count_filter.md

Counting and filtering

• Overview
• Counting
• Filtering
• Count if

Overview

In the course of our Python study, we've learned about some of the basic features of the Python programming language. That includes:

• Basic data types (integers, strings, lists, dicts)
• Expressions and assignment statements
• Variables as storage containers
• Flow control ("for" loops and if/elif/else)
• Built-in functions such as len and print

Let's start tying together this knowledge and applying it in practical contexts.

For these examples, you should fire up an interactive Python interpeter on the command line by typing python or ipython (if you've installed it).

Counting

Counting is one of the most basic and important operations we need to perform.

One of the simplest and most common ways to count items involves using the built-in len function to measure the length of an array:

animals = ['cat', 'dog', 'bird']
len(animals)
3

Another common approach -- one often used when processing data from an external source such as a CSV -- is to use a counter variable.

count = 0
for animal in animals:
	count +=1 # same as writing count = count + 1
print(count)
3

Above, we "initialized" a variable called count, and then used the augmentation operator to increment the count as we loop through the list of animals.

Filtering

Filtering data based on some aspect of the information is another common data wrangling task.

for animal in animals:
    if animal != 'dog':
        print(animal)
cat
bird      

Count if

We can now combine the above techniques to count a filtered list of items. Here are a few different approaches.

# Use a simple counter
count = 0
for animal in animals:
    if animal != 'dog':
        count += 1
print(count)
2

What if we need to keep the results that we filtered for some reason? For example, say we need both the count and the actual data for some additional downstream purpose such as saving it to a new file. In this case, we can adapt our strategy with the help of a list and len.

# Store the filtered items in a new list
noncanines = []
for animal in animals:
    if animal != 'dog':
        noncanines.append(animal)
        
# Now "count" the filtered list
print(len(noncanines))
2

Both of these approaches work, and which one you use will vary based on the end goal.