For Loops

Last updated on 2024-09-03 | Edit this page

Estimated time: 40 minutes

Overview

Questions

  • How can I execute Python code iteratively across a collection of values?

Objectives

  • Explain what for loops are normally used for.
  • Trace the execution of an un-nested loop and correctly state the values of variables in each iteration.
  • Write for loops that use the accumulator pattern to aggregate values.

For loops

Let’s create a short list of numbers in Python, and then attempt to print out each value in the list.

PYTHON 

odds = [1, 3, 5, 7]

One way to print each number is to use a print statement with the index value for each item in the list:

PYTHON 

print(odds[0], odds[1], odds[2], odds[3])

OUTPUT 

1 3 5 7

This is a bad approach for three reasons:

  1. Not scalable. Imagine you need to print a list that has hundreds of elements.

  2. Difficult to maintain. If we want to add another change – multiplying each number by 5, for example – we would have to change the code for every item in the list, which isn’t sustainable

  3. Fragile. Hand-numbering index values for each item in a list is likely to cause errors if we make any mistakes.

PYTHON 

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

We get an IndexError when we try to refer to an item in a list that does not exist.

ERROR 

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-3-7974b6cdaf14> in <module>()
      3 print(odds[1])
      4 print(odds[2])
----> 5 print(odds[3])

IndexError: list index out of range

A for loop is a better solution:

PYTHON 

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

OUTPUT 

1
3
5
7

A for loop repeats an operation – in this case, printing – once for each element it encounters in a collection. The general structure of a loop is:

PYTHON 

for variable in collection:
    # do things using variable, such as print

We can call the loop variable anything we like, there must be a colon at the end of the line starting the loop, and we must indent anything we want to run inside the loop. Unlike many other programming languages, there is no command to signify the end of the loop body; everything indented after the for statement belongs to the loop.

Loops are more robust ways to deal with containers like lists. Even if the values of the odds list changes, the loop will still work.

PYTHON 

odds.append(9)
odds.append(11)
print(odds)
for num in odds:
    print(num)

OUTPUT 

[1, 3, 5, 7, 9, 11]
1
3
5
7
9
11

Using a shorter version of the odds example above, the loop might look like this:

Loop variable 'num' being assigned the value of each element in the list odds in turn and then being printed

Each number (num) variable in the odds list is looped through and printed one number after another.

Loop variables

Loop variables are created on demand when you define the loop and they will persist after the loop finishes. Like all variable names, it’s helpful to give for loop variables meaningful names that you’ll understand as the code in your loop grows. for num in odds is easier to understand than for kitten in odds, for example.

You can loop through other Python objects

You can use a for loop to iterate through each element in a string. for loops are not limited to operating on lists.

PYTHON 

for letter in 'library of babel':
  print(letter)

OUTPUT 

L
i
b
r
a
r
y

o
f

B
a
b
e
l

Use range to iterate over a sequence of numbers.

The built-in function range() produces a sequence of numbers. You can pass a single parameter to identify how many items in the sequence to range over (e.g. range(5)) or if you pass two arguments, the first corresponds to the starting point and the second to the end point. The end point works in the same way as Python index values (“up to, but not including”).

PYTHON 

for number in range(0,3):
    print(number)

OUTPUT 

0
1
2

Accumulators

A common loop pattern is to initialize an accumulator variable to zero, an empty string, or an empty list before the loop begins. Then the loop updates the accumulator variable with values from a collection.

We can use the += operator to add a value to total in the loop below, so that each time we iterate through the loop we’ll add the index value of the range() to total.

PYTHON 

# Sum the first 10 integers.
total = 0

# range(1,11) will give us the numbers 1 through 10
for num in range(1, 11):
    print(f'num is: {num} total is: {total}')
    total += num

print(f'Loop finished. num is: {num} total is: {total}')

OUTPUT 

num is: 1 total is: 0
num is: 2 total is: 1
num is: 3 total is: 3
num is: 4 total is: 6
num is: 5 total is: 10
num is: 6 total is: 15
num is: 7 total is: 21
num is: 8 total is: 28
num is: 9 total is: 36
num is: 10 total is: 45
Loop finished. Num is: 10 total is: 55
  • The first time through the loop, total is equal to 0, and num is 1 (the range starts at 1). After those values print out we add 1 to the value of total (0), to get 1.
  • The second time through the loop, total is equal to 1, and num is 2. After those print out we add 2 to the value of total (1), to get 3.
  • The third time through the loop, total is equal to 3, and num is 3. After those print out we add 3 to the value of total (3), to bring us to 6.
  • And so on.
  • After the loop is finished the values of total and num retain the values that were assigned the last time through the loop. So num is equal to 10 (the last index value of range()) and total is equal to 55 (45 + 10).

Loop through a list

Create a list of three vegetables, and then build a for loop to print out each vegetable from the list.

Bonus: Create an accumulator variable to print out the index value of each item in the list along with the vegetable name.

PYTHON 

vegetables = ['lettuce', 'carrots', 'celery']
for veg in vegetables:
    print(veg)

OUTPUT 

lettuce
carrots
celery

Bonus:

PYTHON 

idx = 0
vegetables = ['lettuce', 'carrots', 'celery']
for veg in vegetables:
    print(idx, veg)
    idx += 1

OUTPUT 

0 lettuce
1 carrots
2 celery

Use range() in a loop

Print out the numbers 10, 11, 12, 13, 14, 15, using range() in a for loop.

PYTHON 


for num in range(10, 16):
    print(num)

OUTPUT 

10
11
12
13
14
15

Use a string index in a loop

How would you loop through a list with the values ‘red’, ‘green’, and ‘blue’ to create the acronym rgb, pulling from the first letters in each string? Print the acronym when the loop is finished.

Hint: Use the + operator to concatenate strings together. For example, lib = 'lib' + 'rary' will assign the value of ‘library’ to lib.

PYTHON 

acronym = ''
for color in ['red', 'green', 'blue']:
    acronym = acronym + color[0]
print(acronym)

OUTPUT 

rgb

You could also concatenate inside of the loop with acronym += color[0].

Subtract a list of values in a loop

  1. Create an accumulator variable called total that starts at 100.
  2. Create a list called numbers with the values of 10, 15, 20, 25, 30.
  3. Create a for loop to iterate through each item in the list.
  4. Each time through the list update the value of total to subtract the value of the current list item from total. Tip: -= works for subtraction in the same way that += works for addition.
  5. Print the value of total inside of the loop to keep track of its value throughout.

PYTHON 

total = 100
numbers = [10, 15, 20, 25, 30]
for num in numbers:
    total -= num
    print(total)

OUTPUT 

90
75
55
30
0

Key Points

  • A for loop executes commands once for each value in a collection.
  • The first line of the for loop must end with a colon, and the body must be indented.
  • Indentation is always meaningful in Python.
  • A for loop is made up of a collection, a loop variable, and a body.
  • 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.