Comparison/Relational Operators

• Generally, only applicable to the same type:
  • 3 > 4 # OK
  • '3' > 4 # TypeError
• We can compare floats to ints. The ints are “promoted” similar to how things worked with arithmetic operators:
  • 2.0 >= 1 # OK
• NEVER use == to compare floats. Floating point arithmetic is not exact, so the results can be confusing.
  • .1 + .1 + .1 == .3 # Evaluates to False

Boolean Operators

• Operands and results should be bools ('A' and 'B' is valid syntax, but don’t do it in this class.)
• and True if and only if both operands are True
• or True if either or both operands are True
• not (unary operator) changes True to False and vice-versa.

if Statements

if EXPRESSION:
   statement1
   statement2

Where EXPRESSION can be anything that evaluates to a Boolean. For example:

x = 10
if x < 100:
   print("X is small!")

if/else Statements

• Example:

      x = 10
      if x < 100:
          print("x is small!")
      else:
          print("x is big!")

• Exactly one branch will be executed.

if/elif statements

• Example

  x = 10
  if x < 20:
     print("x is tiny!")
  elif x < 100:
     print("x is small!")

• At most one branch will be executed.

if/elif/else Statements

• Example:

      x = 10
      if x < 20:
         print("x is tiny!")
      elif x < 100:
         print("x is small!")
      else:
         print("x is big!")

• Exactly one branch will be executed.

Sequential if Statements

• Example:

      x = 10
      if x < 20:
         print("x is tiny!")
      if x < 100:
         print("x is small!")

• No connection between the branches. Some, all, or none may occur depending on the conditions.

Nested if

x = 50
if x < 20:
   print("x is tiny!")
   if x < 100:
       print("x is small!")

What will be printed?

Inappropriate Use of Nesting

• Bad style:

  if a == 0:
      if b == 0:
          if c == 0:
              print("All are zero!")

• Instead:

  if a == 0 and b == 0 and c == 0:
      print("All are zero!")

Comparing to Booleans

• Bad Style:

  if rich == True and age < 25:
      print("Very!")
  else:
      print("Sort of.")

• Instead:

  if rich and age < 25:
      print("Very!")
  else:
      print("Sort of.")