For Loops

Overview

Teaching: 10 min
Exercises: 15 min

Questions

• How can I make a program do many things?

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

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 building in ['Jefferson', 'Madison', 'Adams']:
print(place)

IndentationError: expected an indented block

• Indentation is always meaningful in Python.

first_name = "Thomas"
  last_name = "Jefferson"

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

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

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

for building in ['Jefferson', 'Madison', 'Adams']:
    print(building)

• The collection, ['Jefferson', 'Madison', 'Adams'], is what the loop is being run on.
• The body, print(building), specifies what to do for each value in the collection.
• The loop variable, building, is what changes for each iteration of the loop.
  • The “current thing”.

Loop variable names follow the normal variable name conventions.

• Loop variables will:
  • Be created on demand during the course of each loop.
  • Persist after the loop finishes.
    • Use a new variable name to avoid overwriting a data collection you need to keep for later
  • Often be used in the course of the loop
    • So give them a meaningful name you’ll understand as the body code in your loop grows.
    • Example: for single_letter in ['A', 'B', 'C', 'D']: instead of for blah in ['A', 'B', 'C', 'D']:

For example, you wouldn’t want to do:

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

Show loop using pens exercise and include something that’s not a pen.

• Stops automatically when you’ve run out of things, don’t need to keep track of the size of the collection.
• Doesn’t matter what the loop variable is called.
• It gets replaced each time with the next thing.

The body of a loop can contain many statements.

Math example:

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

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

0
1
2

Or use range to repeat an action an arbitrary number of times.

• You don’t actually have to use the iterable variable’s value.
• Use this structure to simply repeat an action some number of times.
  • That number of times goes into the range function.

for number in range(5):
    print("Again!")

Again!
Again!
Again!
Again!
Again!

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.

Say we have the following list: sizes = [‘xs’,’s’,’m’,’l’,’xl’]

Use the string method .upper() Write a for statement to:

• Create a new list with all the sizes in uppercase.
• Remember! You can’t append to a list that doesn’t exist.

sizes = ['xs','s','m','l','xl']
upper_sizes = []

for s in sizes:
    new_size = s.upper()
    upper_sizes.append(new_size)

print(upper_sizes)

['XS', 'S', 'M', 'L', 'XL']

For loops become more powerful when we can start to do more to test the data.

Practice Accumulating

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.