DataWeave Delight: Splitting An Array Or Object

The module dw::core::Arrays has extra functions that are useful when working with arrays in DataWeave. In this post we will look at the functions splitAt and splitWhere. We can use these functions to split an array into two arrays. The result of both functions is actually a Pair type, which is defined as an object with the keys l and r. An example is { "l": 1, "r": 2 } which we can read as the left side of the pair has value 1 and the right side of the pair has value 2. The result of the splitAt and splitWhere function will have one part of the split array in the left side of the pair and the rest of the array in the right side.

The splitAt function takes the array as first argument and an index value as second argument. The index value should indicate at which position the array should be split. The function can be used with infix notation as well. The splitWhere function takes as first argument also an array, but the second argument is a predicate function. All items starting from the first item for which the predicate function returns true will be assigned to the r key of the Pair result, and all preceding items to the l key. We can use the infix notation here as well.

In the following example code we use the splitAt and splitWhere function on an array:

Source

%dw 2.0

import splitWhere, splitAt from dw::core::Arrays

output application/json
---
{
    // Split the range at index 4. 
    splitAt: (0 to 8) splitAt 4,

    // Split at the position where the predicate returns true for the first time.
    splitWhere: (0 to 8) splitWhere ((item) -> item > 5)
}

Output

{
  "splitAt": {
    "l": [
      0,
      1,
      2,
      3
    ],
    "r": [
      4,
      5,
      6,
      7,
      8
    ]
  },
  "splitWhere": {
    "l": [
      0,
      1,
      2,
      3,
      4,
      5
    ],
    "r": [
      6,
      7,
      8
    ]
  }
}

Although the function work on arrays we can use them on a object as well. We first turn the object into an array of objects with a single key and value using the pluck function. We can use this array with the splitAt and splitWhere functions. Then we can use the reduce function to transform the values in the Pair to an object with multiple keys and values again.

In the next example we use this mechanism on an object:

Source

%dw 2.0

import splitWhere, splitAt from dw::core::Arrays

// Helper functions
// ----------------
// Transform object to array with key/value pairs
fun objectToArray(obj: Object): Array<Object> = obj pluck ((value, key) -> (key): value)

// Transform array with key/value pairs to object
fun arrayToObject(items: Array<Object>): Object = items reduce ((item, acc = {}) -> acc ++ item)

var obj = {
    language: "DataWeave",
    alias: "mrhaki",
    age: 48, 
    country: "NL"
}

output application/json 
---
{
    // We can use splitAt on object if we first transform an object
    // to an array of key/value pairs, apply the splitAt function and 
    // transform the resulting Pair into an object again.
    splitAtObject: objectToArray(obj) 
        splitAt 3 
        mapObject ((value, key) -> (key): arrayToObject(value)),

    // We can use splitWhere in the same manner for objects.
    splitWhereObject: objectToArray(obj) 
        splitWhere ((item) -> typeOf(item[0]) == Number)
        mapObject ((value, key) -> (key): arrayToObject(value))
}

Output

{
  "splitAtObject": {
    "l": {
      "language": "DataWeave",
      "alias": "mrhaki",
      "age": 48
    },
    "r": {
      "country": "NL"
    }
  },
  "splitWhereObject": {
    "l": {
      "language": "DataWeave",
      "alias": "mrhaki"
    },
    "r": {
      "age": 48,
      "country": "NL"
    }
  }
}

Written with DataWeave 2.4.