Javascript list loop/recursion to create an object

Question

I'm trying to create a function to process a list of numbers relating to depth using recursion or loops in JavaScript.

The following "input" needs to be processed into the "output", and it needs to work for arbitary lists.

One thing to note is that numbers increase by either 0 or 1 but may decrease by any amount.

var input = [0, 1, 2, 3, 1, 2, 0]

var output =
  [ { number: 0, children: 
      [ { number: 1, children: 
          [ { number: 2, children: 
              [ { number: 3, children: [] } ]
            } 
          ] 
        } 
      , { number: 1, children: 
          [ { number: 2, children: [] } ]
        } 
      ] 
    } 
  , { number: 0, children: [] } 
  ]

I worked it out myself, although it needs some refinement.

var example = [0, 1, 2, 2, 3, 1, 2, 0]
var tokens = []
var last = 0
const createJSON = (input, output) => {
  if (input[0] === last) {
    output.push({ num: input[0], children: [] })
    createJSON(input.splice(1), output)
  } 
  else if (input[0] > last) {
    last = input[0]
    output.push(createJSON(input, output[output.length-1].children))
  } 
  else if (input[0] < last) {
    var steps = input[0]
    var tmp = tokens
    while (steps > 0) {
      tmp = tmp[tmp.length-1].children
      steps--
    }
    tmp.push({ num: input[0], children: [] })
    createJSON(input.splice(1), tmp)
  }
}
createJSON(example, tokens)
console.log(tokens)

I'm trying to create a function Can you show us how you tried to implement your function? Where specifically are you having trouble? — user47589
– user47589, Commented Jan 9, 2020 at 23:31
Can you elaborate on what you are trying to do? It is very unclear. — user47589
– user47589, Commented Jan 9, 2020 at 23:33
We expect that you'll do research and make an attempt before posting. Then, when you do post, that you'll show what you've tried and ask a specific question about what you are having trouble with. We're not a code writing service, so help us help you. — Scott Marcus
– Scott Marcus, Commented Jan 9, 2020 at 23:41
Sorry for not adding my code initially. I didn't add commas as it's only intended to be a representation and I think it's more readable. — user3462522
– user3462522, Commented Jan 10, 2020 at 1:14

John Montgomery · Accepted Answer · 2020-01-11 00:24:56Z

1

In fact, it's a very simple problem to solve...

var input   = [0, 1, 2, 3, 1, 2, 0]
  , output  = []
  , parents = [output]
  ;
for(el of input)
  {
  let nv = { number:el, children:[] }
  parents[el].push( nv )
  parents[++el] = nv.children  // save the  @ddress of children:[] for adding items on
  }
console.log( output )

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Jan 11, 2020 at 0:24

John Montgomery

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answered Jan 10, 2020 at 3:18

Mister Jojo

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4 Comments

user3462522 Over a year ago

Thanks for the answer, much more simple than my solution. What does this line do "parents[++el] = nv.children"?

Mister Jojo Over a year ago

@user3462522 you mean ++el ? this the same incrementation operator as C see = developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

user3462522 Over a year ago

Ah, I know this one. I just thought something clever might be happening inside a list. How do I learn to solve recursive problems like this so simply, are there any recommended resources or books?

Mister Jojo Over a year ago

@user3462522 it is not a recursive problem, the only question was just to know to which element to attach an arriving according to its indentation, from where the idea of a table to keep the addresses of the tables before collecting them. Solving a problem well means first asking the right questions

tex · Accepted Answer · 2020-01-11 19:42:19Z

0

Here's a functional solution based on recursion and Array.prototype.reduce:

const data = [0, 1, 2, 3, 1, 2, 0]
const last = xs => xs.length > 0 ? xs[xs.length - 1] : undefined
const lastD = (d, t, i = 0) => i > d ? t : lastD(d, last(t).children, i + 1)
const buildIt = xs => xs.reduce(
  (a, x) =>((x === 0 ? a : lastD(x - 1, a)).push({ number: x, children: [] }), a),
  []
)

console.log(buildIt(data))

.as-console-wrapper { max-height: 100% !important; top: 0; }

Note: This solution does not depend on the mutation of additional variables for bookkeeping purposes.

Turns out the actual problem was significantly simpler to solve than my initial misinterpretation!

edited Jan 11, 2020 at 19:42

answered Jan 10, 2020 at 0:19

tex

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4 Comments

user3462522 Over a year ago

Thanks Tex, it's along the right lines but your first solution doesn't handle decreasing numbers correctly. The second "1" ends up at the root, not as a child of the previous "0".

tex Over a year ago

Ah, I misread the desired output format. Thought that 1 was at the root. Might take another look tomorrow.

user3462522 Over a year ago

It's close, I'm also having difficulties with the decreasing numbers.

tex Over a year ago

user3462522 I've fixed my snippet based on your comment.

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