Python / Making decisions

elif chains

Choose between more than two paths with an if, elif, else chain, and put the conditions in the right order. Baccalaureate

if - else gives only two paths. But a cinema ticket might be number 1, 2 or 3, each with its own prize. To add another condition use elif, short for "else if": otherwise, if...

Python 
ticket = 2
if ticket == 1:
    print("Free drink")
elif ticket == 2:
    print("Includes popcorn")
elif ticket == 3:
    print("Mini snack set")
Console
Includes popcorn

Add as many elif lines as you need, then an optional else at the end for every case no condition matched:

Python 
ticket = 10
if ticket == 1:
    print("Free drink")
elif ticket == 2:
    print("Includes popcorn")
else:
    print("No prize")
Console
No prize

How the chain runs

The computer checks the conditions from top to bottom and stops at the first True one; it runs that block and skips the rest of the chain. So at most one block runs:

Python 
cup = 0
if cup <= 10:
    print("10 or fewer")
elif cup <= 5:
    print("5 or fewer")
elif cup <= 3:
    print("3 or fewer")
Console
10 or fewer
All three conditions are True when cup = 0, but only the first prints. So put the narrowest condition first: == 0, then <= 3, then the rest.
Python 
cup = 2
if cup == 0:
    print("We need to go buy some!")
elif cup <= 3:
    print("It's almost gone")
else:
    print("There is still some")
Console
It's almost gone

A chain always starts with one if, and else, if present, comes last. Every elif has a condition; else does not.

Practice

1Edit the programCurriculum practice

Add at the end: if ticket equals 4, print Goods voucher; if no condition matches, print No prize.

Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor 
ticket = 10
if ticket == 1:
    print("Free drink")
elif ticket == 2:
    print("Includes popcorn")
elif ticket == 3:
    print("Mini snack set")
Console
Press Run to see the result.
2Write the programCurriculum practice

Assign 3 to people. If people equals 2, print 10% off; if it is 3 or more, print 20% off; otherwise print Regular price.

Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor 
  
 
Console
 
Press Run to see the result.
 
   
 
 
3Fill in the blankCurriculum practice
 
Complete the elif line: if score is 80 or more, print Pass.
  
 
 Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor     
 
score = 82
if score == 100:
    print("Perfect score")
___
    print("Pass")
  
 
Console
 
Press Run to see the result.
 
   
 
 
4Write the programCurriculum practice
 
Assign 5 to egg. If egg equals 0, print Out of stock; else if it is 3 or less, print Low stock; otherwise print In stock.
  
 
 Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor     
 
  
 
Console
 
Press Run to see the result.
 
   
 
 
5Write the programCurriculum practice
 
Assign 77 to english. If it is 100, print Perfect score; else if it is 75 or more, print Pass; otherwise print Make-up exam.
  
 
 Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor     
 
  
 
Console
 
Press Run to see the result.
 
   
 
 
6Edit the programExtra practice
 
The program prints C for every mark of 65 or more because the conditions are in the wrong order. Reorder them (A from 85, B from 75, C from 65, otherwise D) so it prints B for a mark of 78.
 
HintStart with the narrowest condition: the highest mark first.
 
 
 Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor     
 
mark = 78
if mark >= 65:
    print("C")
elif mark >= 75:
    print("B")
elif mark >= 85:
    print("A")
else:
    print("D")
  
 
Console
 
Press Run to see the result.
 
   
 
 
7Write the programExtra practice
 
Assign 38 to temp. If it is 35 or more, print Very hot: drink water; else if 25 or more, print Nice day; else if 15 or more, print Take a jacket; otherwise print Cold: stay warm.
  
 
 Python Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor     
 
  
 
Console
 
Press Run to see the result.
 
   
 
 
Key points
  • Shape: if, then any number of elif condition:, then an optional else: at the end.
  • Conditions are checked from the top, and only the first True one runs.
  • Put the narrowest condition first so a wider one does not hide it.