Ifs and Booleans

if statements

Simplest form:

	if expression:
	    body

• Evaluate expression
• If expression's value is True, then execute body
• Otherwise, skip body

General form:

	if expression1:
	    body1
	elif expression2:
	    body2
	# ...
	elif expressionN:
	    bodyN
	else:
	    body

• Evaluate expression1
• If expression1's value is True, then execute body1
• Otherwise, evaluate expression2
• If expression2's value is True, then execute body2
• Otherwise, ...
• Otherwise, evaluate expressionN
• If expressionN's value is True, then execute bodyN
• Otherwise, execute body
• Important: expression2 is evaluated ONLY if expression1's value was False, and similarly for all other expressions— in particular, the final body is executed only if every expression evaluated to False

Compare:

	if sunny:
	    print("It's lovely out!")
	elif warm:
	    print("At least it's not cold.")

with:

	if sunny:
	    print("It's lovely out!")
	if warm:
	    print("At least it's not cold.")

Booleans

• If-statements need a condition: a yes/no value that determines what to perform next.
• We call a yes/no value a boolean. (Comes from George Boole, a philosopher/logician of the early 19th century.)
• So far, we've seen conditions like

  my_average >= 90

  This is a boolean expression: its value is of type bool.
  >= is a boolean operator: it takes two operands and gives a bool back.
• There are lots of other boolean operators in Python.
• Comparison operators: operators that compare two things and give back a boolean

  	3 < 4
  	3 > 8
  	8 > 3
  	3.5 >= 3.5
  	7 == 7   # Why not just one equals sign?
  	x == 7
  	y == 7.0
  	x == y
  	3 != 4
  

• Logical operators: operators that take two booleans and give back a boolean

  	snowing = False     # The basic boolean values need a capital
  	sunny = True
  	not snowing         # "not" is a unary operator: 1 operand
  	not sunny
  	sunny and snowing   # "and" and "or" are binary operators: 2 operands
  	sunny or snowing
  	a = False
  	b = True
  	not a or b          # precedence?
  	(not a) or b
  	not (a or b)        # "not" has priority: it's like "-"
  	-3 + 7
  	(-3) + 7
  	-(3 + 7)
  	not not a           # double negative
  	not not not a       # or more
  

• Of course, we can combine these

  	x < 0 or x > 100
  

• Python allows chaining, e.g.,

  	0 < x <= 20
  

  is a shorthand for

  	0 < x and x <= 20
  

  NOTE: The original version of these notes mistakenly used or instead of and to combine the two inequalities— and is the correct operator

Boolean representation

• False is stored internally as 0, and True as 1, but you should never need to use this directly
• Python gives a Boolean value to many different types— you can easily figure out whether or not a value is treated as True by using it in an if-statement

Tips for writing code with booleans

• If you are unsure of precedence, use brackets! That way, you won't make a mistake, and the reader will probably appreciate it.
• Use the simplest expression possible
• Example 1: Avoid double negatives like not not a; a is much clearer
• Example 2: A classic stylistic error

  	# Suppose you have a boolean variable below_zero.
  	if below_zero == True:
  	    print("Brrrr!")
  	
  	# That was analogous to saying:
  	if (temperature < 0) == True:       # Obviously silly, but equivalent!
  	    print("Brrr!")
  	
  	# below_zero is already a boolean (just like "temperature < 0");
  	# there's no need to make it into one.  This is simpler:
  	if below_zero:
  	    print("Brrr!")
  

• Example 3: Another classic

  	def is_odd(x):
  	    if x % 2 == 1:
  	        return True
  	    else:
  	        return False
  

  can be much simplified:

  	def is_odd(x):
  	    return x % 2 == 1