20  Filtering Lists

A filter operation applies a filter condition to arrive at a subset of the data, where only the items that match the filter condition are retained.

We saw we can access a particular item in a list by its numeric position, and we can access a sequential subset using list slicing, but a filter condition will allow us to access only the items that match some condition.

The simplest way to implement this is by introducing an "if" statement into the scope of the loop:

my_numbers = [1, 2, 3, 4, 5, 6, 7]

for n in my_numbers:
    if n > 3: # FILTER CONDITION
        print(n)

4
5
6
7

We see we are only printing numbers that match the condition.

However in this case we lose access to the matching items. To retain access for later, we can implement a familiar collection operation using the append method, similar to the mapping operation:

my_numbers = [1, 2, 3, 4, 5, 6, 7]

matching_nums = []

for n in my_numbers:
    if n > 3: # FILTER CONDITION
        #print(n)
        matching_nums.append(n) # COLLECT FOR LATER

print(matching_nums)

[4, 5, 6, 7]

After performing a filter operation, we wind up with a subset of the data. Depending on the condition, is possible for the resulting list to contain no items, one item, some items, or all items:

my_numbers = [1, 2, 3, 4, 5, 6, 7]

matching_nums = []

for n in my_numbers:
    if n > 0:
        matching_nums.append(n)

print(matching_nums)

[1, 2, 3, 4, 5, 6, 7]

my_numbers = [1, 2, 3, 4, 5, 6, 7]

matching_nums = []

for n in my_numbers:
    if n == 5:
        matching_nums.append(n)

print(matching_nums)

[5]

my_numbers = [1, 2, 3, 4, 5, 6, 7]

matching_nums = []

for n in my_numbers:
    if n > 100:
        matching_nums.append(n)

print(matching_nums)

[]