Iterations: ‘for’ and ‘while’loops

for loop

Python for Loop Syntax

for individual_item in iterator:
statements…

• individual item: just a variable name
• iterator: something that can be iterated; like lists, tuples, dictionaries, range etc.

Examples

# loops and iteration

# 1. for loop
# for each item in some collection, do some stuff

# You list
programs_to_write = [
    "slightly better bash script",
    "web crawler",
    "port scanner",
    "web application",
    "cloud provisioning tool",
]

# Basic Form - IDENTATION is the key
for program_name in programs_to_write:
    print("I'm going to write a", program_name)  # capitalize?

print("\n...We're done!")
print("but it's not all clean fun -- the loop variable --", program_name, "-- still exists!")

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Range() Function

• The range() function accepts a start, stop, and optional step argument so you can get fancy with the sequences it can generate

# for loop with range() function
>>> for i in range(0,3):
print(i)

0
1
2

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Iterating Through a Dictionary

• If you try to iterate through a dictionary, you might be surprised by the results

# Iterating through a Dictionary
>>> fruit_inventory = {"apples": 5, "pears": 2, "oranges": 9}
>>> for fruit in fruit_inventory:
print(fruit)

oranges
apples
pears

• Note that the iterator for a dictionary only iterates over the dictionary's keys

• To iterate over both the key and value, use the .items() dictionary method which returns a list of tuples

# Iterating through a dictionary using .items() method
>>> list(fruit_inventory.items())
[('oranges', 9), ('apples', 5), ('pears', 2)]
>>> for fruit in fruit_inventory.items():
print(fruit)

('oranges', 9)
('apples', 5)
('pears', 2)

• To split up the Key and Value from the Tuple, use Unpacking

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Unpacking

• Assign a collection of items to a collection of variables

# Unpacking
>>> a, b, c = [1, 2, 3]
>>> a
1
>>> b
2
>>> c
3

# Unpacking Key and Value from Tuple
>>> for fruit, quantity in fruit_inventory.items():
print("You have {} {}.".format(quantity, fruit))

You have 5 apples.
You have 2 pears.
You have 9 oranges.

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Nested ‘for’ loops

• Avoid nested ‘for’ loops with large collections because this will make the program runs slow

# Nested 'for' loops and range() function
grid = []

for x in range(0,3):
    for y in range(0,3):
        grid.append([x,y])
        print(x,y)

grid = []
for x in range(0,3):
    for y in range(0,3):
        grid.append([x,y])
        print(x,y)

for emotion in ["sometimes I hate", "sometimes I love"]:
for activity in ["flying", "travel", "eating", "sleeping"]:
print(emotion, activity)

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While Loop

Python while Loop Syntax

# Syntax
while expression:
statements…

• expression can be any Python expression that evaluates to True or False value

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Examples

# while loop
>>> i = 0
>>> while i < 5:
print(i)
i += 1

0
1
2
3
4

# while loop - a traditional loop
print("Welcome to my game")

something_true = True
while (something_true):
    print("Going through the loop")

    # possibly make something_true = False
    something_true = False

print("we're done! something_true is now", something_true)

print("Welcome to my game")

choice = ""
while (choice != "quit"):
    choice = input("What would you like to do?\n")
    print("Going through the loop")
    print("Your choice was: ", choice)

print("we're done! choice is now", choice)

Index