if/elif/else statements

if statements have multiple ways that we can customize them (other than changing the logical expression): the second of these ways is by adding an elif clause to our statement.

The syntax of if/elif/else statements is:

if logical_expression:
    # do something
elif logical_expression2:
    # do something different
else:
    # do something different again

Let’s take a look at our previous example code again.

# TODO: run this code!
# Under what circumstances is "big number" printed out?
# What about "small number"? What about "medium number"?
# Answer:

number = int(input("number? "))

if number > 100:
    print("big number")
elif number < 0:
    print("small number")
else:
    print("medium number")
    
print("done")

if/elif/else statement as a flowchart

Notice that for each branch in our if/elif/else statement, we only take that branch if all of the previous conditional statements evaluated to False.

# TODO: run this code!
# What is different about this statement from the
# previous example?
# Answer:

number = int(input("number? "))

if number > 1000:
    print("extra big number")
elif number > 100:
    print("big number")
else:
    print("other number")
    
print("done")

Notice that “big number” is only printed out if the first test of number > 1000 was False.

Order of testing/execution

For all if/elif/else statements, the program always executes the tests from top to bottom and stops as soon as one test evaluates to True.

Variations the structure of if/elif/else

Any combination of if/elif/else together is considered one control statement. The requirements for each part of this control statement are as follows:

  1. if - there must always be 1 and only 1 if statement at the beginning of your selection statement followed by a logical expression.
  2. elif - there may be 1 or more elif statements. They must contain logical expressions and they must follow the if statement.
  3. else - there may be 1 or 0 else statements. The else statement can never contain a logical expression. (Implicitly, the logical expression here is that all of the previous logical expressions were False.)

At the bottom of this set of course notes are a series of different, valid variations on a single if/elif/else statement.

Note that all of these examples are for only 1 control statement.

# TODO: run this code!

# we'll talk about this code in the modules section
# -- basically we're getting 
# some random numbers so that the behavior of this code can change 
# when you run it different times
import random 

# random int from the list [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# just like the range() function that we use with for loops!
x = random.randrange(0, 10)
y = random.randrange(0, 10)

print("x is: " + str(x))
print("y is: " + str(y))


# TODO: run this code!
# We'll be using the values of x and y generated in the
# code above for the rest of the code in this notebook!

# just an if statement
if x < y or x == 0:
    print("either [x is less than y] or [x is 0] or both!")


# if with an else
if x < y or x == 0:
    print("either [x is less than y] or [x is 0] or both!")
else:
    print("both of [x is greater than or equal to y] and [x is not 0]")


# if with an elif and no else
if x < y:
    print("[x is less than y]")
elif y % 2 == 0:
    print("[x is not less than y] and [y is even]") 


# if with an elif and an else
if x < y:
    print("[x is less than y]")
elif y % 2 == 0:
    print("[x is not less than y] and [y is even]") 
else:
    print("[x is not less than y] and [y is not even]") 


# if with more than 1 elif and an else
if x < y:
    print("[x is less than y]")
elif y % 2 == 0:
    print("[x is not less than y] and [y is even]") 
elif y < 5:
    print("[x is not less than y] and [y is not even] and [y is less than 5]") 
elif x < 5:
    print("[x is not less than y] and [y is not even] and [y is not less than 5] and [x is less than 5]") 
else:
    print("[x is not less than y] and [y is not even] and [y is not less than 5] and [x is not less than 5]")