Conditionals

Last updated on 2023-05-08 | Edit this page

Overview

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)

PYTHON 

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

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

OUTPUT 

3.54 is larger

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.

PYTHON 

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

OUTPUT 

3.54 is larger
9.22 is larger

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.

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for mass in masses:
    if mass > 3.0:
        print(mass, 'is larger')
    else:
        print(mass, 'is smaller')

OUTPUT 

3.54 is larger
2.07 is smaller
9.22 is larger
1.86 is smaller
1.71 is smaller

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”).

PYTHON 

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

OUTPUT 

3.54 is larger
2.07 is smaller
9.22 is HUGE
1.86 is smaller
1.71 is smaller

Conditions are tested once, in order.

  • Python steps through the branches of the conditional in order, testing each in turn.
  • So ordering matters.

PYTHON 

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

OUTPUT 

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

PYTHON 

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

OUTPUT 

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

PYTHON 

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)

OUTPUT 

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.

i 0 . 1 . 2 . 3 . 4 .
velocity 10.0 20.0 . 30.0 . 25.0 . 20.0 . 30.0
  • 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 Usingand,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

PYTHON 

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 <= 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:

PYTHON 

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

write one of these:

PYTHON 

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)

{: python}

OUTPUT 

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.

PYTHON 

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)

OUTPUT 

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

PYTHON 

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

Processing Small Files

Modify this program so that it only processes files with fewer than 50 records.

PYTHON 

import glob
import pandas
for filename in glob.glob('data/*.csv'):
    contents = pandas.read_csv(filename)
    ____:
        print(filename, len(contents))

PYTHON 

import glob
import pandas
for filename in glob.glob('data/*.csv'):
   contents = pandas.read_csv(filename)
   if len(contents) < 50:
       print(filename, len(contents))

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.

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

PYTHON 

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)

PYTHON 

values = [1, 3, 4, 5, 10]
smallest, largest = None, None
for v in values:
   if largest is None:
       smallest, largest = v, v
   else:
       smallest = min(smallest, v)
       largest = max(largest, v)
print(smallest, largest)

Using Functions With Conditionals in Pandas

Functions will often contain conditionals. Here is a short example that will indicate which quartile the argument is in based on hand-coded values for the quartile cut points.

PYTHON 

def calculate_life_quartile(exp):
    if exp < 58.41:
        # This observation is in the first quartile
        return 1
    elif exp >= 58.41 and exp < 67.05:
        # This observation is in the second quartile
       return 2
    elif exp >= 67.05 and exp < 71.70:
        # This observation is in the third quartile
       return 3
    elif exp >= 71.70:
        # This observation is in the fourth quartile
       return 4
    else:
        # This observation has bad data
       return None

calculate_life_quartile(62.5)

OUTPUT 

2

That function would typically be used within a for loop, but Pandas has a different, more efficient way of doing the same thing, and that is by applying a function to a dataframe or a portion of a dataframe. Here is an example, using the definition above.

PYTHON 

data = pd.read_csv('Americas-data.csv')
data['life_qrtl'] = data['lifeExp'].apply(calculate_life_quartile)

There is a lot in that second line, so let’s take it piece by piece. On the right side of the = we start with data['lifeExp'], which is the column in the dataframe called data labeled lifExp. We use the apply() to do what it says, apply the calculate_life_quartile to the value of this column for every row in the dataframe.

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.