5) shows just range(0, 5), because a range makes its numbers one at a time as the loop asks for them. To see them all at once, wrap it in list(): list(range(5)) shows [0, 1, 2, 3, 4].

range(start, stop, step)

How many numbers will I get?

With a step of 1, the count is simply stop − start. range(1, 6) gives 6 − 1 = 5 numbers. range(3, 10) gives 7 numbers.

Reading a range out loud

Say range(2, 11, 2) as "start at 2, go up by 2, and stop before reaching 11." Then list the numbers: 2, 4, 6, 8, 10. The next one, 12, would pass the stop, so the range ends.

Try it: range in a loop

Change start, stop, and step, then run the loop. Try 1, 11, 1 (1 to 10), 0, 21, 5 (count by fives), and 10, 0, -1 (count down).

Predict before you run

A range that makes no numbers is not an error. The loop body just never runs, and Python goes straight to the line after it. That makes this bug easy to miss.

Counting down and adding up

Tracing the running total (limit = 4)

Two placement rules for an accumulator: total = 0 goes before the loop (or it would reset every time), and print goes after the loop (or it would print every partial total).

Looping through a string

for letter in "cat":
    print(letter)

Doing something with each character

The loop variable is a one-character string, so you can use it in any expression:

for letter in "hey":
    print(letter.upper() * 2)

HH
EE
YY

The original string is not changed. The loop only reads each character. letter.upper() makes a new uppercase copy each time.

Looping through a list

fruits = ["apple", "mango", "kiwi"]
scores = [88, 92, 75]

for fruit in fruits:
    print("I like", fruit)

Lists can hold numbers too

scores = [88, 92, 75]
for score in scores:
    print(score + 5)     # a 5-point curve

93
97
80

The loop variable takes the type of each item. Here each score is an int, so math works. If the list held strings, each item would be a string.

The in operator: is it inside?

Using in to replace a long or

# long way
if day == "Sat" or day == "Sun":
    print("Weekend!")

# with in
if day in ["Sat", "Sun"]:
    print("Weekend!")

Both work the same, but the in version is shorter and easier to extend: just add more items to the list.

Ignoring capital letters: "A".lower() in "cat" is True, because .lower() turns "A" into "a" first.

Using in inside a loop

Trace word = "idea"

This is called the counting pattern: start a counter at 0, loop through every item, and add 1 only when an if is True. You can count anything this way: spaces, scores above 90, names that start with "A."

Printing on one line with end=

for i in range(1, 6):
    print(i, end=" ")

How end= works

Every print adds something at the end. Normally that is a new line ("\n"). end= lets you replace it with anything else. end="" adds nothing at all, so values touch: 12345.

Why the extra print() after the loop? The last value was printed without a new line, so the cursor is still on the same line. Without that empty print(), the next output (Done!) would end up stuck to the numbers.

A comma problem: end=", " prints 1, 2, 3, 4, 5, with an extra comma at the end. Later you will learn ", ".join(...), which puts separators only between items.

for or while?

scores = [88, 92, 75]
total = 0
for score in scores:
    total += score
print("Total:", total)

answer = input("Type quit: ")
while answer != "quit":
    answer = input("Type quit: ")
print("Bye!")

Some jobs can use either

Printing 1 to 5 works with both loops (you saw it on slide 2). When both work, prefer for: it is shorter and cannot become an infinite loop by accident.

A quick test: can you say how many times (or which items) before the loop starts? If yes, use for. If the answer is "it depends on what happens inside the loop," use while.

Common mistakes

Error messages shown are from Python 3.13. Older versions word some messages a little differently.

Checking a range in 10 seconds

Most range bugs are "off by one": the loop runs one time too many or too few. Before you run a loop, check just two things:

1. First value: is the start what you want? (Remember the default is 0.)
2. Last value: is it one step before the stop? If you want to include a number, the stop must be past it.

If you are unsure, test with print(list(range(...))) to see every number the range makes.

"Not iterable" in an error means "you tried to loop over something that is not a sequence," like a plain number. Wrap the number in range().

Check yourself

Predict the answer, then click Check. This is practice only. Nothing is graded or recorded.

Cheat sheet

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

for i in range(3):
    print(i)
for letter in "hey":
    print(letter)
for pet in ["dog", "cat"]:
    print(pet)

"a" in "cat"          # True
"dog" in ["dog", "cat"] # True
"x" not in "cat"      # True

Before you run a for loop, check:

• The line reads for name in sequence: with a colon at the end
• The body is indented 4 spaces
• Your range starts where you want, and the stop is one past the last number you want
• When counting down, the step is negative and the stop is smaller than the start
• range only gets whole numbers
• You used for for a known count or a sequence, and while for "repeat until"

Key words to know

for loop · iteration · loop variable · sequence · range(start, stop, step) · off-by-one error · accumulator · counting pattern · in (membership) · infinite loop