📝Week 6 - Lab Intro

In this lab introduction we will briefly discuss the topic of looping, which is the focus of the assigned homework.

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)

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)

with 2 arguments: (start, stop, 1)

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

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

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

looping over a list

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

for d in days:
    print(d)

looping over a string

message = "Hello World!"

for letter in message:
    print(letter)

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)

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)

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

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

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

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.")  

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)

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)

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}")