A bit of Python basics. Day 6 - Exchanging variable values

In Python, you can swap variables after they have already been assigned objects. Below, we first assign 20 to the variable x and 30 to the variable y, and then swap them: x becomes 30, and y becomes 20. This method is called tuple packing/unpacking.

x, y = 20, 30
x, y = y, x

print('x is: ', x)
print('y is: ', y)

x is 30
y is 20

You can also use the XOR (exclusive or) operator to swap variables. This is a three-step method. In the example below, we swap the values of x and y.

x = 20
y = 30

# step one
x ^= y
# step two
y ^= x
# step three
x ^= y

print(f'x is: {x}')
print(f'y is: {y}')

x is: 30
y is: 20

You can also use arithmetic operations (addition and subtraction) to swap variables without a temporary variable. However, this method is recommended for swapping numeric data types. Here's an example:

# Use arithmetic operations
x = 5
y = 10

x = x + y
y = x - y
x = x - y

print("After swapping:")
print("x =", x)
print("y =", y)

After swapping:
x = 10
y = 5

As a result of these arithmetic operations, the values of x and y have actually been swapped. After the swap, x contains the original value of y (10), and y contains the original value of x (5).

This method of swapping variables without a temporary variable is based on the fact that when you add or subtract the value of one variable from another, you can effectively swap their values without the need for additional storage.

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A bit of Python basics. Day 7. Counting the number of occurrences of an element

If you need to find out how many times an element appears in an iterable collection, you can use the Counter class from the collections module. Counter() returns a dictionary with the number of times each element appears in the sequence. Let's say we want to find out how many times the name Peter appears in the following list. We can use Counter(). See below:

from collections import Counter

list1 = ['John', 'Kelly', 'Peter', 'Moses', 'Peter']

count_peter = Counter(list1).get("Peter")

print(f'The name "Peter" appears in the list '
      f'{count_peter} times.')

Output:

The name "Peter" appears in the list 2 times.

Another way to do this is with a regular for loop. We create a count variable and increase it by 1 each time we find the name Peter in the sequence. This is a naive approach. See below:

list1 = ['John', 'Kelly', 'Peter', 'Moses', 'Peter']
# Create a count variable
count = 0
for name in list1:
    if name == 'Peter':
        count +=1
print(f'The name "Peter" appears in the list'
      f' {count} times.')

Output:

The name "Peter" appears in the list 2 times.

Lists and other iterable data structures in Python have a built-in count() method, which allows us to count the number of occurrences of a specific element. We can use count() to count how many times Peter appears in the list.

list1 = ['John', 'Kelly', 'Peter', 'Moses', 'Peter']

print(f'The name "Peter" appears in the list '
      f'{list1.count("Peter")} times.')

Output:

The name "Peter" appears in the list 2 times.

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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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