Operators, Control Statements, and Loops

Assignment cont.

• Python is sort-of pointer-y
• example:

   x = []
   y = x
   y.append("frog")
   print(x) #["frog"]
   print(y) #["frog"]
  

Operators

• Arithmetic
• +,-,/,*
• / <- floating-point division
  • 5/2 -> 2.5
  • 5.5/3.3 -> 1.66667
• // <- floored quotient
• ** - power
• +=, -=, /=, //=, *=, **=

• There is no ++ nor --

• relational
  • ==,!=,>=,<=,<,>
• logical
  • not -> like ! (there is no ! in Python)

  • or -> like

  • and -> like &&
• membership operator
  • in
  • returns True or False
  • x in y returns True if x is a member in y, otherwise returns False
  • Example:

      x = [ 1,2,3,4 ]
      4 in x #True
      50 in x #False
      y = {'a':10,'b':50}
      'a' in y #True
      50 in y #False, 50 is not a key
      50 in y.values() #True
    

• is
  • Returns True or False
  • Tests object identity
  • Don’t use it unless you have a good reason to.
  • not the same as ==
  • okay to use to compare with None
  • Pyflakes recommends using is with True and False
    • don’t

Control Statements

• if / elif / else
  • Example:

    if x == 5:
      stuff() #called the body
    elif x == 6:
      stuff()
    else:
      stuff()
    

  • No parentheses around expression
  • don’t forget colons
  • no curly braces -> watch your indentation
    • Python is whitespace delimited

Truthiness/Falsiness

• Falsey values
  • False
  • None
  • numeric zero
  • ””
  • empty built-in data structures
• Truthy values
  • True
  • not Falsey
• bool()
  • bool(truthy) #True
  • bool(falsey) #False
• be careful when comparing against True and False using ==
  • 1 == True #True
  • [] == False #False
  • bool([]) == False #True
  • aside from numeric types, objects of different types do not compare equal
  • Prime test material

Loops

• while
  • pre-check loop
    • no post-check loop
  • leave parentheses off of expression
  • don’t forget the colon
• for
  • different than c++
  • Iterable -> can iterate with for
    • Iterator object can yield/return its members one at a time
    • File objects are iterable
  • for <var> in <iterable>:
  • for x in range():
  • leave parentheses off of expression
  • don’t forget the colon
  • break
    • breaks out of the innermost loop
  • continue
    • continues to next iteration of loop
  • pass <- placeholder
  • else
    • executes if loop completes uninterrupted