For Loops

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"

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

• This for loop is equivalent to:

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

• And the for loop's output is:

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2
3
5

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

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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.

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

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IndentationError: expected an indented block

• Indentation is always meaningful in Python.

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firstName = "Jon"
  lastName = "Smith"

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  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.

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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.

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

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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

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

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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.

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

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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.

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total = 0
for char in "tin":
    total = total + 1

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

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

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

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total = 0
for word in ["red", "green", "blue"]:
    total = total + len(word)
print(total)

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

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lengths = []
for word in ["red", "green", "blue"]:
    lengths.append(len(word))
print(lengths)

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

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words = ["red", "green", "blue"]
result = ""
for word in words:
    result = result + word
print(result)

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

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

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

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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)

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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)

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

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