How to subtract two array values (from different keys) using a ramda pipe?

Question

In a ramda pipe, I want to subtract the values of two array keys, to ultimately end up with an array of those differences.

For example, consider the following mice_weights array. I want to get an array with the differences weight_post minus weight_pre, for the male mice only.

const mice_weights = [
    {
        "id": "a",
        "weight_pre": 20,
        "weight_post": 12,
        "is_male": true
    },
    {
        "id": "b",
        "weight_pre": 25,
        "weight_post": 19,
        "is_male": false
    },
    {
        "id": "c",
        "weight_pre": 15,
        "weight_post": 10,
        "is_male": true
    },
    {
        "id": "d",
        "weight_pre": 30,
        "weight_post": 21,
        "is_male": false
    }
]

So based on this answer, I can construct 2 equivalent pipes, get_pre() and get_post():

const R = require("ramda");

filter_males = R.filter(R.path(["is_male"])) // my filtering function

const get_pre = R.pipe(
    filter_males,
    R.map(R.prop("weight_pre"))
)

const get_post = R.pipe(
    filter_males,
    R.map(R.prop("weight_post"))
)

res_pre  = get_pre(mice_weights) // [20, 15]
res_post = get_post(mice_weights) // [12, 10]


const res_diff = res_pre.map((item, index) => item - res_post[index]) // taken from: https://stackoverflow.com/a/45342187/6105259

console.log(res_diff); // [8, 5]

Although [8, 5] is the expected output, I wonder if there's a shorter way using ramda's pipe such as:

// pseudo-code

const get_diff = R.pipe(
    filter_males,
    R.subtract("weight_pre", "weight_post")
)

get_diff(mice_weights) // gives [8, 5]

Is it possible to achieve something similar using ramda? Perhaps there's a built-in functionality for such a task?

Answer 1

Sorry, I don't know about ramda pipes, but this is a trivial matter for array filtering and mapping.

const get_diff = (n, v) => // this takes a field and value to filter
       mice_weights 
       .filter(f => f[n] === v) // this keeps only datasets that have the field/value combo you're seeking
       .map(f => f.weight_pre - f.weight_post) // this gets the diff

const mice_weights = [{
    "id": "a",
    "weight_pre": 20,
    "weight_post": 12,
    "is_male": true
  },
  {
    "id": "b",
    "weight_pre": 25,
    "weight_post": 19,
    "is_male": false
  },
  {
    "id": "c",
    "weight_pre": 15,
    "weight_post": 10,
    "is_male": true
  },
  {
    "id": "d",
    "weight_pre": 30,
    "weight_post": 21,
    "is_male": false
  }
]

const get_diff = (n, v) => mice_weights.filter(f => f[n] === v).map(f => f.weight_pre - f.weight_post)

console.log(get_diff('is_male', true)) // gives [8, 5]

Answer 2

I would propose to use props and reduceRight functions in order to achieve that:

const getProps = R.props(['weight_pre', 'weight_post'])

const subtract = R.reduceRight(R.subtract)(0)

const get_diff = R.pipe(
    R.filter(R.path(['is_male'])),
    R.map(R.pipe(getProps, subtract))
)

console.log(get_diff(mice_weights));