Lecture 2, Tue 10/02

Elements of simple programs in Python

Resources from lecture

• Link to all code written in lecture

Announcements:

• Complete ic00 and bring it to your lab section tomorrow
• During the first 10 minutes of the lab, try to find a suitable lab partner
• My office hours: Wed 3:30 - 5:00 pm (HFH 1155)

Concept Questions

• Check your understanding: Lect 02 Concept Questions
• You’ll need your iclickers to participate in class

Today’s lecture:

• Python as a calculator
• Storing data using variables
• More than just a calculator (working with other data types)
• Interacting with a program using print and input

Python as a calculator

• Numerical data types
  • Integer representing non-decimal values
  • float: Floating point number representing a decimal (fractional) value
• Operations with numeric types
  • Arithmetic (+ - * /), Comparison(== < > <= >=)
• Evaluating expressions:
  • Just like writing math expressions
  • Mixed types are okay

Numerical type examples (type these in the shell to see what happens)

>>> 1

>>> 1+2

>>> 4/2

>>> 1/3

>>> 6/2

>>> 1/0

>>> 1+2*4

Storing data using variables

• Think of a variable as a box with a name
• Just like you can store things in a box, you can store values in a variable (for now we’ll only store numeric values, but you can store other types as well)
• Names of variables must…
  • Start with a letter or underscore (the former is more common)
  • Remaining letters in variables names can consist of letters, numbers, or underscores
  • Names are case-sensitive (name and NAME are considered two different variables).

>>>x = 10 #What is the value of x?

>>>x = x * 10 # 10 * current value of x is stored back into x

More than just a calculator

• Other data types
  • int: Integer representing non-decimal values
  • float: Floating point number representing a decimal (fractional) value.
  • string: Represents a collection of characters
    • Examples of characters: ‘A’, ‘a’, ‘1’, ‘ ‘, …
  • bool: Evaluates to either True or False
    • Ex: 4 <= 6 True
    • Ex: 1 == 2 False
  • Note: 3 and 3.0 are considered different types
    • 3 is an integer
    • 3.0 is a float
  • Python knows what type these numbers are based on its value.
• Numerical type examples

x = 1
print(x)
print(type(x))

x = 4 / 2
print(x)
print(type(x))

y = 4 * 2
print(y)
print(type(y))

z = 4 * 2.0
print(z)
print(type(z))

x = "CS 8"
print(x)
print(x*2)

x = "8.0" # string not float
print(x) #8.0
print(type(x)) #str

print(x + 2) #ERROR
print(x + "2") # No error, uses concatenation

print(float(x) + 2) # No error, 10.0

x = "8.0" # Be careful ...
print(int(x)) #crashes

x = "8"
print(int(x))

x = "8.0"
y = "8.0"
z = "8.00"

print(x == y) #True
print(x == z) #False
print(float(x) == float(z)) #True
print(2 * 3 > 5) #True
print(type(2 * 3 > 5)) # bool

• Indexing strings and substrings
  • In a string, we can extract certain pieces from it.
  • This is known as “parsing” a string
  • Positions in a string start at index 0

schoolName = "UCSB"
print(len(schoolName)) # 4
print(type(schoolName)) # str
print(schoolName[0])
print(schoolName[3])
#print(schoolName[4]) #ERROR
print(schoolName[-1]) # B - refers to the last index
#print(schoolName[-5]) # ERROR

#Extract a substring
print(schoolName[1:3]) # from position 1 up to (but not
                       # including) position 3
                       
# compute salary
hours = 40
rate = 10
salary = hours * rate
print("Salary is $", salary) # Notice quotes aren't displayed on the string in the outpout

Interacting with a program using print and input

• When we write software, we’re modeling the real world
  • … or at least we do it as best we can
  • You can think of everything with respect to things and actions.
    • Things (nouns) - Objects
    • Actions (verbs) - Functions, operators, …
  • Python (and generally all languages) gives some way to represent and combine these.
  • There are also ways to interact with the program
    • Generally, we call this “user input”
    • Reading files
    • clicking on buttons
    • keyboard characters
  • In order to interact with our program using text, Python has the input() function for us to use

# Example
print("Hi, please enter your name: ")
userName = input()
print("Hello", userName)


# We use something called ESCAPE CHARACTERS to print
# special characters including quotes.

print("\"Hi!\"") # \" is the double quote characters
print("Hi\nthere\n!") # \n is the newline character


# Another larger example
TAX_RATE = 0.1
print("Hi, please enter your name: ")
userName = input()
print("Hi", userName, ". What is the amount of your bill \
(not including tax and tip)?")
totalBill = float(input()) #be careful, will crash if not float
print("What is the tip percentage you would like to leave?")
tipPercentage = float(input())
taxAmount = totalBill * TAX_RATE
tipAmount = totalBill * (tipPercentage / 100)
print("The total amount to pay is $", totalBill + taxAmount + tipAmount)