For Loops

Estimated time: 25 minutes

Overview

Questions

• How can I make a program do many things?
• How can I do something for each thing in a list?

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.

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.

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”

PYTHON

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

• This for loop is equivalent to:

PYTHON

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

• And the for loop’s output is:

OUTPUT

2
3
5

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

---

PYTHON

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.

PYTHON

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

OUTPUT

IndentationError: expected an indented block

• Indentation is always meaningful in Python.

PYTHON

firstName = "Jon"
  lastName = "Smith"

ERROR

  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.

PYTHON

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.

PYTHON

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

OUTPUT

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

PYTHON

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

OUTPUT

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.

PYTHON

# Sum the first 5 integers.
my_sum = 0 # Line 1
for number in range(5): # Line 2
   my_sum = my_sum + (number + 1) # Line 3
print(my_sum) # Line 4

OUTPUT

15

• 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.
  • You could also have used number and range(11).

We can trace the program output by looking at which line of code is being executed and what each variable’s value is at each line:

Line No	Variables
1	my_sum = 0
2	my_sum = 0 number = 0
3	my_sum = 1 number = 0
2	my_sum = 1 number = 1
3	my_sum = 3 number = 1
2	my_sum = 3 number = 2
3	my_sum = 6 number = 2
2	my_sum = 6 number = 3
3	my_sum = 10 number = 3
2	my_sum = 10 number = 4
3	my_sum = 15 number = 4
4	my_sum = 15 number = 4

Let’s double check our work by visualizing the code.

Classifying Errors

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

Show me the solution

An indentation error (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.

Tracing Execution

Trace through the following code and create a table showing the lines that are executed when this program runs, and the values of the variables after each line is executed.

PYTHON

total = 0 # Line 1
for char in "tin": # Line 2
    total = total + 1 # Line 3

Show me the 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’

Reversing a String

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

PYTHON

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

Show me the solution

PYTHON

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

Practice Accumulating

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

PYTHON

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

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

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

Show me the solution

PYTHON

# A
# 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)

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

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

String accumulation

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

Give me a hint

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

Show me the solution

PYTHON

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

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].

PYTHON

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

Show me the solution

PYTHON

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

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.

This is a first taste of if statements. We will be going into more detail on if statements in future lessons. No errors in this code have to do with how the if statement is being used.

PYTHON

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)

Show me the 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.

PYTHON

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)

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.

PYTHON

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

Show me the solution

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

PYTHON

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