DEV Community: Zeeshan Asgar

Custom sorting in Python

Zeeshan Asgar — Fri, 20 Nov 2020 01:51:19 +0000

Consider the following list:

employees = ['Bill Gates', 'Mark Zukerberg', 'Jeff Bezos', 'Elon Musk', 'Larry Page']

To sort the list `employees`

employees.sort()

Output

['Bill Gates', 'Elon Musk', 'Jeff Bezos', 'Larry Page', 'Mark Zukerberg']

As we can see here the normal sorting sort the elements by alphabets. Let's say we want to sort the list by the length of each string element in the list. We can easily do that using the key parameter of the sort method.

To sort the original list `employees` by the length of elements

employees.sort(key=len)
print(employees)

Output

['Elon Musk', 'Bill Gates', 'Jeff Bezos', 'Larry Page', 'Mark Zukerberg']

In the example above we are passing a built-in len() method to sort the list by the length of elements. We can also supply our custom sort function as well instead of a built-in function.
This key function takes each element of the list, transforms it and return one value which is then used to sort the list.

sort()

sorts a list in-place in ascending or descending order. sort() method takes two parameters which are both optional:

key: function which serves as the basis for comparison.
reverse: if True, then it will sort the list in descending order, otherwise in ascending order. Defaults to False.

Syntax

list.sort(key=..., reverse=...)

Let's look at another example of sorting a list of tuples using the second element:

cart =[('Mango', 5), ('Grapes', 10), ('Banana', 3), ('Melon', 5), ('Strawberry', 7)]

To sort the `cart` list using the second element of each tuple

cart.sort(key=lambda x : x[1])
print(cart)

Output

[('Banana', 3), ('Mango', 5), ('Melon', 5), ('Strawberry', 7), ('Grapes', 10)]

There is one more method that is used to sort the iterable called sorted().

`sorted()`

returns a sorted list of elements in ascending or descending order.

It takes 3 parameters:

iterable: Either a sequence of elements like list, string or tuple or a collection like dict, set etc.
key (optional): function which serves as the basis for comparison.
reverse (optional): if True, then it will sort the iterable in descending order, otherwise in ascending order. Defaults to False.

Syntax

sorted(iterable, key=..., reverse=...)

Looping over a collection

Zeeshan Asgar — Tue, 10 Nov 2020 01:47:47 +0000

Consider the following collection(list):

fruits = ['apple', 'grapes', 'mango', 'banana', 'orange']

To loop over the list `fruits`

for fruit in fruits:
    print(fruit)

To loop in reverse over the list `fruits`

for fruit in reversed(fruits):
    print(fruit)

To get the indices of the list `fruits`

for i, fruit in enumerate(fruits):
    print(f'{i} - {fruit}')

Output of `enumerate()`

0 - apple
1 - grapes
2 - mango
3 - banana
4 - orange

In the previous code snippet, we have used enumerate to loop over the list.

enumerate(iterable, start=0)

enumerate() allows us to loop over an iterable and provides an automatic counter. It takes two parameters:

iterable - an object that supports iteration like list, tuple etc.
start - specifies the starting index of the counter. It is optional and defaults to 0.

Example

for i, fruit in enumerate(fruits, start=1):
    print(f'{i} - {fruit}')

print("-----------")

for i, fruit in enumerate(fruits, start=100):
    print(f'{i} - {fruit}')

Output

1 - apple
2 - grapes
3 - mango
4 - banana
5 - orange
-----------
100 - apple
101 - grapes
102 - mango
103 - banana
104 - orange

Looping over a range of numbers in Python3

Zeeshan Asgar — Tue, 20 Oct 2020 02:32:01 +0000

for i in range(7):
    print(i**3)

We have used range here to generate the sequence of numbers.

range(stop)

returns the sequence of numbers up to stop argument.

range takes two other arguments as well

start
step

range(start, stop[, step])

start argument tells where to start the sequence from. Defaults to 0.
step argument tells the increment value between each number in the sequence. Defaults to 1.

Note - range in Python3 returns a range object, not list.

DEV Community: Zeeshan Asgar

Custom sorting in Python

To sort the list employees

Output

To sort the original list employees by the length of elements

Output

sort()

Syntax

To sort the cart list using the second element of each tuple

Output

sorted()

Syntax

Looping over a collection

To loop over the list fruits

To loop in reverse over the list fruits

To get the indices of the list fruits

Output of enumerate()

enumerate(iterable, start=0)

Example

Output

Looping over a range of numbers in Python3

range(stop)

range(start, stop[, step])

To sort the list `employees`

To sort the original list `employees` by the length of elements

To sort the `cart` list using the second element of each tuple

`sorted()`

To loop over the list `fruits`

To loop in reverse over the list `fruits`

To get the indices of the list `fruits`

Output of `enumerate()`