Conditionals

Last updated on 2024-09-24 | Edit this page

Estimated time: 25 minutes

Overview

Questions

  • How can programs do different things for different data?

Objectives

  • Correctly write programs that use if and else statements using Boolean expressions.
  • Trace the execution of conditionals inside of loops.

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

An if statement is a conditional statement that controls whether a block of code is executed or not. The syntax of an if statement is similar to a for statement:

  • The first line opens with if and ends with a colon.
  • The body is indented (usually by 4 spaces)

PYTHON 

checkouts = 11
if checkouts > 10.0:
    print(f'{checkouts} is over the limit.')

checkouts = 8
if checkouts > 10.0:
    print(f'{checkouts} is over the limit.')

OUTPUT 

11 is over the limit.

Conditionals are often used inside loops.

There is not much of a point using a conditional when we know the value (as above), but they’re useful when we have a collection to process.

PYTHON 

checkouts = [0, 3, 10, 12, 22]
for checkout in checkouts:
    if checkout > 10.0:
        print(f'{checkout} is over the limit.')

OUTPUT 

12 is over the limit.
22 is over the limit.

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

An else statement can be used following if to allow us to specify an alternative code block to execute when the if branch is not taken.

PYTHON 

for checkout in checkouts:
    if checkout > 10.0:
        print(f'{checkout} is over the limit.')
    else:
        print(f'{checkout} is under the limit.')

OUTPUT 

0 is under the limit.
3 is under the limit.
10 is under the limit.
12 is over the limit.
22 is over the limit.

Notice that our else statement led to a false output that says 10 is under the limit. We can address this by adding a different kind of else statement.

Use elif to specify additional tests.

You can use elif (short for “else if”) to provide several alternative choices, each with its own test. An elif statement should always be associated with an if statement, and must come before the else statement (which is the catch all).

PYTHON 

for checkout in checkouts:
    if checkout > 10.0:
        print(f'*Warning*: {checkout} is over the limit.')
    elif checkout == 10:
        print(f'{checkout} is at the exact limit.')
    else:
        print(f'{checkout} is under the limit.')

OUTPUT 

0 is under the limit.
3 is under the limit.
10 is at the exact limit.
*Warning*: 12 is over the limit.
*Warning*: 22 is over the limit.

Conditions are tested once, in order and are not re-evaluated if values change. Python steps through the branches of the conditional in order, testing each in turn, so the order of your statements 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

Compound conditionals using and and or

Often, you want some combination of things to be true. You can combine relations within a conditional using and and or.

We can also check if something is less/greater than or equal to a value by using >= and <= operators.

PYTHON 

checkouts = [3, 50, 120]
users = ['fac', 'grad']

for user in users:
    for checkout in checkouts:
        #faculty checkout limit is 100
        if checkout >= 100 and user == 'fac':
            print(f"*Warning*: {checkout} is over the {user} limit.")
            
        #grad limit is 50
        elif checkout >= 50 and user == 'grad':
            print(f"{checkout} is over the {user} limit.")
        
        else:
            print(f"{checkout} is under the {user} limit.")
    
    # print an empty line between users
    print()

OUTPUT 

3 is under the fac limit.
50 is under the fac limit.
*Warning*: 120 is over the fac limit.

3 is under the grad limit.
*Warning*: 50 is over the grad limit.
*Warning*: 120 is over the grad limit.

Conditional logic: Fill in the blanks

Fill in the blanks in the following program to check if both the name variable is present in the names list and the password variable is equal to ‘true’ before giving a user access to a library system.

If you have extra time after you solve the fill in the blanks, change the value of password and re-run the program to view the output.

PYTHON 

names = ['Wang', 'Garcia', 'Martin']
name = 'Martin'
password = 'true'

___ item in names:
    print(item)
    if name == item ___ password == _____:
        print('Login successful!')
    elif password __ 'true':
        print(f'Your password is incorrect. Try again.')
    ____ name __ item:
        print(f'- Name does not match. Testing the next item in the list for {name}...')

PYTHON 

names = ['Wang', 'Garcia', 'Martin']
name = 'Martin'
password = 'true'

for item in names:
    print(item)
    if name == item and password == 'true':
        print('Login successful!')
    elif password != 'true':
        print(f'Your password is incorrect. Try again.')
    elif name != item:
        print(f'- Name does not match. Testing the next item in the list for {name}...')

OUTPUT 

Wang
- Name does not match. Testing the next item in the list for Martin...
Garcia
- Name does not match. Testing the next item in the list for Martin...
Martin
Login successful!

Processing Files Based on Record Length

Modify this program so that it only processes files with fewer than 85 records.

PYTHON 

import glob
import pandas
for filename in glob.glob('data/*.csv'):
    contents = pandas.read_csv(filename)
    ____ ___(______) < ____:
        print(f'{filename} : {len(contents)}')

PYTHON 

import glob
import pandas
for filename in glob.glob('data/*.csv'):
   contents = pandas.read_csv(filename)
   if len(contents) < 85:
       print(f'{filename} : {len(contents)}')

Write a Python program that checks the age of a user to determine if they will receive a youth or adult library card. The program should:

  1. Store age in a variable.
  2. Use an if statement to check if the age is 16 or older. If true, print “You are eligible for an adult library card.”
  3. Use an else statement to print “You are eligible for a youth library card” if the age is less than 16.

If you finish early, try this challenge:

  • In a new cell, adapt your program to loop through a list of age values, testing each age with the same output as above.

For parts 1 to 3:

PYTHON 

age = 25

if age >= 16:
  print('You are eligible for an adult library card.')
else:
  print('You are eligible for a youth library card.')

For the challenge:

PYTHON 

ages = [10, 16, 30, 65]

for age in ages:
  if age >= 16:
    print('You are eligible for an adult library card.')
  else:
    print('You are eligible for a youth library card.')

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.
  • Use and and or to check against multiple value statements.