Arrays in practice

Arrays in practice

Arrays are useful when several values form a collection and the same operations need to be applied across that collection.

A common example is calculating a total from a series of amounts.

Suppose the following values represent order amounts before tax:

{12.50, 4.00, 27.50, 8.00, 19.00}

We want to:

1. keep only orders worth at least 10;
2. add 21% VAT to each remaining amount;
3. round each amount to two decimal places;
4. calculate the total.

This can be expressed as:

{12.50, 4.00, 27.50, 8.00, 19.00}
| filter(greater-than-or-equal(10))
|> (multiply(1.21) | round(2))
| sum

The result is:

71.40

Starting with a collection

The initial value is an array:

{12.50, 4.00, 27.50, 8.00, 19.00}

An array represents an ordered collection of values.

In this example, every element has the same role: it is an order amount that will go through the same processing pipeline.

Filtering the array

The first step removes amounts below 10:

| filter(greater-than-or-equal(10))

filter evaluates the predicate against every element and keeps only those for which it succeeds.

The array becomes:

{12.50, 27.50, 19.00}

The operation changes which elements belong to the array, but the result is still an array.

Transforming every element

Next, VAT is added and each resulting amount is rounded to two decimal places:

|> (multiply(1.21) | round(2))

The |> operator applies the complete expression between parentheses to every element of the array.

For each amount, Expressif first multiplies it by 1.21 and then rounds the result:

12.50 → 15.125 → 15.13
27.50 → 33.275 → 33.28
19.00 → 22.99  → 22.99

The resulting array is:

{15.13, 33.28, 22.99}

This illustrates an important pattern with arrays: |> can map an entire pipeline over every element, not just a single function call.

Reducing the array

Finally, the individual amounts are no longer needed. We only need their total:

| sum

sum consumes the array and combines its elements into a single value:

15.13 + 33.28 + 22.99 = 71.40

The final result is therefore:

71.40

Why arrays?

This example illustrates a common way to work with arrays in Expressif.

The data evolves through several stages:

order amounts
     ↓
filtered amounts
     ↓
transformed amounts
     ↓
total

At each stage, the array represents the collection currently being processed.

filter changes which elements belong to the collection. The |> operator applies a transformation to every element while preserving the array structure. Finally, sum reduces the array to a single value.

Arrays are therefore a natural fit when several values need to be filtered, transformed, or combined as part of the same computation.