📝Week 7 - Lab Intro

In this lab introduction we will briefly discuss the topic of looping.

Looping

for loops

for loops let you run the same block of code for a predetermined number of iterations.

syntax: for [variable name] in [iterable]

for i in range(10):
    print(i)

0
1
2
3
4
5
6
7
8
9

the range function

The range() function creates an iterable over integers. It can have up to three arguments (start, stop, step).

# loop from 2 up to (not including) 20, stepping by 3
for i in range(2,20,3):
    print(i)

2
5
8
11
14
17

with 2 arguments: (start, stop, 1)

# loop from 6 up to (not including) 10
for i in range(6,10):
    print(i)

6
7
8
9

As before, with 1 argument: (0, stop, 1)

# loop from 0 up to (not including) 5
for i in range(5):
    print(i)

0
1
2
3
4

looping over a list

days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

for d in days:
    print(d)

Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday

looping over a string

message = "Hello World!"

for letter in message:
    print(letter)

H
e
l
l
o
 
W
o
r
l
d
!

looping over a dictionary

A for loop over a dictionary iterates over the keys.

fruit_inventory = {"apple" : 6,
        "banana" : 2,
        "orange" : 4}

for f in fruit_inventory:
    print(f)

apple
banana
orange

If we want the values as well, we can use [dict_name].items().

fruit_inventory = {"apple" : 6,
                "banana" : 2,
                "orange" : 4}

for f in fruit_inventory.items():
    print(f)

('apple', 6)
('banana', 2)
('orange', 4)

To separate each (key, value) tuple, we can provide to variables to the for loop.

fruit_inventory = {"apple" : 6,
                "banana" : 2,
                "orange" : 4}

for fruit_name, quantity in fruit_inventory.items():
    print(f"There are {quantity} {fruit_name}s in stock.")

the enumerate function

We can use the enumerate function to loop over an iterable with (index, object) pairs.

days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

for d in enumerate(days):
    print(d)

We can use two variables to “unpack” the tuple.

days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

for i, d in enumerate(days):
    print(f"{d} is day #{i+1}.")

while loops

while loops keep iterating while some condition is True.

syntax: while [condition]

i = 0
while i < 10:
    print(i)
    i += 1

0
1
2
3
4
5
6
7
8
9

Notice that this is the same as the for loop we saw previously:

for i in range(10):
    print(i)

0
1
2
3
4
5
6
7
8
9

Make sure the while loop condition will eventually become False. You don’t want to loop forever!

Run this at your peril:

# loops forever

i = 0
while i < 10:
    print(i)

If you already know how many times the loop needs to run, it is easier/cleaner to use a for loop.

Here’s an example where while is required.

input_string = input("Please enter your favorite number:")
while not input_string.isnumeric():
    input_string = input("Not a number. Try again:")


print(f"{input_string} is your favorite number.")  

Please enter your favorite number: five
Not a number. Try again: six
Not a number. Try again: 6

6 is your favorite number.

We need a while loop (rather than a for loop) because we don’t know how many times the user will mess up and enter a non-number.

break and continue

break statements

The keyword break causes a loop to immediately terminate.

days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

for d in days:
    if len(d) > 6:
        break
    print(d)

Sunday
Monday

continue statements

The keyword continue causes a loop to immediately terminate the current iteration, and move to the next one.

days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

for d in days:
    if len(d) > 6:
        continue
    print(d)

Sunday
Monday
Friday

Nested loops

Loops can be nested. The following example uses nested for loops to count in binary from 0 to 7.

for i in range(2):
    for j in range(2):
        for k in range(2):
            print(f"{i}{j}{k}")

000
001
010
011
100
101
110
111

Counting with dictionaries

Dictionaries are a natural choice of data structure when you want to count different things.

Consider the following example with counting suits in a hand of playing cards.

# cards are represented by a tuple: `(value, suit)`
hand_of_cards = [("ace", "spades"),("jack", "diamonds"),("queen", "spades"),("four", "spades"), ("jack", "hearts")]


# initialize a dictionary for counting the suits, all starting at 0
suit_counts = {"clubs" : 0,
                "diamonds" : 0,
                "hearts" : 0,
                "spades" : 0}

# loop through the hand of cards, unpacking the tuple into value and suit variables
for value, suit in hand_of_cards:
    # increment the value located at key=suit
    suit_counts[suit] += 1


# print the final dictionary of counts
print(suit_counts)

{'clubs': 0, 'diamonds': 1, 'hearts': 1, 'spades': 3}