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For Loops

Overview

Teaching: 10 min
Exercises: 5 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.

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

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

2
3
5

The first line of the for loop must end with a colon, and the body must be indented.

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

IndentationError: expected an indented block

firstName="Jon"
  lastName="Smith"

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

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

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

Loop variables can be called anything.

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

The body of a loop can contain many statements.

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.

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

The Accumulator pattern turns many values into one.

# Sum the first 10 integers.
total = 0
for number in range(10):
   total = total + (number + 1)
print(total)

55

Classifying Errors

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

Solution

An IndentationError is a syntax error. Programs with syntax errors cannot be started. A program with a runtime will start but an error will be thrown under certain conditions.

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 char in "tin":
    total = total + 1

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’ | {. solution}

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

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.

# 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 = 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

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

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 = ""
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 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
sum = 0
print(cumulative)
data = [1,2,2,5]

Solution

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

Solution

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

Solution

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

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.

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