for Loops and range()

A teacher handing back tests walks down the row, desk by desk. Nobody has to count laps. When the last desk is done, so is the job.

In while Loops, every loop needed three pieces you had to manage yourself: a counter, a condition, and a step. Forget the step, and the loop never ends. Start the counter in the wrong place, and it runs once too often.

For the most common kind of repetition, "do this for each thing" or "do this 10 times," Python has a loop that handles all of that for you: the for loop.

Desk by desk

A teacher handing back tests doesn't think "set counter to 1, check whether the counter is past the last desk, add one to the counter." They just walk down the row and hand a test to each student, one after another, until there are no more desks.

A for loop works the same way. You give it a collection of things, and it hands you each one in turn, until they run out.

Looping through a string

A string is a collection of characters, so a for loop can visit each one:

for letter in "CAT":
    print(letter)

C
A
T

Read it almost like English: "for each letter in CAT, print the letter."

• for starts the loop.
• letter is the loop variable. On each iteration, it holds the next item: first "C", then "A", then "T".
• in "CAT" names the collection to loop through.
• A colon and an indented block, just like if and while.

There's no counter and no condition to get wrong. The loop ends by itself when the string runs out of characters. Choose a loop variable name that says what each item is: letter, student, score.

Counting with range()

To repeat something a set number of times, use range(). It produces a sequence of numbers for the loop to walk through:

for i in range(5):
    print("Iteration", i)

Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4

range(5) gives the numbers 0, 1, 2, 3, 4: five numbers, starting at 0 and stopping before 5. That "stop before" rule is the same one slicing uses in Working with Strings, and it means range(n) always runs exactly n times.

A plain counter is traditionally called i. When you don't need the number at all, just the repetition, name it _, which tells readers "I'm not using this":

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

Hip hip hooray!
Hip hip hooray!
Hip hip hooray!

Choosing where to start and stop

Give range() two numbers, and the first is where to start:

for lap in range(1, 6):
    print("Lap", lap)

Lap 1
Lap 2
Lap 3
Lap 4
Lap 5

range(1, 6) starts at 1 and stops before 6. That's the same loop as the while version in while Loops, in two lines instead of four, with no way to forget the step.

A third number sets the step: how much to count by. A negative step counts down:

for n in range(2, 11, 2):
    print(n, end=" ")
print()

for n in range(3, 0, -1):
    print(n, end=" ")
print("Liftoff!")

2 4 6 8 10
3 2 1 Liftoff!

Notice range(2, 11, 2) needs to stop at 11 to include 10, and range(3, 0, -1) stops before 0, which is exactly right for a countdown.

You write	Numbers produced
range(4)	0, 1, 2, 3
range(1, 4)	1, 2, 3
range(0, 10, 3)	0, 3, 6, 9
range(5, 0, -1)	5, 4, 3, 2, 1

Running totals

Adding things up works just like with while: create the total before the loop, add to it inside:

total = 0
for number in range(1, 101):
    total += number
print("1 + 2 + ... + 100 =", total)

1 + 2 + ... + 100 = 5050

And the same pattern counts things. How many vowels are in a sentence?

sentence = "Python is fun to learn"
vowels = 0
for letter in sentence.lower():
    if letter in "aeiou":
        vowels += 1
print("Vowels:", vowels)

Vowels: 6

That's a loop with an if inside, and it's one of the most common shapes in all of programming: go through everything, and do something with the items that match.

Using the position too

Sometimes you want each character and its position. range(len(word)) gives you every valid index, which you can use to look characters up:

word = "JAVA"
for i in range(len(word)):
    print(i, word[i])

0 J
1 A
2 V
3 A

You'll learn a neater way to get both, called enumerate, in Lists.

for or while?

Both loops can do the same jobs, but each one fits some jobs better:

• Use for when you're going through a collection, or repeating a known number of times: every character, every student, 10 rounds.
• Use while when you're repeating until something happens, and you don't know when that will be: until the savings reach a goal, until the user types a valid answer.

When in doubt, reach for for. It can't loop forever by accident, and it can't be off by one because of a forgotten step.

Don't change what you're counting

One warning. Inside a for loop over range(), changing the loop variable does not change how many times the loop runs:

for i in range(3):
    print(i)
    i = 100

0
1
2

On each iteration, for hands i the next number from the range, overwriting whatever you did. The loop always walks the sequence it was given. If you need to jump around, that's a job for while, or for break and continue, coming up next.

Try it

This program prints a times table row, counts the letters in a word, adds up the even numbers from 1 to 20, and builds a staircase of stars. Predict every line.

Running Python here

Running Python right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have Python installed on your computer (see Setting Up), save it as main.py and run it with python main.py (or python3 main.py on macOS and Linux).

Check your prediction

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
'banana' has 3 a's
Sum of even numbers up to 20: 110
*
**
***
3 2 1 Go!

range(1, 6) gives 1 to 5, so the times table stops at 7 x 5. range(2, 21, 2) needs the stop value 21 to include 20. And "*" * step repeats the star once, then twice, then three times.

Try it yourself

1. Change the times table to show 7 x 1 through 7 x 10.
2. Write a for loop that prints every other letter of "Syntaxia": S n a i. (Hint: range with a step of 2, and indexes.)
3. Count how many digits are in the string "Room 204, Floor 3", using a loop and the string method isdigit().

Check your understanding

What numbers does range(4) produce?

1, 2, 3, 4

0, 1, 2, 3

0, 1, 2, 3, 4

4

Which range counts down 5, 4, 3, 2, 1?

range(5, 1)

range(5, 0)

range(1, 5, -1)

range(5, 0, -1)

You want to keep asking for a password until it's correct. Which loop fits best?

A while loop, because you don't know how many tries it will take

A for loop over range(10)

A for loop over the password

No loop is needed

Up next

Sometimes you want to leave a loop early, like when you've found what you're looking for, or skip one item without stopping the whole loop. Python has two small keywords for that: break and continue.