Lesson 3 Notes: Variables, Expressions, Statements

• Overview
  • Values
  • Numbers in Python
  • Variables
  • Operations
  • Composition
  • Comments
  • Input
• Values
  • Strings
    • Sets of text, usually
    • Denoted by “quotation marks”
  • Integers
    • Whole numbers, no decimal places
  • Floating-point
    • Numbers with decimal point values
  • Long Integers
    • Numbers larger than about 2 billion
• Numbers
  • Integers
    • Cannot be larger than fixed size (about 2 billion on 32-bit system using +/-)
    • When divided, give only whole number results
  • Long Integers
    • Signified by L at end of number (23434235234L)
• Working with Numbers
  • Use the type you want the results in.
    • If you want decimal values, use at least one decimal number in computation.
    • Example: 15 / 2.0 will yield 7.5, 15/2 will yield 2
  • If you want large values, use at least one Long in number computation
    • Example: 23423908423908430L / 2
• Strings
  • Strings must be inside single or double quotation marks
    • name = “This is your name”
    • name2 = ‘This is also your name.’
  • If the string needs quotation marks, precede the extra “ with a \.
    • title = “He said \”I Love You\” “
    • title2 = ‘It\’s So Nice’
• String Backslash Characters
  • \\ = Include backslash
  • \’ = Single quote
  • \” = Double quote
  • \n = New line
• Variables
  • Variable = Name that refers to a value
  • Limitations
    • Cannot be a keyword (i.e. print, and, or, not)
    • Cannot start with a number
    • Case sensitive
    • Cannot include illegal characters (i.e. $, %, +, =)
  • Create variable names that mean something
    • Bad: diy = 365
    • Good: days_in_year = 365
• Math Operations
  • Basic operations:
    Add +, Subtract -, Multiply *, Divide /, Exponent **, Modulus %
  • Order of precedence
    • Parenthesis, Exponents, Multiply/Divide, Add/Subtract
    • Left to right
    • 2*(3-1) = 4
    • 2*3-1=5
• String Operations
  • Concatenation: Adding two strings together
    • x = “Hello”
    • y = “World”
    • print x+y
    • >>>Hello World
  • Repetition: Repeating a string
    • z = “Howdy”
    • print z*3
    • >>>Howdy Howdy Howdy
• Composition
  • Composition is the ability to combine simple statements into compound ones
  • Can be used with commands
    • x = “mud”
    • print “Your name is:”, x
    • >>>Your name is: mud
  • Can be used with assignments
    • average = (scores + extra_credit)/possible
• Comments
  • Comments are notes you write to help explain what is going on in the program
  • Comments begin with #
  • Everything after # is ignored by interpreter
    • days=12 #Days available for project
  • Use comments to help you remember
  • First lines of codes should always have your name in a comment and brief description of program
• Input
  • For strings, use raw_input()
    • name = raw_input(“What is your name? “)
  • For numbers, use input()
    • number = input(“How many? “)
  • For both, write prompt in quotations inside the parenthesis. The prompt is optional.
• Review
  • Describe the following values:
    Strings, integers, floating point, longs
  • Why does the division below yield 0?
    print 9/10
  • Explain the guidelines for naming variables.
  • What operations are available on strings?
  • How are comments created and used?
  • What is the difference between raw_input() and input()?