For Loops

Explain what for loops are normally used for. Trace the execution of a simple (unnested) loop and correctly state the values of variables in each iteration. Write for loops that use the Accumulator pattern to aggregate values.

Recognition and Attribution

This page is adapted from the Software Carpentry Python Gapminder lesson, Copyright (c) The Carpentries. The original material is licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0).

Changes made: Content has been modified and expanded by Innovations for Poverty Action (IPA) to include IPA-specific examples and context relevant to research data analysis.

Original citation: Achterberg, et al. (2024). swcarpentry/python-novice-gapminder: Software Carpentry: Plotting and Programming in Python (v2024.6.27.1). Zenodo. https://doi.org/10.5281/zenodo.12167686

Learning Objectives

Explain what for loops are normally used for.
Trace the execution of a simple (unnested) loop and correctly state the values of variables in each iteration.
Write for loops that use the Accumulator pattern to aggregate values.

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

Doing calculations on the values in a list one by one is as painful as working with pressure_001, pressure_002, etc.
A for loop tells Python to execute some statements once for each value in a list, a character string, or some other collection.
“for each thing in this group, do these operations”

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

This for loop is equivalent to:

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

And the for loop’s output is:

2
3
5

A `for` loop is made up of a collection, a loop variable, and a body

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

The collection, [2, 3, 5], is what the loop is being run on.
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.
- The “current thing”.

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.
Python uses indentation rather than {} or begin/end to show nesting.
- Any consistent indentation is legal, but almost everyone uses four spaces.

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

IndentationError: expected an indented block

Indentation is always meaningful in Python.

firstName = "Jon"
  lastName = "Smith"

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

This error can be fixed by removing the extra spaces at the beginning of the second line.

Loop variables can be called anything

As with all variables, loop variables are:
- Created on demand.
- Meaningless: their names can be anything at all.

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

The body of a loop can contain many statements

But no loop should be more than a few lines long.
Hard for human beings to keep larger chunks of code in mind.

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

2 4 8
3 9 27
5 25 125

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
- Exactly the legal indices of a list or character string of length N

print('a range is not a list: range(0, 3)')
for number in range(0, 3):
    print(number)

a range is not a list: range(0, 3)
0
1
2

The Accumulator pattern turns many values into one

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.

# Sum the first 10 integers.
total = 0
for number in range(10):
   total = total + (number + 1)
print(total)

Read total = total + (number + 1) as:
- Add 1 to the current value of the loop variable number.
- Add that to the current value of the accumulator variable total.
- Assign that to total, replacing the current value.
We have to add number + 1 because range produces 0..9, not 1..10.

Exercise: Classifying Errors

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

Solution

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.

Exercise: Tracing Execution

Create a table showing the numbers of the lines that are executed when this program runs, and the values of the variables after each line is executed.

total = 0
for char in "tin":
    total = total + 1

Solution

Line no	Variables
1	total = 0
2	total = 0 char = ‘t’
3	total = 1 char = ‘t’
2	total = 1 char = ‘i’
3	total = 2 char = ‘i’
2	total = 2 char = ‘n’
3	total = 3 char = ‘n’

Exercise: Reversing a String

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

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

Solution

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

Exercise: Practice Accumulating

Fill in the blanks in each of the programs below to produce the indicated result.

# Total length of the strings in the list: ["red", "green", "blue"] => 12
total = 0
for word in ["red", "green", "blue"]:
    ____ = ____ + len(word)
print(total)

Solution

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

# List of word lengths: ["red", "green", "blue"] => [3, 5, 4]
lengths = ____
for word in ["red", "green", "blue"]:
    lengths.____(____)
print(lengths)

Solution

lengths = []
for word in ["red", "green", "blue"]:
    lengths.append(len(word))
print(lengths)

# Concatenate all words: ["red", "green", "blue"] => "redgreenblue"
words = ["red", "green", "blue"]
result = ____
for ____ in ____:
    ____
print(result)

Solution

words = ["red", "green", "blue"]
result = ""
for word in words:
    result = result + word
print(result)

Create an acronym: Starting from the list ["red", "green", "blue"], create the acronym "RGB" using a for loop.

Hint: You may need to use a string method to properly format the acronym.

Solution

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

Exercise: 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].

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

Solution

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

Exercise: Identifying Variable Name Errors

Read the code below and try to identify what the errors are without running it.
Run the code and read the error message. What type of NameError do you think this is? Is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
Fix the error.
Repeat steps 2 and 3, until you have fixed all the errors.

for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + "b"
print(message)

Solution

Python variable names are case sensitive: number and Number refer to different variables.
The variable message needs to be initialized as an empty string.
We want to add the string "a" to message, not the undefined variable a.

message = ""
for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (number % 3) == 0:
        message = message + "a"
    else:
        message = message + "b"
print(message)

Exercise: Identifying Item Errors

Read the code below and try to identify what the errors are without running it.
Run the code, and read the error message. What type of error is it?
Fix the error.

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[4])

Solution

This list has 4 elements and the index to access the last element in the list is 3.

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[3])

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.
The Accumulator pattern turns many values into one.

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

Loop variables can be called anything

The body of a loop can contain many statements

Use range to iterate over a sequence of numbers

The Accumulator pattern turns many values into one

Exercise: Practice Accumulating

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

Use `range` to iterate over a sequence of numbers