Else And If Python

Else And If Python

In the last exercise, you worked out some if- statements and then tried to guess what they are and how they work. Before you learn more, I’ll explain what everything is by answering the questions you had from the Study Drills. You did the Study Drills, right?

  1. What do you think the if does to the code under it? The if- statement tells your script, “If this boolean expression is True, then run the code under it, otherwise skip it.”
  2. Why does the code under the if need to be indented four spaces? A colon at the end of a line is how you tell Python you are going to create a new “block” of code, and then indenting four spaces tells Python what lines of code are in that block. This is exactly the same thing you did when you made functions in the fi rst half of the book.
  3. Python expects you to indent something after you end a line with a : (colon).
  4. Can you put other boolean expressions from Exercise 27 in the if- statement? Try it. Yes, you can, and they can be as complex as you like, although really complex things generally are bad style.
  5. What happens if you change the initial values for people, cats, and dogs? Because you are comparing numbers, if you change the numbers, different if- statements will evaluate to True, and the blocks of code under them will run. Go back and put different numbers in and see if you can figure out in your head what blocks of code will run. 
1 people = 30
2 cars = 40
3 buses = 15
4
5
6 if cars > people:
7 print "We should take the cars."
8 elif cars < people:
9 print "We should not take the cars."
10 else:
11 print "We can't decide."
12
13 if buses > cars:
14 print "That's too many buses."
15 elif buses < cars:
16 print "Maybe we could take the buses."
17 else:
18 print "We still can't decide."
19
20 if people > buses:
21 print "Alright, let's just take the buses."
22 else:
23 print "Fine, let's stay home then."

Output

We should take the cars.
Maybe we could take the buses.
Alright, let's just take the buses.

Study Drills

  1. Try to guess what elif and else are doing.
  2. Change the numbers of cars, people, and buses, and then trace through each ifstatement to see what will be printed.
  3. Try some more complex boolean expressions like cars > people and buses < cars.
  4. Above each line, write an English description of what the line does.

What happens if multiple elif blocks are True?

Python starts at the top and runs the first block that is True, so it will run only the first one.

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