Conditionals and Loops¶

Topics covered¶

Boolean operations
Conditional logic
Introduction to loops

Boolean Operators¶

We have seen that Python has a type bool that can take two values True or False
Boolean operators return True and False (or yes/no) answers

Operation	Description	Operation	Description
`a == b`	`a` equal to `b`	`a != b`	`a` not equal to `b`
`a < b`	`a` less than `b`	`a > b`	`a` greater than `b`
`a <= b`	`a` less than or equal to `b`	`a >= b`	`a` greater than or equal to `b`

Tip: Remember that the = operator is for assignment
use == when you want to compare equality

age = 25
print(age < 40)
print(age >= 40)
print(age == 25)

True
False
True

Conditional Logic¶

Most code makes extensive use of if then statements
This makes use of the if, elif, and else keywords

if <Boolean Operation 1>:
    do something
elif <Boolean Operation 2>:
    do something different
else:
    take default action

Example 1:

Is the number less than 2?

number = 5

if number > 2:
    print("Number > 2!")
else:
    print("Number <=2")

Number > 2!

Python whitespace¶

The indenting in the previous example is mandatory in Python
If you do not indent your if then statements then Python will throw an IndentationError exception

number = 1
if number > 2:
print("Number > 2!")
else:
print("Number <=2")

  File "<ipython-input-3-44d8c9689021>", line 3
    print("Number > 2!")
    ^
IndentationError: expected an indented block

A more complex if then

lower_limit = 10
upper_limit = 15

number_to_check = 12

if number_to_check >= lower_limit and number_to_check <= upper_limit:
    print('number is inside range')
else:
    print('number is outside of range')

number is inside range

An example using elif

age = 32
pensionable_age = 68

if age < 0:
    print('Error. Please enter an age greater than zero')
elif age < pensionable_age:
    years_to_pension = pensionable_age - age
    print(f'There are {years_to_pension} years until you can draw your pension!')
else:
    print('You are eligible to draw your_pension') 

There are 36 years until you can draw your pension!

Functions and if statements¶

we could also use a function within an if statement

def eligible_for_pension(age):
    '''
    Return boolean True or False to indicate if
    person is eligible for their pension
    
    Parameters:
    ----------
    age: int
        a persons age in integer years (e.g. 32 or 45)
    '''
    return age >= 68    

age = 68

if eligible_for_pension(age):
    print('Congratulations you can retire!')
else:
    print('You are not yet eligible for your pension')

Congratulations you can retire!

We can also use the not operator in Python

age = 68

if not eligible_for_pension(age):
    print('You are not yet eligible for your pension')
else:
    print('Congratulations you can retire!')

Congratulations you can retire!

Nested if statements¶

def stamp_duty(house_price, first_time_buyer):
    '''
    First time buyers recieve more tax relief 
    than people buying their next home.
    Returns float representing the stamp duty owed.
    '''
    if first_time_buyer:
        #this if statement is nested within the first
        if house_price <= 300000:
            return 0.0
        else: 
            return house_price * 0.05     
    else:
        if house_price < 125000:
            return 0.0
        else:
            return house_price * 0.05

my_first_house_price = 310_000
owed = stamp_duty(my_first_house_price, True)
print('stamp duty owed = £{0}'.format(owed))

stamp duty owed = £15500.0

Introduction to iterating over data using loops¶

Algorithms often need to do the same thing again and again

For example, an algorithm making three servings of toast

Making Toast Algorithm:

Put a slice of bread in the toaster
Push lever down to turn on the toaster
Remove the toasted bread from the toaster
Put a slice of bread in the toaster
Push lever down to turn on the toaster
Remove the toasted bread from the toaster
Put a slice of bread in the toaster
Push lever down to turn on the toaster
Remove the toasted bread from the toaster

A better way to do this in code is to use a LOOP

Do the following 3 steps, 3 times:

Put a slice of bread in the toaster
Push lever down to turn on the toaster
Remove the toasted bread from the toaster

There are two types of loop in Python
for loops and while loops
We generally use while if we do not know the number of iterations in advance
We generally use for if we know the number of iterations in advance

While loops¶

age = 15

while age <= 18:
    print(f'you are currently {age} years old')
    age += 1

you are currently 15 years old
you are currently 16 years old
you are currently 17 years old
you are currently 18 years old

While Loop Structure¶

All while loops have the same structure
You use the while keyword
Followed by a boolean operation (e.g. age <= 18)
Beware of infinite loops!
Let’s test a more complex while loop using a function

def fizzbuzz(n):
    """
    For multiples of three print "Fizz" instead of the number 
    and for the multiples of five print "Buzz". 
    For numbers which are multiples of both three 
    and five print "FizzBuzz".
    
    Parameters:
    ----------
    n: int
        the number to test
    """
    if n % 3 == 0 and n % 5 == 0:
        print('fizzbuzz')
    elif n % 3 == 0:
        print('fizz')
    elif n % 5 == 0:
        print('buzz')
    else:
        print(n)

n = 1
limit = 15

while n <= limit:
    fizzbuzz(n)
    n += 1

1
2
fizz
4
buzz
fizz
7
8
fizz
buzz
11
fizz
13
14
fizzbuzz

A while loop example where we do not know the number of iterations in advance.

list_to_search = ['we', 'are', 'the', 'knights', 'who', 'say', 'ni']
not_found = True
word_to_find = 'knights'
current_index = 0
index_of_word = -1

while not_found:
    if list_to_search[current_index] == word_to_find:
        not_found = False
        index_of_word = current_index
    current_index += 1

print(f"the word '{word_to_find}' is located in index {index_of_word}")

the word 'knights' is located in index 3

The previous example can easily lead to an IndexError.
If word_to_find was ‘shrubbery’ then the loop would exhaust all list elements
Although we do not know the number of iterations needed, we can easily modify the while loop to take account of the maximum allowable iterations.

list_to_search = ['we', 'are', 'the', 'knights', 'who', 'say', 'ni']
not_found = True
word_to_find = 'shrubbery'
current_index = 0 
index_of_word = -1
word_count = len(list_to_search)

while not_found and current_index < word_count:
    if list_to_search[current_index] == word_to_find:
        not_found = False
        index_of_word = current_index
    current_index += 1

if not_found: 
    print(f"'{word_to_find}' could not be found")
else:
    print(f"'{word_to_find}' is located in index {index_of_world}")

'shrubbery' could not be found

For loops¶

To create a for loop you need the following:
for keyword
a variable
the in keyword
the range() function - which is a built-in function in the Python library to create a sequence of numbers

for age in range(5):
    print(f'you are currently {age} years old')

you are currently 0 years old
you are currently 1 years old
you are currently 2 years old
you are currently 3 years old
you are currently 4 years old

notice that the loop sets age to 0 initially!
range() takes keyword arguments to set the start (inclusive, default = 0), end (exclusive) and step

for age in range(1, 5):
    print(f'you are currently {age} years old')

you are currently 1 years old
you are currently 2 years old
you are currently 3 years old
you are currently 4 years old

for age in range(1, 5, 2):
    print(f'you are currently {age} years old')

you are currently 1 years old
you are currently 3 years old

limit = 15

for n in range(1, limit+1):
    fizzbuzz(n)

1
2
fizz
4
buzz
fizz
7
8
fizz
buzz
11
fizz
13
14
fizzbuzz

Watch out for python whitespace rules!¶

Remember to indent the next line after :

limit = 15

for n in range(1, limit+1):
fizzbuzz(n)

  File "<ipython-input-25-a9a12e790b42>", line 4
    fizzbuzz(n)
    ^
IndentationError: expected an indented block

Nested Loops¶

for and while loops can be nested within each other.
Think of nested loops as a ‘loop of loops’
Remember that for each iteration outer loop will consist of multiple iterations of an inner loop
Don’t panic if you do not understand straight away!

for outer_index in range(3):
    print(f'Outer loop iteration: {outer_index}')
    
    for inner_index in range(5):
        print(f'\tInner loop iteration: {inner_index}')
        

Outer loop iteration: 0
	Inner loop iteration: 0
	Inner loop iteration: 1
	Inner loop iteration: 2
	Inner loop iteration: 3
	Inner loop iteration: 4
Outer loop iteration: 1
	Inner loop iteration: 0
	Inner loop iteration: 1
	Inner loop iteration: 2
	Inner loop iteration: 3
	Inner loop iteration: 4
Outer loop iteration: 2
	Inner loop iteration: 0
	Inner loop iteration: 1
	Inner loop iteration: 2
	Inner loop iteration: 3
	Inner loop iteration: 4

Example 2: The inner loop now iteratures backwards

for outer_index in range(2):
    print(f'Outer loop iteration: {outer_index}')
    
    for inner_index in range(5, 0, -1):
        print(f'\tInner loop iteration: {inner_index}')

Outer loop iteration: 0
	Inner loop iteration: 5
	Inner loop iteration: 4
	Inner loop iteration: 3
	Inner loop iteration: 2
	Inner loop iteration: 1
Outer loop iteration: 1
	Inner loop iteration: 5
	Inner loop iteration: 4
	Inner loop iteration: 3
	Inner loop iteration: 2
	Inner loop iteration: 1

Nested Loops Example 2.

Let’s use a nested for loop to create the pattern below

for outer_index in range(1, 6):
    #remember this is a loop of loops. 
    #the loop below execute all iterations each 
    # time the outer loop iterates
    for inner_index in range(1, outer_index + 1):
        
        #we use the end='' option of print so 
        #that it prints on the same line as previous
        print(inner_index, end='')
    
    #new line
    print('')
        

Introduction to Python