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

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

Let’s visualize this loop using Python Tutor. In the notebook, write

%load tutor.py

This loads the following code:

from metakernel import register_ipython_magics
register_ipython_magics()

Execute the code (it gives no output).

Now you can rerun the previous for loop in a new cell that starts with %%tutor:

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

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

A string is also a collection that can be looped over.

organ = "liver"
for char in organ:
    print(char)

This means that if we have a DNA string, we can loop over the individual bases:

DNA = "ACGTGC"
for base in DNA:
    print(base)

As an example, let’s use a for loop to de the same as the .upper() function. .upper() changes all characters to upper case:

dna1 = "acgtgc"
print(dna1)
dna2 = dna1.upper()
print(dna2)

With a for loop, we could do it like this:

dna1 = "acgtgc"
dna2 = ''
print(dna1)
for base in dna1:
    dna2 = dna2 + base.upper()
print(dna2)

Rerun this loop using Python Tutor as described above.

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)

55

• 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.

Exercises on slides

• Reversing a String
• Summarising DNA strings
• Cumulative Sum

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.