part-3-control-flow.md

Control Flow

Control flow is the idea of deciding when, how, what, and how many times code should run

if statements

Syntax: a set of if, elif (else if), and else blocks. The first condition that matches is run. elif can be repeated multiple times. If else is used, it must be the last block.

Whitespace is important! Indentation decides what is run when a condition is True.

# eg 1
if something is True:
    do_something()
elif something else is True:
    do_something_else()
elif some other thing is True:
    ...
else:
    print("none of the above blocks must have been True")

# eg 2
if True:
    do_something()

# eg 3
x = 4
if x < 3:
    print("x is less than 3")
elif x == 3:
    print("x equals 3")
else:
    print("x is greater than 3)

# eg 4
x = "momentum"
# if the first character is "a"
if x[0] == "a":
    print("starts with a")
elif x[0] == "b":
    print("starts with b")
elif x[0] == "c":
    print("starts with c")
else:
    print("it doesn't start with a, b, or c")

for loops

for loops allow you to repeat a statement some number of times. A common usage is

for i in range(5):
    print(i) # 0 1 2 3 4

Here, we're asking python to loop over a list of numbers from 0 to 4. In its simplest form, the range function produces a list of numbers from 0 to the argument, exclusive. For [0, 5), this is 0, 1, 2, 3, 4. (See official docs on range).

You can also iterate over the elements in a list

us_cities = ["Anaheim", "Boston", "Chicago", "Detroit", "El Paso"]

for city in us_cities:
    print(city + " is in the United States")

Alternatively, you can iterate over list indices

for c in range(len(us_cities)):
    # this uses f-string syntax
    # https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literals
    print(f"City {c}: {us_cities[c]}")

while loops

while loops allow you to write code that keeps running until some condition is met. For example, the two loops below are equivalent

for i in range(5):
    print(i)

i = 0
while i < 5:
    print(i)
    i = i + 1 # don't forget or you'll have an infinite loop!

When you need to write a loop, for should be your first instinct, but a while loop can come in handy too.

Exercise 3.1

Dictionaries can be elements in a list. Write a function, old_school, that takes the given list of dictionaries and outputs the number of universities that are more than 150 years old.

university_stats = [
    {
        "name": "MIT",
        "year_founded": 1861
    },
    {
        "name": "Harvard",
        "year_founded": 1636
    },
    {
        "name": "Vanderbilt",
        "year_founded": 1873
    },
    {
        "name": "Stanford",
        "year_founded": 1885
    },
    {
        "name": "Emory",
        "year_founded": 1836
    }
]

old_school(university_stats) # 3

Exercise 3.2

To append, or add, an element to the end of list, use the append function. For example,

drinking_apparatuses = ["cup"]
drinking_apparatuses.append("mug")
drinking_apparatuses # ['cup', 'mug']

Write a function, fibo, that takes a number n and uses a for-loop that produces the first n numbers of the Fibonacci sequence.

fibo(10) # [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

Exercise 3.3

You can slice strings, not just lists. Given the following list of names, write a function, ends_in_y, that prints only the names that end with a "y".

names = ["amy", "becky", "cyrano", "daisy", "evelyn", "franny", "ginny", "lydia"]

ends_in_y(names)
# amy
# becky
# daisy
# franny
# ginny

Exercise 3.4 (Harder than you might think)

Write a function, mode, that returns the mode of a list of numbers. If there is no mode, return None. Hint, there are many ways to solve this problem. You can use control flow logic alone or you can try using more advanced dictionary features like dict.setdefault and dict.get, and other built-in functions like max.

list_of_numbers = [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
mode(list_of_numbers) # 1

another_list = [1, 2, 3]
mode(another_list) # None

Head over to part-4-objects-classes to learn how to make your own types.