CS50's Introduction to Programming with Python: Lesson 0

LECTURE 0

>Creating Code with Python<
*VS Code - Special type of text editor, a compiler
         - Top: Text Editor
         - Bottom: Terminal
*In terminal: Execute ``code hello.py`` to start coding
*In text editor: Type ``print("hello world")`` - Famous canonical program that nearly all coders write during the learning process
*In terminal window: Can execute commands
                     To run the program, cursor needs to be moved to the bottom of the screen. Type ``python hello.py`` next to the dollar sign to type a second command in the terminal and press the enter key
*Result of running ``python hello.py`` program: ``hello, world``

>Functions<
*Functions - Verbs or actions that the computer will already know how to perform
*``Print`` function - Knows how to print to the terminal program
                    - Takes arguments

>Bugs<
*Bugs - Natural part of coding

>Improving Your First Python Program<
*``input`` - Function that takes a prompt as an argument

        input("What's your name? ")
        print("hello, world")

>Variables<
*Variable - Container for a value within your own program

    *To introduce the variable in the program:

    name = input("What's your name? ")
    print ("hello, world")

        ``=`` - Assigns what is on the right to what is on the left
              - Value returned by ``input("What's your name? ")`` is assigned to ``name``

    *Further editing of the code:

    name = input("What's your name? ")
    print("hello,")
    print(name)

        >Result of the terminal window:
            What's your name? |David|
            |hello|
            |David|

>Comments<
*Comments - Way for programmers to track what they're doing in their programs; Notes

    # Insert comment here

>Pseudocode<
*Pseudocode - Important type of comment that becomes a special type of to-do list, especially when you don't understand how to accomplish a coding task

    # Ask the user for their name
    name = input("What's your name? ")

    # Print hello
    print("hello,")

    # Print the name inputted
    print(name)

>Further Improving Your First Python Program<

    *Can further edit the code as follows:

    # Ask the user for their name
    name = input("What's your name? ")

    # Print hello and the inputted name
    print("hello, " + name)

    *Can use a comma ``,`` to pass in multiple arguments by editing the code as follows:

    # Ask the user for their name
    name = input("What's your name? ")

    # Print hello and the inputted name
    print("hello, ", name)

>Strings and Parameters<
*String ``str`` - Sequence of text
*``print`` function - Automatically include a piece of code ``end='\n'``
                    - Takes an argument called  ``end`` and the default is to create a new line
*``\n`` - Indicates that the print function will automatically create a line break when run
*Can also provide an argument for ``end`` such that a new line isn't created

    *Can modify the code as follows:

    # Ask the user for their name
    name = input("What's your name? ")
    print("hello, ", end="")

*``end=""`` - Over-writes the default value of ``end``, making it not create a new line after this first print statement
*Parameters - Arguments that can be taken by a function

>A Small Problem with Quotation Marks<
*Adding quotation marks as part of the string is challenging as ``print("hello,"friend"")`` won't work and compiler will throw an error
    >2 Approaches to Fix the Issue:
        1. Change the quotes to single quote marks
        2. Code as ``print("hello, \"friend\"")``
            ->Backslashes tell the compiler that the following character should be considered a quotation mark in the string

>Formatting Strings<
    *Possibly the most elegant way to use strings would be as follows:

    # Ask the user for their name
    name = input("What's your name? ")
    print(f"hello, {name}")

        ->``f`` - Special indicator to Python to treat this string a special way, different than previous approaches

>More on Strings<
*Built into strings is the ability to remove whitespace from a string
*Method ``strip`` on ``name`` as ``name = name.strip()`` - Strips all the whitespace on the left and right of the user's input:

    # Ask the user for their name
    name = input("What's your name? ")

    # Remove whitespace from the str
    name = name.strip()

    # Print the output
    print(f"hello, {name}")

*``title`` method - Title case the user's name:

    # Ask the user for their name
    name = input("What's your name? ")

    # Remove whitespace from the str
    name = name.strip()

    # Capitalize the first letter of each word
    name = name.title()

    # Print the output
    print(f"hello, {name}")

*Can up arrow to recall the most recent terminal commands that was made
    *Code can be more efficient:

    # Ask the user for their name
    name = input("What's your name? ")

    # Remove whitespace from the str and capitalize the first letter of each word
    name = name.strip().title()

    # Print the output
    print(f"hello, {name}")

*An even more efficient way:

    # Ask the user for their name, remove whitespace from the str, and capitalize the first letter of each word
    name = input("What's your name? ").strip().title()

    # Print the output
    print(f"hello, {name}")

>Intergers or int<
*``int``  - Interger

    x = 1
    y = 2

    z = x + y

    print(z)

    *Can make it more interactive using the ``input`` function:

    x = input("What's x? ")
    y = input("What's y? ")

    z = x + y

    print(z)

    *Needs to convert this input from a string into an integer:

    x = input("What's x? ")
    y = input("What's y? ")

    z = int(x) + int(y)

    print(z)

*Using int(x) is "Casting" - Where a value is temporarily changed from one type of variable [string] to another [interger]

    *Can further improve the program:

    x = int(input("What's x? "))
    y = int(input("What's y? "))

    print (x + y)

    *Most inner function is run first, then the outer one is run

>Float Basics<
*Floating Point Value - Real number that has a decimal point in it
    *Can change the code to support floats:

    x = float(input("What's x? "))
    y = float(input("What's y? "))

    print (x + y)

*``round`` - Available argument: ``round(number[n, digits])``
                                    - Square brackets indicate that something optional can be specified
                                    - Could do ``round(n)`` to round a digit to its nearest interger
    *Alternatively, code can be as follows:

    # Get the user's input
    x = float(input("What's x? "))
    y = float(input("What's y? "))

    # Create a rounded result
    z = round(x + y)

    # Print the result
    print(z)

    *Formatting the output of long numbers, from 1000 to 1,000:

    # Get the user's input
    x = float(input("What's x? "))
    y = float(input("What's y? "))

    # Create a rounded result
    z = round(x + y)

    # Print the result
    print(f"{z:,}")

>More on Floats<
    *To round floating point values:

    # Get the user's input
    x = float(input("What's x? "))
    y = float(input("What's y? "))

    # Create a rounded result [to the nearest 2 decimal points]
    z = (x / y, 2)

    # Print the result
    print(z)

    *Can use ``fstring`` to format the output below:

    # Get the user's input
    x = float(input("What's x? "))
    y = float(input("What's y? "))

    # Calculate the result
    z = x / y

    # Print the result
    print(f"{z:.2f}")

        *``fstring`` - Displays the same as the prior strategy

>Def<
    *Starting from scratch:

    name = input("What's your name? ")
    hello()
    print(name)

        >Compiler will error because ``hello`` has no defined function

    *Creating own function called ``hello``:

    def hello():
        print("hello")

    name = input("What's your name? ")
    hello()
    print(name)

        >Python is an idented language - Uses idention to understand what is part of the above function
                                       - Everything in the ``hello`` function must be idented

    *Can further improve code:

    # Create own function
    def hello(to):
        print("hello,", to)

    # Output using own function
    name = input("What's your name? ")
    hello(name)

        >First lines: Creating ``hello` function; however, compiler is being told that this function takes a single parameter: a variable called ``to``
            -> How values are passed into functions

    *Can add a default value to ``hello``:

    # Create or own function
    def hello():
        print("hello,", to)

    # Output using own function
    name = input("What's your name? ")
    hello(name)

    # Output without passing expected arguments
    hello()

    *Functions don't need to be at the start of the program; they can be moved down but compiler needs to be told that there's a ``main`` function and a separate ``hello`` function:

    def main():

        # Output using own function
        name = input("What's your name? ")
        hello (name)

        # Output without passing expected arguments
        hello ()

    # Create own function
    def hello(to="world"):
        print("hello,",to)

        >However, this alone will create an error as nothing in the code is actually running the ``main`` function

    *To call the main function:

    def main():

        # Output using own function
        name = input("What's your name? ")
        hello (name)

        # Output without passing expected arguments
        hello ()

    # Create own function
    def hello(to="world"):
        print("hello,",to)

    main()

>Returning Values<
*``return`` value   - "Passing back" of a value
                    - Function to return the calculation's value back to another part of the program

    *Calculator.py; rework the code as follows:

    def main():
        x = int(input("What's x? "))
        print("x squared is", square(x))

    def square(n):
        return n * n

    main()

        >``x`` is passed to ``square``; Calculation of ``x * x`` is returned back to the main function