Conditionals

Overview

Teaching: 10 min
Exercises: 15 min

Questions

• How can programs do different things for different data?

Objectives

• Correctly write programs that use if and else statements and simple Boolean expressions (without logical operators).

• Trace the execution of unnested conditionals and conditionals inside loops.

Use if statements to control whether or not a block of code is executed.

• An if statement (more properly called a conditional statement) controls whether some block of code is executed or not.
• Structure is similar to a for statement:
  • First line opens with if and ends with a colon
  • Body containing one or more statements is indented (usually by 4 spaces)

filesize = 520
if filesize > 500:
    print(filesize, 'is large')

filesize = 34
if filesize > 500:
    print (filesize, 'is large')

520 is large

Conditionals are often used inside loops.

• Not much point using a conditional when we know the value (as above).
• But useful when we have a collection to process.

filesizes = [34, 800, 230, 1200, 11.5]
for f in filesizes:
    if f > 500:
        print(f, 'is large')

800 is large
1200 is large

Use else to execute a block of code when an if condition is not true.

• else can be used following an if.
• Allows us to specify an alternative to execute when the if branch isn’t taken.

filesizes = [34, 800, 230, 1200, 11.5]
for f in filesizes:
    if f > 500:
        print(f, 'is large')
    else:
        print(f, 'is small')

34 is small
800 is large
230 is small
1200 is large
11.5 is small

Use elif to specify additional tests.

• May want to provide several alternative choices, each with its own test.
• Use elif (short for “else if”) and a condition to specify these.
• Always associated with an if.
• Must come before the else (which is the “catch all”).

filesizes = [34, 800, 230, 1200, 11.5]
for f in filesizes:
    if f > 1000:
        print(f, 'is huge')
    elif f > 500:
        print(f, 'is large')
    else:
        print(f, 'is small')

34 is small
800 is large
230 is small
1200 is huge
11.5 is small

Conditions are tested once, in order.

• Python steps through the branches of the conditional in order, testing each in turn.
• So ordering matters.
• Often use conditionals in a loop to “evolve” the values of variables.

Compound Relations Using and, or, and Parentheses

Often, you want some combination of things to be true. You can combine relations within a conditional using and and or. Continuing the example above, suppose you have

num_pages = [120,  50,  452,  98,  850]
height = [26, 28, 45, 15, 35]

for i in range(5):
    if num_pages[i] > 300 and height[i] > 30:
        print("Thick heavy book!")
    elif num_pages[i] > 100 and num_pages[i] <= 200 and height[i] <= 30:
        print("Typical size.")
    elif num_pages[i] <= 100 and height <= 30:
        print("Light object.")
    else:
        print("Whoa!  Something is up with the data.  Check it")

Just like with arithmetic, you can and should use parentheses whenever there is possible ambiguity. A good general rule is to always use parentheses when mixing and and or in the same condition. That is, instead of:

if num_pages[i] <= 100 or num_pages[i] >= 200 and height[i] > 30:

write one of these:

if (num_pages[i] <= 100 or num_pages[i] >= 200) and height[i] > 30:
if num_pages[i] <= 100 or (num_pages[i] >= 200 and height[i] > 30):

so it is perfectly clear to a reader (and to Python) what you really mean.

Tracing Execution [SLIDE]

What does this program print?

filesize = 71
if filesize > 50.0:
    filesize = 25.0
elif filesize <= 50.0:
    filesize = 0.0
print(filesize)

Solution

25.0

Key Points

• Use if statements to control whether or not a block of code is executed.

• Conditionals are often used inside loops.

• Use else to execute a block of code when an if condition is not true.

• Use elif to specify additional tests.

• Conditions are tested once, in order.

• Create a table showing variables’ values to trace a program’s execution.