For Loops

Last updated on 2026-10-19 | Edit this page

Estimated time: 25 minutes

Overview

Questions

  • How can I make a program repeat things?

Objectives

  • Explain what a for loop does.
  • Correctly write for loops to repeat simple calculations.
  • Trace changes to a loop variable as the loop runs.
  • Trace changes to other variables as they are updated by a for loop.

The power of programming comes from repetition

So far, we’ve been looking manipulating one value at a time. To write programs that are useful for manipulating real data, we need to make Python perform repetitive tasks for us.

An example task that we might want to repeat is accessing numbers in a list. We could do this printing each number on a line of its own.

PYTHON 

odds = [1, 3, 5, 7]
print(odds[0])
print(odds[1])
print(odds[2])
print(odds[3])

OUTPUT 

1
3
5
7

This is a bad approach for three reasons:

  1. It’s not scalable. We’d have to write hundreds of print() statements for a longer list.
  2. It’s difficult to maintain.
  3. It’s fragile. What happens if a list is shorter than we expected? (An error.)

We can solve this problem using a for loop that steps through each item in a collection.

PYTHON 

odds = [1, 3, 5, 7]
for num in odds:
    print(num)

OUTPUT 

1
3
5
7

A for loop executes commands once for each value in a collection.

A for loop tells Python to execute some statements once for each value in a list, a character string, or some other collection.

The loop has three parts: an iterable, a loop variable, and a body that is repeated once for each item in the iterable. The name of a loop variable can be any valid variable name.

PYTHON 

for number in [2, 3, 5]: # the colon is important
    print(number) # so is the indentation

Let’s anatomize this loop:

  • The iterable, [2, 3, 5], is what the loop is being run on. It is followed by a :.
  • The body, print(number), specifies what to do for each value in the collection.
  • The loop variable, number, is what changes for each iteration of the loop.

This for loop is equivalent to:

PYTHON 

print(2)
print(3)
print(5)

And the for loop’s output is:

OUTPUT 

2
3
5

The first line of the for loop must end with a colon, and the body must be indented.

The colon at the end of the first line signals the start of a block of statements. Any consistent indentation is grammatical, but almost everyone uses four spaces or the tab key.

PYTHON 

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

ERROR 

IndentationError: expected an indented block

Python is picky about indentation and will not parse unexpected indents.

PYTHON 

firstName = "Jon"
  lastName = "Smith"

ERROR 

  File "<ipython-input-7-f65f2962bf9c>", line 2
    lastName = "Smith"
    ^
IndentationError: unexpected indent
Challenge

Classifying Errors

Is an indentation error a syntax error or a runtime error?

An IndentationError is a syntax error. Programs with syntax errors cannot be started. A program with a runtime error will start but an error will be thrown under certain conditions.

Loop variables can be called anything.

As with all variables, loop variables are: * Created on demand. * Can have any valid variable name.

PYTHON 

for kitten in [2, 3, 5]:
    print(kitten)

OUTPUT 

2
3
5

You are not required to use loop variables in the body

You are not required to use the loop variable in the body of your loop. You can do anything len(iterable) times.

PYTHON 

for number in [1, 2, 3]:
    print("Hello")

OUTPUT 

Hello
Hello
Hello

The body of a loop can contain many statements.

The body of a loop can contain multiple statements, as long as they’re all indented appropriately. All indented lines (an indented block) before the first unintended line are treated as part of the same loop and are evaluated in order.

PYTHON 

primes = [2, 3, 5] # First three primes
for p in primes:
    squared = p ** 2
    cubed = p ** 3
    print(p, squared, cubed)

OUTPUT 

2 4 8
3 9 27
5 25 125
Challenge

Debugging Loops

Your friend wants to compute the square roots of 1, 4, 9, and 16 using a loop. They do not get the answer they expect with the following code.

What is the problem and how can it be fixed?

PYTHON 

squares = [1, 4, 9, 16]
for s in squares:
    print("The square root is", squares ** 0.5)

This code has a common mistake: the iterable (squares) is used in place of the loop variable. While raising a number to the 0.5 power is equivalent to applying the square root of a number, you can’t get the square root of a list!

PYTHON 

squares = [1, 4, 9, 16]
for square in squares:
    print(square ** 0.5)

OUTPUT 

1.0
2.0
3.0
4.0

The range() function iterates over a sequence of numbers.

Use range() to iterate over a sequence of numbers.

  • The built-in function range produces a sequence of numbers.
  • Not a list: the numbers are produced on demand to make looping over large ranges more efficient.
  • range(N) is the numbers 0..N-1

PYTHON 

for i in range(5):
    print(i)

OUTPUT 

0
1
2
3
4

The range() function is convenient when you want to do an operation a fixed number of times.

PYTHON 

housemates = ["joe", "james", "yao"] # Iterated in order
for housemate in range(len(housemates)):
    name = housemates.pop()
    print(name, "moved out")
print(housemates)

OUTPUT 

yao moved out
james moved out
joe moved out
[]

The numbers in a range can be adjusted by their start, stop, and step arguments. If a single integer is supplied, it is assumed to be the stop. The default value of step is 1, ie. it is incremented by 1.

PYTHON 

one_to_five = range(1, 6) # Start at 1, stop before 6 (exclusive)
for num in one_to_five:
    print(num)

OUTPUT 

1
2
3
4
5

Set the step parameter to -1 to have the range count down rather than up. The start is still inclusive and the stop is still exclusive.

PYTHON 

five_to_one = range(5, 0, -1) # Start at 5 (inclusive), stop at 0 (exclusive), -1 to step
for num in five_to_one:
    print(num)

OUTPUT 

5
4
3
2
1

Programming pattern: accumulation

A common pattern in programs is to:

  1. Initialize an accumulator variable to zero, the empty string, or the empty list.
  2. Update the variable with values from a collection

Let’s say we want to compute the sum of the first five integers starting at 1. You will need to initalize a varable called total to 0. This will be our accumulator variable.

PYTHON 

# Sum the first 10 integers.
total = 0 # Initialize a variable called sum
for num in range(1, 6):
    print("Total", total)
    print("Adding", num)
    total = total + num # same as total += num
print(total)

OUTPUT 

Total 0
Adding 1
Total 1
Adding 2
Total 3
Adding 3
Total 6
Adding 4
Total 10
Adding 5
15

Let’s find the total length (in characters) of the strings in the following list: ["red", "green", "blue"].

PYTHON 

total = 0
for word in ["red", "green", "blue"]:
    total = total + len(word)
print(total)

OUTPUT 

12
Challenge

Making an Acronym

Using that same ["red", "green", "blue"] list, how could we derive the acronym "rgb" using an accumulator pattern?

PYTHON 

acronym = _____
for word in ["red", "green", "blue"]:
    acronym = ___________ + ________
print(acronym)

Instead of accumulating a number starting at 0, accumulate a string one character at a time starting with the empty string.

PYTHON 

acronym = ""
for word in ["red", "green", "blue"]:
    acronym = acronym + word[0]
print(acronym)
Challenge

Reversing a String

Fill in the blanks in the program below so that it prints “nit” (the reverse of the original character string “tin”).

PYTHON 

original = "tin"
result = ____
for char in original:
    result = ____
print(result)

PYTHON 

original = "tin"
result = ""
for char in original:
    result = char + result
print(result)
Challenge

Cumulative Sum

Reorder and properly indent the lines of code below so that they print a list with the cumulative sum of data. The result should be [1, 3, 5, 10].

PYTHON 

cumulative.append(total)
for number in data:
cumulative = []
total = total + number
total = 0
print(cumulative)
data = [1,2,2,5]

PYTHON 

total = 0
data = [1,2,2,5]
cumulative = []
for number in data:
    total = total + number
    cumulative.append(total)
print(cumulative)
Challenge

Debugging Accumulators

Let’s say you want to compute the factorial of 6 or \(6!\) by hand. This is equal to 6 * 5 * 4 * 3 * 2 * 1. We can use a for-loop and an accumulator pattern to do this.

Let’s say your first draft looks like this.

PYTHON 

accumulator = 0
for i in range(1, 7): # 1 inclusive to 7 exclusive
    print(i)
    accumulator = accumulator * i
print("6! = ", accumulator)

Is the answer correct? What needs to be changed about this block of code in order for it to compute \(6!\) correctly?

You’re multiplying rather than adding. Making your accumulator start at 0 will 0 out your results, as the product of any number * 0 is 0. Therefore, to get the correct result, you want \(1 * (6 * 5 * 4 * 3 * 2 * 1)\). Setting accumulator to 1 fixes this.s

Solution

PYTHON 

accumulator = 1
for i in range(1, 7): # 1 inclusive to 7 exclusive
    accumulator = accumulator * i
print("6! = ", accumulator)

OUTPUT 

6! =  720
Key Points
  • A for loop executes commands once for each value in a collection.
  • A for loop is made up of a collection, a loop variable, and a body.
  • The first line of the for loop must end with a colon, and the body must be indented.
  • Indentation is always meaningful in Python.
  • Loop variables can be called anything (but it is strongly advised to have a meaningful name to the looping variable).
  • The body of a loop can contain many statements.
  • Use range() to iterate over a sequence of numbers.