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)

mass = 3.54
if mass > 3.0:
    print(mass, 'is large')

mass = 2.07
if mass > 3.0:
    print (mass, 'is large')

3.54 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.

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 3.0:
        print(m, 'is large')

3.54 is large
9.22 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.

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 3.0:
        print(m, 'is large')
    else:
        print(m, 'is small')

3.54 is large
2.07 is small
9.22 is large
1.86 is small
1.71 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”).

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 9.0:
        print(m, 'is HUGE')
    elif m > 3.0:
        print(m, 'is large')
    else:
        print(m, 'is small')

3.54 is large
2.07 is small
9.22 is HUGE
1.86 is small
1.71 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.

grade = 85
if grade >= 70:
    print('grade is C')
elif grade >= 80:
    print('grade is B')
elif grade >= 90:
    print('grade is A')

grade is C

• Does not automatically go back and re-evaluate if values change.

velocity = 10.0
if velocity > 20.0:
    print('moving too fast')
else:
    print('adjusting velocity')
    velocity = 50.0

adjusting velocity

• Often use conditionals in a loop to “evolve” the values of variables.

velocity = 10.0
for i in range(5): # execute the loop 5 times
    print(i, ':', velocity)
    if velocity > 20.0:
        print('moving too fast')
        velocity = velocity - 5.0
    else:
        print('moving too slow')
        velocity = velocity + 10.0
print('final velocity:', velocity)

0 : 10.0
moving too slow
1 : 20.0
moving too slow
2 : 30.0
moving too fast
3 : 25.0
moving too fast
4 : 20.0
moving too slow
final velocity: 30.0

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

• The program must have a print statement outside the body of the loop to show the final value of velocity, since its value is updated by the last iteration of the loop.

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

mass     = [ 3.54,  2.07,  9.22,  1.86,  1.71]
velocity = [10.00, 20.00, 30.00, 25.00, 20.00]

i = 0
for i in range(5):
    if mass[i] > 5 and velocity[i] > 20:
        print("Fast heavy object.  Duck!")
    elif mass[i] > 2 and mass[i] <= 5 and velocity[i] <= 20:
        print("Normal traffic")
    elif mass[i] <= 2 and velocity[i] <= 20:
        print("Slow light object.  Ignore it")
    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 mass[i] <= 2 or mass[i] >= 5 and velocity[i] > 20:

write one of these:

if (mass[i] <= 2 or mass[i] >= 5) and velocity[i] > 20:
if mass[i] <= 2 or (mass[i] >= 5 and velocity[i] > 20):

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

Tracing Execution

What does this program print?

pressure = 71.9
if pressure > 50.0:
    pressure = 25.0
elif pressure <= 50.0:
    pressure = 0.0
print(pressure)

Solution

25.0

Trimming Values

Fill in the blanks so that this program creates a new list containing zeroes where the original list’s values were negative and ones where the original list’s values were positive.

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = ____
for value in original:
    if ____:
        result.append(0)
    else:
        ____
print(result)

[0, 1, 1, 1, 0, 1]

Solution

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = []
for value in original:
    if value<0.0:
        result.append(0)
    else:
        result.append(1)
print(result)

Initializing

Modify this program so that it finds the largest and smallest values in the list no matter what the range of values originally is.

values = [...some test data...]
smallest, largest = None, None
for v in values:
    if ____:
        smallest, largest = v, v
    ____:
        smallest = min(____, v)
        largest = max(____, v)
print(smallest, largest)

What are the advantages and disadvantages of using this method to find the range of the data?

Solution

values = [-2,1,65,78,-54,-24,100]
smallest, largest = None, None
for v in values:
    if smallest==None and largest==None:
        smallest, largest = v, v
    else:
        smallest = min(smallest, v)
        largest = max(largest, v)
print(smallest, largest)

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.