A bit of #Python basics. Day 8 - Flatten a nested list

I'll show you three (3) ways to flatten a two-dimensional list. The first method uses a for loop, the second uses the itertools module, and the third uses list comprehension.

⚙️ Using a for loop:

For this method, we use a nested for loop. The outer loop iterates over the inner lists, and the inner loop accesses the elements in the inner lists.

# In [19]:
list1 = [[1, 2, 3],[4, 5, 6]]

newlist = []
for list2 in list1:
    for j in list2:
        newlist.append(j)

print(newlist)

[1, 2, 3, 4, 5, 6]

⚙️ Using the itertools module:

The itertools.chain.from_iterable() function from the itertools module can be used to flatten a nested list. This method may not be suitable for deeply nested lists.

# In [20]:
import itertools

list1 = [[1, 2, 3],[4, 5, 6]]

flat_list = list(itertools.chain.from_iterable(list1))
print(flat_list)

[1, 2, 3, 4, 5, 6]

You can see that the nested loop has been flattened.

⚙️ Using list comprehension

If you don't want to import itertools or write a regular for loop, you can simply use list comprehension.

# In [21]:
list1 = [[1, 2, 3], [4, 5, 6]]

flat_list = [i for j in list1 for i in j]
print(flat_list)

[1, 2, 3, 4, 5, 6]

List comprehension is well suited for moderately nested lists. For deeply nested lists, it is not suitable, as the code becomes harder to read.

⚙️ Using a generator function

You can create a generator function that yields elements from the nested list, and then convert the generator into a list.

# In [22]:
def flatten_generator(nested_list):
    for sublist in nested_list:
        for item in sublist:
            yield item

list1 = [[1, 2, 3], [4, 5, 6]]

flat_list = list(flatten_generator(list1))
flat_list

Out[22]: [1, 2, 3, 4, 5, 6]

The generator method is suitable for flattening large or deeply nested lists. This is because generators are memory-efficient.

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Absolute value (module) of a number

Let's say you have a negative number and you want to get its absolute value. For this, you can use the abs() function. The abs() function returns the absolute value of any number (positive, negative, and complex). Below is shown how to get a list of absolute values from a list that contains both negative and positive numbers. We use list comprehension.

list1 = [-12, -45, -67, -89, 34, 67, -13]

print([abs(num) for num in list1])

[12, 45, 67, 89, 34, 67, 13]

Also, abs() can be applied to a floating-point number, and it will return the absolute value. See below:

num = -23.12

print(abs(num))

23.12

➡️ Using the math module

If you need more advanced mathematical functions, you can use fabs() from the math module. This function always returns a float.

import math

num = -23.12
absolute_value = math.fabs(num)
absolute_value

23.12

➡️ Using a lambda function

You can also use lambda to turn a negative number into its absolute value. The code below checks if x is less than zero (that is, if it's a negative value). If so, it returns -x, essentially removing the minus and making the number positive. If x is not negative (greater than or equal to 0), it returns x as it is.

num = -23.12
absolute_value = (lambda x: -x if x < 0 else x)(num)
absolute_value

23.12

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