11.5. Using if and else

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.

weight = 0.5 if weight < 1: price = 1.45 else: price = 1.15 # happens if weight is NOT < 1 total = weight * price print(weight) print(price) print(total)

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

Flow of execution for both an if and else

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

Mixed up programs

The following program should print out “x is even” if the remainder of x divided by 2 is 0 and “x is odd” otherwise, but the code is mixed up. Remember that the % symbol gives the remainder after the first number is divided by the second number. Drag the blocks from the left and place them in the correct order on the right. Be sure to also indent correctly!

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

The following function should calculate the shipping cost for an order. Orders over $50 ship free. Otherwise, it is $5 base shipping plus $0.75 per pound. Drag the blocks from the left and place them in the correct order on the right. Be sure to also indent correctly!

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

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