Iterate over an Array

Exercise

Given the following input payload:

<?xml version="1.0"?>
<catalog>
   <book id="bk101">
      <author>Gambardella, Matthew</author>
      <title>XML Developer's Guide</title>
      <genre>Computer</genre>
      <price>44.95</price>
      <publish_date>2000-10-01</publish_date>
      <description>An in-depth look at creating applications
      with XML.</description>
   </book>
   <book id="bk102">
      <author>Ralls, Kim</author>
      <title>Midnight Rain</title>
      <genre>Fantasy</genre>
      <price>5.95</price>
      <publish_date>2000-12-16</publish_date>
      <description>A former architect battles corporate zombies,
      an evil sorceress, and her own childhood to become queen
      of the world.</description>
   </book>
   <book id="bk103">
      <author>Corets, Eva</author>
      <title>Maeve Ascendant</title>
      <genre>Fantasy</genre>
      <price>5.95</price>
      <publish_date>2000-11-17</publish_date>
      <description>After the collapse of a nanotechnology
      society in England, the young survivors lay the
      foundation for a new society.</description>
   </book>
</catalog>

Write a DataWeave transformation that produces the following output:

[
  {
    "index": 0,
    "id": "bk101",
    "title": "XML Developer's Guide",
    "author": "Gambardella, Matthew"
  },
  {
    "index": 1,
    "id": "bk102",
    "title": "Midnight Rain",
    "author": "Ralls, Kim"
  },
  {
    "index": 2,
    "id": "bk103",
    "title": "Maeve Ascendant",
    "author": "Corets, Eva"
  }
]

Optional:

• Use a lambda to write the transformation.

Hints

• To iterate over a list of elements we use the map(items: Array<T>, mapper: (item: T, index: Number) -> R) function, which takes an Array of elements and a function in charge of transforming each element.
• There are several ways to solve it; the easiest and simplest one is using a lambda. Remember that when using a lambda, to refer to the parameters received by the function we use $ and $$, which stand for the item and index parameters.

---

🔒🔓Show solution

Using a lambda

%dw 2.0
output application/json
---
payload.catalog.*book map {
    index: $$,
    id: $.@id,
    title: $.title,
    author: $.author
}

Using a function

%dw 2.0
output application/json

fun transformar(item: Object, index: Number): Object = {
    index: index,
    id: item.@id,
    title: item.title,
    author: item.author
}
---
map(payload.catalog.*book, transformar)

Notes:

1. When using a lambda, we refer to the element of the list simply with $, since it is the first parameter (the item) of the mapper the map function expects, just as $$ is the second parameter, the one standing for the index.
2. If you use a function instead, you have to write one with the shape map expects: (item: T, index: Number) -> R. So, since the Transformar function takes an item and an index and returns an object, it is valid to use inside the mapping function.