Welcome to CS

The first program we saw on the last page is a bit repetitive. It compares weight to the number 1 twice, once to see if it is below that number, once to see if it is not below that number:

if weight < 1:
    price = 1.45
if weight >= 1:
    price = 1.15

In situations like this, where there are exactly two mutually exclusive options (we either have to do one or the other), there is an easier way to write the logic. That is to use else. else provides a way to say “if the condition for the preceding if was False, do this”. The body of the else - the code that will be executed if the if’s test fails - is indented after the else.

Using an if with an else makes sure that either the if block is executed or the else block is executed, but never both.

We never have to use an else. We can always write two separate if statements. But using else can help avoid bugs where there is a “gap” between the two options like the one we saw on the previous page.

Mixed up programs

x = input("Enter a number")
if x % 2 == 0:
 print("x is even")
else:
 print("x is odd")

def calculateShipping(orderCost, weight):
 if orderCost > 50:
  shipping = 0
 else:
  shipping = 5 + 0.75 * weight
 return shipping