Pattern: Counter

One of the most common things you'll do in programming is count things. How many even numbers? How many words longer than 5 letters? How many players are online? This is the counter pattern

The idea is simple: start with a variable at 0, loop through the data, and increase by 1 every time we find what we're looking for

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_count = 0

for n in numbers:
    if n % 2 == 0:
        even_count = even_count + 1

print(f"Even numbers: {even_count}")

Output Even numbers: 5

The structure is always the same:

Initialize the counter to 0
Loop through the data
Check a condition
If the condition is true, increase the counter

Let's count how many characters in a name are uppercase

name = "Tommy Vercetti"
upper_count = 0

for char in name:
    if char.isupper():
        upper_count = upper_count + 1

print(f"Uppercase letters: {upper_count}")

Output Uppercase letters: 2 (T and V)

.isupper() is a string method that returns True if the character is uppercase. There's also .islower(), .isdigit(), .isalpha(), and more

We can count multiple things at once

text = "Hello World 123"
letters = 0
digits = 0
spaces = 0

for char in text:
    if char.isalpha():
        letters = letters + 1
    elif char.isdigit():
        digits = digits + 1
    elif char == " ":
        spaces = spaces + 1

print(f"Letters: {letters}, Digits: {digits}, Spaces: {spaces}")

Output Letters: 10, Digits: 3, Spaces: 2

By the way, Python has a shortcut for x = x + 1. We can write x += 1 instead. Same thing, less typing

count = 0
count += 1
count += 1
count += 1
print(count)

Output 3. Works with other operators too: x -= 1, x *= 2, x /= 3

Mission: Reading Bands

You have a list of reactor readings named readings. Using the counter pattern, count how many fall into each band and print the totals:

Low: then how many readings are below 50
Mid: then how many are from 50 to 99 (inclusive)
High: then how many are 100 or above

Output

Low: 2
Mid: 4
High: 2