Pattern: Accumulator

The counter pattern counts how many things match a condition. The accumulator pattern is its cousin: instead of counting, we build up a result. It could be a sum, a product, a string, or a list

We already saw the simplest accumulator when we summed prices

prices = [10, 20, 30, 40]
total = 0
for price in prices:
    total += price
print(total)

Output 100. We accumulated the sum step by step

But accumulators aren't just for numbers. We can build strings

words = ["Vice", "City", "Stories"]
sentence = ""
for word in words:
    sentence += word + " "
print(sentence)

Output Vice City Stories

Or build a new list from an existing one

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = []
for n in numbers:
    if n % 2 == 0:
        evens.append(n)
print(evens)

Output [2, 4, 6, 8, 10]

The pattern is always:

Initialize the accumulator (0 for sum, "" for string, [] for list)
Loop through the data
Add to the accumulator in each iteration (or some iterations)

A fun example: let's build a reversed string

original = "Tommy"
reversed_str = ""
for char in original:
    reversed_str = char + reversed_str
print(reversed_str)

Output ymmoT

How does this work? Each new character goes at the beginning instead of the end. First iteration: "T". Second: "oT". Third: "moT". And so on. Think about it, it's a beautiful trick

Another example: product of all numbers in a list

numbers = [2, 3, 4, 5]
product = 1
for n in numbers:
    product *= n
print(product)

Output 120 (2 × 3 × 4 × 5). Notice that we start with 1 not 0. Why? Because multiplying by 0 would give us 0 forever. The starting value depends on the operation: 0 for sum, 1 for product, "" for strings, [] for lists

Mission: Signal Accumulator

You have a list of signal strengths and a code word. Using the accumulator pattern for each step:

Sum: then the total of all strengths (accumulate from 0)
Strong: then a list of only the strengths above 50 (accumulate into an empty [])
Reversed: then the code word reversed character by character (accumulate into an empty "", putting each new character at the front)

Output

Sum: 255
Strong: [65, 90]
Reversed: TIBRO