Lecture 2, Thu 01/11

Basic Input / Output, Variables and Assignments, Python Data Types

Recorded Lecture: 1_11_24

Print Functions

• In general, functions are pieces that perform a specific functionality.
• For example, the print() function will display contents inside its parenthesis (())
  • We will go more in-depth in various functions and creating our own later in this course
• Example

print("Hello CSW8!")
print("Python is amazing!")

• The text in between double-quotes (") is what we call a string
  • Think of a string as a collection of characters (something we can type on the keyboard)
  • Technically, single quotes (')can also be used to represent a string (and the interactive shell will display strings with single quotes, but print functions display strings without quotes)
• Note that each of these print statements display on a separate line
• There’s a special character called a newline character (\n) inserted at the end of the print function by default (more on some other special characters later)
• So, we could achieve the same output with the following single print function:

print("Hello CSW8!\nPython is amazing!")

• Whenever a newline character is displayed, it moves the rest of the string to a new line in the output
• We can also print multiple strings in a single print function:

print("Hello CSW8!", "Python is amazing!", "Keep the strings going!")

• Note that each string separated by commas in the print function is separated by a single whitespace (" ") character in the outout by default
• Sometimes, we may not want a newline character at the end of our print function, so we can explictly tell Python what we want at the end (or nothing). For example:

print("Hello CSW8!", end="") # note: "" is an empty string
print("Python is amazing!")

• Also, we may not want a whitespace character separating our strings in a print function, so we can explicitly tell Python not to use anything. For example:

print("Hello CSW8!", "Python is amazing!", "Keep the strings going!", sep="")

Python Data Types

• Python gives us many types to use out-of-the-box
  • Types are important depending on what you need to do with data
  • Some types support certain functionality while others don’t
  • Using certain types can affect the accuracy / performance of your program

Some Common Python Data Types

• int: integers representing non-decimal value (whole numbers)
• float: floating point numbers representing a fractional value
• str: string representing a collection of characters (or no characters (empty string ("")))
  • Examples: "A", "a", "1", "-", “abc”
• And many more types we’ll cover soon!

type() Function

• We can use the type() function to determine what type a value is
• Example (don’t worry about the class keyword for now):

print(type(1)) #int
print(type("1")) #str
print(type(1.0)) #float

Some Numerical Operators

• Python gives us certain operators to perform on numerical types
  • Examples: +, -, *, /, **, //, %
• Follows algebraic order-of-operations (PEDMAS) (Python org: https://docs.python.org/3/reference/expressions.html#operator-precedence)
• Example

  print(2 + 3 * 3 + (2 - 1))
  

• Either ints or floats are returned depending on the values used
• Example

print(2 + 2) #4 (int)
print(2 + 2.0) #4.0 (float)
print(2 - 1) #1 (int)
print(2 - 1.0) #1.0 (float)
print(2 * 3) #6 (int)
print(2 * 3.0) #6.0 (float)
print(1 / 2) #0.5 (float)
print(2 / 2) #1.0 (float - doesn't matter if using only ints)
print(2 ** 3) #8 (int)
print(2 ** 3.0) #8.0 (float)
print(10 // 3) #3 (int)
print(10.0 // 3) #3.0 (float)
print(10 % 3) #1 (int)
print(10.0 % 3) #1.0 (float)

Variables and Assignments

• Variables are useful for assigning values of any type
• Names of variables (known as identifiers) must:
  • Start with a letter or underscore (_) (former is more common)
  • Remaining letters in variable names can consist of letters, numbers, or underscores
• Variable names are case-sensitive (x and X are considered two different variables)
• Assigning values to variables can be broken into left-hand-side (variable) and right-hand-side (expression) using the assignment operator (=)
  • Right-hand-side expressions are computed first to produce a value
  • This value then gets assigned to the variable (left-hand-side)
• Example

x = 10
print(x)
x = x * 10 #10 * current value of x replaces existing value of x
print(x) #100

Another Note on Operators and Types

• Some operators work with non-numerical types like strings. For example:

print("CSW" + "8") #CSW8 (concatenation)

• But what happens when we add a number to a string?

print("CSW" + 8) #ERROR!

• Always knowing the types you’re working with are extremely important in Python!
• If you’re not careful, this can lead to unintended side-effects / errors

Conversion Functions

• We can convert some types to other types using conversion functions
  • Such as int(), float(), str()
• Example

x = "10.0"
print(type(x)) # string type

x = float(x)
print(type(x)) # float type

x = int(x) #OK, removes decimal portion from float number
print(type(x)) # int type

x = str(x)
print(type(x)) # string type

# When converting a string to an int, it doesn't expect the decimal portion
# in the string
x = "10.0"
x = int(x) # Error

Input Function

• Useful programs generally require input from the user
  • Imagine if your web browser ONLY took you to https://www.ucsb.edu !
• There are many forms of input into a program (keyboard, mouse movement / clicks, files, …)
• For an interactive shell application, we can accept user keyboard input using the input function (input())
  • When the input function is called, it displays what’s inside, then waits until the user enters something and hits return
  • Whatever the user typed before hitting return is inputted into the program as a string type
    • Important to convert this string type to an appropriate numerical type if necessary
• A tip calculator example:

TAX_RATE = 0.1
userName = input("Hi, please enter your name: ")

print("Hi,", userName, ". What’s the amount of your bill (not including tax and tip)?")
totalBill = float(input()) #take the input() string and convert it to a float.

print("What tip percentage would you like to leave?")
tipPercentage = float(input())

taxAmount = totalBill * TAX_RATE
tipAmount = totalBill * (tipPercentage / 100)
print("The total amount to pay is $", totalBill + taxAmount + tipAmount)