For Loops

Overview

Teaching: 20 min
Exercises: 20 min

Questions

• How can I make a program repeat a task?

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 accumulator 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 temperature_001, temperature_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 variable name ‘number’ is a name that you choose, when you create the for loop.
• You can choose any variable name you want.
• But don’t use an already existing variable name, as this will then be overwritten.
• The variable persists after the loop is done and will contain the last used value.
• In the case above, ‘number’ will contain the integer 5 after the for loop is finished.

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 to show nesting (rather than {} or begin/end, which are used in some other programming languages).
  • Any consistent indentation is legal, but almost everyone uses four spaces or tab.
  • IDEs like Jupyter Lab will automatically create an indentation after the colon.

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

    print(number)
    ^
IndentationError: expected an indented block after 'for' statement on line 1

• Indentation is always meaningful in Python.

firstName = "Jon"
  lastName = "Smith"

    lastName = "Smith"
    ^
IndentationError: unexpected indent

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

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.

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.
  • 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 asdf in ['A', 'B', 'C', 'D']:.
• Use a new variable name to avoid overwriting a data collection you need to keep for later

The body of a loop can contain many statements

• You can have as much code as you like inside of a loop.
• But keep in mind, the more code you have, the more difficult it can be to keep track of.

numbers = [2, 3, 5]
for n in numbers:
    squared = n ** 2
    cubed = n ** 3
    print("The number", n, "squared is:", squared, ", and the number", n, "cubed is:", cubed)

The number 2 squared is: 4 , and the number 2 cubed is: 8
The number 3 squared is: 9 , and the number 3 cubed is: 27
The number 5 squared is: 25 , and the number 5 cubed is: 125

Use range to go through 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, 1, 2, … N-1

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

Or use range to repeat an action a set 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!

Using accumulator variables

• 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 all of the integers from 1 to 10.
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.
    • We have to add number + 1 because range produces the numbers 0…9, not 1…10.
  • Add that to the current value of the accumulator variable total.
  • Assign that to total, replacing the current value.

Exercises

Classifying Errors

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

Solution

It is a syntax error. The problem has to do with the placement of the code, not its logic.

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 animal in ['cat', 'dog', 'elephant', 'fish']:
    total = total + 1

Solution

Code line #	total	animal
1	0	N/A
2	0	'cat'
3	1	'cat'
2	1	'dog'
3	2	'dog'
2	2	'elephant'
3	3	'elephant'
2	3	'fish'
3	4	'fish'

Explanation:

• Line 1 initializes the variable total with a value of 0.
• Line 2 begins a for loop that iterates over the list ['cat', 'dog', 'elephant', 'fish'].
  • Line 2 is the first iteration, where animal takes the value ‘cat’, and total retains 0.
  • Line 3 increments total by 1, making it 1, and animal retains the value ‘cat’.
  • Line 2 is the second iteration, where animal takes the value ‘dog’, and total retains 1.
  • Line 3 increments total by 1, making it 2, and animal retains the value ‘dog’.
  • Line 2 is the third iteration, where animal takes the value ‘elephant’, and total retains 2.
  • Line 3 increments total by 1, making it 3, and animal retains the value ‘elephant’.
  • Line 2 is the fourth iteration, where animal takes the value ‘fish’, and total retains 3.
  • Line 3 increments total by 1, making it 4, and animal retains the value ‘fish’.

After the program finishes, total is 4, and animal retains its value from the last iteration, which is ‘fish’.

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

• result is an empty string because we use it to build or accumulate on our reverse string.
• char is the loop variable for original.
• For each iteration of the loop, char takes on one value (character) from original.
• Add char to the beginning of result to change the order of the string.
• Our loop code should look like this:

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

If you were to explain the loop step by step, the iterations would look something like this:

#First loop
char = "t"
result = ""
char + result = "t"
#Second loop
char = "i"
result = "t"
char + result = "it"
#Third loop
char = "n"
result = "it"
char + result = "nit"

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 length of the strings in the list: ["red", "green", "blue"] => 12
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

# List of word lengths: ["red", "green", "blue"] => [3, 5, 4]
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

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

# Create acronym: ["red", "green", "blue"] => "rgb"
# write the whole thing

Solution

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

Cumulative Sum

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

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

Solution

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

[1, 3, 5, 10]

Identifying Variable Name Errors

1. Read the code below and try to identify what the errors are without running it.
2. 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?
3. Fix the error.
4. 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)

Identifying Item Errors

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

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

Solution

It is an index error:

IndexError: list index out of range

The problem is that 4 points to an item that doesn’t exist in the list. Remember the first item of a list in Python is 0.
Replace seasons[4] with seasons[0], seasons[1], seasons[2] or seasons[3] to have the different items of the list printed.

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.