The Accumulator Pattern

Concept: for loops

An accumulator stores a result built up as a loop runs. Initialize it before the loop, update it when needed inside the loop, and use the completed result after the loop. A total changes by addition, a collection gains items, and a best-so-far value changes only when a better candidate appears.

Summing

Start a total at 0, then add each item to it:

total = 0
for roll in [3, 5, 2]:
    total = total + roll
print(total)

total starts at 0, becomes 3, then 8, then 10. The starting value has to be set before the loop — otherwise there is nothing to add the first item to.

Collecting results in a list

Create the result list before the loop. Append one result on each repetition:

updated = []
for score in [3, 7, 4]:
    updated.append(score + 1)
print(updated)

The result grows from [] to [4], then [4, 8], then [4, 8, 5]. Creating the list inside the loop would discard earlier entries. In a function that builds a complete list, return it after the loop finishes.

Counting

To count how many items match a condition, start at 0 and add 1 each time the condition holds:

big_rolls = 0
for roll in [3, 5, 2, 6]:
    if roll >= 4:
        big_rolls = big_rolls + 1
print(big_rolls)

Here the counter records how many rolls are at least 4. A count adds 1 for each match; a sum adds the matching value itself.

Your turn: count matches

Count how many values are at least 4. Print the count, not the sum of those values.

rolls = [2, 6, 4, 1]
count = 0
# Count the matching values, then print count.

Expected output

The shortcut for a plain sum

If you only need to add numbers up, sum() does it in one line:

rolls = [3, 5, 2]
print(sum(rolls))

Use sum() when adding existing numbers is the whole task. In these exercises, use an explicit loop when practising how a result is updated step by step, especially when collecting values into a list.

See also: For loops, The range() function.

Optional practice: sum matching values

Add a loop that adds up only the rolls of 4 or more, then print the total. The accumulator is already started for you.

rolls = [3, 5, 2, 6, 4]
total = 0
# add a loop with an if that adds the roll to total, then print total (four lines)

Expected output

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