🐥 Padding and making a list cumulative in Elixir

Sometimes you need to work with lists of a fixed length, or compute a running total from a series of values. Let’s look at how to do both in Elixir using a short, practical example.

Suppose you start with this list:

list = [19, 21, 27, 16, 37, 34, 27, 32, 33, 10]

Padding a list to a fixed length

You can easily extend this list to a fixed size (e.g. 12 elements) by adding zeros until it reaches the desired length:

target_length = 12
pad = max(target_length - length(list), 0)

list = list ++ List.duplicate(0, pad)

Here’s what happens:

• length(list) gets the current size.
• target_length - length(list) calculates how many zeros are needed.
• List.duplicate(0, pad) creates a list with that many zeros.
• Finally, list ++ ... concatenates them together.

After running this code, you’ll have:

[19, 21, 27, 16, 37, 34, 27, 32, 33, 10, 0, 0]

To compute a running total from your list, use Enum.scan/2:

cumulative = Enum.scan(list, &+/2)

This function walks through your list and accumulates values as it goes:

• 19
• 19 + 21 = 40
• 40 + 27 = 67
• and so on.

The resulting list is:

[19, 40, 67, 83, 120, 154, 181, 213, 246, 256, 256, 256]

By combining simple functions from Elixir’s standard library:

• You can pad a list to a specific length using List.duplicate/2.
• You can calculate a running total using Enum.scan/2.

These small, composable functions let you transform lists cleanly and expressively—one of the many strengths of functional programming in Elixir.