04-Review (Fundamentals)

04-Review (Fundamentals)#

Goals of this lecture#

This lecture is a quick refresher on Python fundamentals.

Variables.
Defining and using functions.
Conditional statements.
Loops.

Hopefully, much of this will be review from CSS 1 (and lab 1).

Variables#

A variable stores a particular value.

Unlike literals (e.g., an int), the value of a variable can change (i.e., it can vary).
Variables can be assigned using the assignment operator (=).
Once you’ve assigned a variable, you can use it by referencing that variable name.

## Example variable
my_int = 5

## Using this variable
my_int + 2

## Changing the value of the variable
my_int += 10
my_int

Rules on assigning variables#

Names on the left, values on the right (e.g., test_var = 2).
Names are case sensitive (the variable test_var cannot be accessed with test_VAR).
Variable names must begin with a letter.
- They can contain a number (e.g., test1) or under-score, but can’t begin with a number or under-score.
Python mostly doesn’t care how you name your variables, though you should!
- Remember that code is intended to be read by others––so make sure it’s clear!

`type`s of objects#

The value assigned to a variable can have different types.

Here are some of the possible types in Python:

Type	Description	Example
`str`	String/text	`"A String"`
`int`	Integer	`2`
`float`	Float	`2.6789`
`list`	List	`[1, 2, 3]`
`dict`	Dictionary	`{'a': 2}`
`bool`	Boolean	`True`
`NoneType`	None	`None`

Some `type`s have special functions#

In Python, different types of objects have different functions.

The str object has the replace and split function.
The list object has the append and join function.

my_str = "CSS 1"
my_str = my_str.replace("1", "2")
my_str

'CSS 2'

my_str.split(" ")

['CSS', '2']

Some `type`s are “collections”#

Both lists and dictionaries store collections of items.

### example list
my_list = [1, 2, "a"]
my_list

[1, 2, 'a']

### example dictionary
my_dictionary = {'a': 1, 'b': 2}
my_dictionary

{'a': 1, 'b': 2}

Functions#

A function is a re-usable piece of code that performs some operation (typically on some input), and then typically returns a result (i.e., an output).

A function is defined using the def keyword:

def func_name(arg1):
    ### Whatever the function does
    return some_value

Once a function has been created, you can use it.

Functions: a simple example#

The function square below returns the result of multiplying the input x by itself.

def square(x):
    return x * x

square(2)

square(3)

Check-in: functions vs. variables#

Suppose we define a function called cube, which cubes some input x. How would we call that function, e.g., on an input like 2?

cube
cube()
cube(2)
cube 2

Check-in: decoding a function#

What does the following function do?

def mystery_func(x, n):
    return x % n == 0

Solution#

Here, mystery_func checks whether x is divisible by n.

mystery_func(10, 2)

True

mystery_func(10, 3)

False

Default values#

A default value is the value taken on by an argument by default. If no other value is specified, this is the value assumed by the function.

In the function definition, a default value can be specified by setting: arg_name = default_value.

In the example below, x is required.
But n has a default value of 2.

def is_divisible(x, n = 2):
    return x % n == 0

is_divisible(10) ### Assumes n = 2

True

is_divisible(10, 3) ### Override with n = 3

False

Check-in: positional vs. keyword arguments#

An argument to a function can be indicated using its position or a keyword. Which of these is demonstrated in the code below?

Positional
Keyword
Both

is_divisible(24, n = 3)

True

Control Flow#

Control flow refers to tools we can use to control which lines of code get executed, when.

In Python, there are two main ways to control the “flow” of our program:

Conditional statements: if/elif/else
Loops: for/while

Conditional statements#

In a nutshell, a conditional is a statement that checks for whether some condition is met.

We can use the if command to control which lines of code are executed.

x = "One string"
y = "One string"
if x == y:
    print("These strings are the same.")

These strings are the same.

`else`#

An else statement tells Python what to do if an if statement evaluates to False.

x = "One string"
y = "Different string"
if x == y:
    print("These strings are the same.")
else:
    print("These strings are different")

These strings are different

Conditionals and functions#

Conditional statements become particularly useful when we combine them with functions.

def square_if_even(x):
    if x % 2 == 0:
        return x ** 2
    else:
        return x

Check-in#

How would you write a function fizzbuzz that:

Takes a number x as input.
returns “Fizz” if the number is divisible by 3.
returns “Buzz” if the number is divisible by 5.
returns “FizzBuzz” if the number is divisible by both 3 and 5.

### Your code here

Solution#

def fizzbuzz(x):
    if x % 5 == 0 and x % 3 == 0:
        return "FizzBuzz"
    if x % 3 == 0: 
        return "Fizz"
    if x % 5 == 0:
        return "Buzz"

Check-in#

How would you write a function called contains_letter that:

Takes both a word and a letter as inputs.
Checks whether the word contains that letter.
- if it does, it prints out “Yes”.
- Otherwise, it prints out “No”.

### Your code here

Solution#

def contains_letter(word, letter):
    if letter in word:
        print("Yes")
    else:
        print("No")

contains_letter("dog", "o")

Yes

contains_letter("cat", "o")

No

Loops#

A loop is a way to repeat the same piece of code multiple times.

When should you use a loop?#

Rule of thumb: if you find yourself copying/pasting the same code many different times…you might think about using a loop!

More generally: in programming, we often want to execute the same action multiple times.

Apply the same instruction to every item on a list.
Continue running some code until a condition is met.

`for` loops in action#

A for loop is used for iterating over a sequence.

A for loop uses the syntax:

for elem in list_name:
    # Do something

## This is a list in Python
numbers = [1, 2, 3]
### This is a for loop
for number in numbers:  
    print(number)

1
2
3

`for` loops and functions#

Like if statements, a for loop becomes especially powerful when combined with a function.

def multiply_list(numbers):
    product = 1 ## why do I start at 1, not 0?
    for i in numbers:
        product *= i
    return product ## note the indentation!

multiply_list([1, 2, 3])

Check-in#

Write a function called find_vowels, which:

Takes as input a str.
returns a list of the vowels in that string.

### Your code here.

Solution#

def find_vowels(s):
    vowels = []
    for c in s:
        if c.lower() in "aeiou":
            vowels.append(c)
    return vowels

find_vowels("programming")

['o', 'a', 'i']

find_vowels("cat")

['a']

Conclusion#

This was intended as a rapidfire review of Python fundamentals, including:

Basic Python syntax.
Defining functions.
Using if statements.
Using for loops.

Next time, we’ll review packages that will be helpful for working with data, such as pandas.

04-Review (Fundamentals)

Contents

04-Review (Fundamentals)#

Goals of this lecture#

Variables#

Rules on assigning variables#

types of objects#

Some types have special functions#

Some types are “collections”#

Functions#

Functions: a simple example#

Check-in: functions vs. variables#

Check-in: decoding a function#

Solution#

Default values#

Check-in: positional vs. keyword arguments#

Control Flow#

Conditional statements#

else#

Conditionals and functions#

Check-in#

Solution#

Check-in#

Solution#

Loops#

When should you use a loop?#

for loops in action#

for loops and functions#

Check-in#

Solution#

Conclusion#

`type`s of objects#

Some `type`s have special functions#

Some `type`s are “collections”#

`else`#

`for` loops in action#

`for` loops and functions#