JavaScript mutating arrays in loop

Question

I want to duplicate certain arrays in arrays using JavaScript, for example:

var res = [["1,2,3",100,50],["4,5",75,10],["6",20,90]];
var res2 = [];
for(var z in res) {
  var row = res[z];
  var keys = row[0].split(',')
  for(var y in keys) {
    var key = keys[y];
    res2.push([key,row[1]/keys.length,row[2]/keys.length]);
  }
}
/* 
[ [ '1', 33.333333333333336, 16.666666666666668 ],
  [ '2', 33.333333333333336, 16.666666666666668 ],
  [ '3', 33.333333333333336, 16.666666666666668 ],
  [ '4', 37.5, 5 ],
  [ '5', 37.5, 5 ],
  [ '6', 20, 90 ] ]
*/

The arrays are really-really long, is it possible to do this in-place (res, without res2)?

you can try with reduce

Grundy
– Grundy

2015-03-26 11:45:14 +00:00
Commented Mar 26, 2015 at 11:45 — Grundy
– Grundy, Commented Mar 26, 2015 at 11:45
@Grundy I don't think reduce qualifies as in-place

JuniorCompressor
– JuniorCompressor

2015-03-26 11:56:02 +00:00
Commented Mar 26, 2015 at 11:56 — JuniorCompressor
– JuniorCompressor, Commented Mar 26, 2015 at 11:56
oh, @JuniorCompressor you right!

Grundy
– Grundy

2015-03-26 12:01:21 +00:00
Commented Mar 26, 2015 at 12:01 — Grundy
– Grundy, Commented Mar 26, 2015 at 12:01

JuniorCompressor · Accepted Answer · 2015-03-26 12:34:17Z

7

You can use splice for a true in-place processing of res:

for (var i = 0; i < res.length; ) {
  var row = res[i];
  var keys = row[0].split(',')
  res.splice(i, 1); // Remove old element
  // Insert new elements at current position
  for (var j in keys)
    res.splice(i++, 0, [keys[j], row[1] / keys.length, row[2] / keys.length]);
}

Result:

[
 ["1", 33.333333333333336, 16.666666666666668],
 ["2", 33.333333333333336, 16.666666666666668],
 ["3", 33.333333333333336, 16.666666666666668],
 ["4", 37.5, 5],
 ["5", 37.5, 5],
 ["6", 20, 90]
]

EDIT:

Another trick to avoid splices is to extend the size of res and start filling it from the end to the beginning:

var n = res.length;

// Precalculate new length
var length = 0;
for (var i = 0; i < res.length; i++)
    length += res[i][0].split(',').length;

// Change length of array
res.length = length;

// Start filling from end to start
for (var i = n - 1, k = length - 1; i >= 0; i--) {
    var row = res[i];
    var keys = row[0].split(',');
    for (var j = keys.length - 1; j >= 0; j--)
        res[k--] = [keys[j], row[1] / keys.length, row[2] / keys.length];
}

edited Mar 26, 2015 at 12:34

answered Mar 26, 2015 at 12:12

JuniorCompressor

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3 Comments

vp_arth Over a year ago

inserting to array(which is very long) is heavy, but yes, it seems like in-place.

JuniorCompressor Over a year ago

@vp_arth I updated with a splice-less implementation

Kokizzu Over a year ago

last one is beauty :3

Grundy · Accepted Answer · 2015-03-26 12:17:22Z

You can use reduce function something like this

var res = [["1,2,3",100,50],["4,5",75,10],["6",20,90]];
var r = res.reduce(function(acc,curr){
            return acc.concat(curr[0].split(',').map(function(el,_,cursplit){
                return [el, curr[1]/cursplit.length,curr[2]/cursplit.length];
            }));
        },[]);

but i'm not sure that it more readable

var res = [["1,2,3",100,50],["4,5",75,10],["6",20,90]];
    var r = res.reduce(function(acc,curr){
        return acc.concat(curr[0].split(',').map(function(el,_,cursplit){
            return [el, curr[1]/cursplit.length,curr[2]/cursplit.length];
        }));
    },[]);

document.getElementById('d').innerHTML = r.join('<br>');

<div id="d"></div>

UPDATE another variant with mutate res

for(var i=0,len=res.length;i<len;i++){
    var cur = res.shift();
    cur[0].split(',').forEach(function(el,_,cursplit){
            res.push([el, cur[1]/cursplit.length,cur[2]/cursplit.length]);
    });
}

var res = [["1,2,3",100,50],["4,5",75,10],["6",20,90]];
for(var i=0,len=res.length;i<len;i++){
    var cur = res.shift();
    cur[0].split(',').forEach(function(el,_,cursplit){
            res.push([el, cur[1]/cursplit.length,cur[2]/cursplit.length]);
    });
}

document.getElementById('d').innerHTML = res.join('<br>');

<div id="d"></div>

Grundy · Accepted Answer · 2015-03-26 12:19:55Z

2

You can get it with Array.reduce

var res = [["1,2,3",100,50],["4,5",75,10],["6",20,90]];
res = res.reduce(function(a, b){
   var parts = b[0].split(',');
   parts.forEach(function(i){
     a.push([i, b[1]/parts.length, b[2]/parts.length]);
   });
   return a;
}, []);

But, I believe this still eat double memory :)

Another solution (with res2, but more economy):
It get source entities one-by-one, process it and gc allowed to clean memory...

var res2 = [];
while (res.length) {
    var cur = res.shift();
    var ids = cur[0].split(',')
    ids.forEach(function(i){
      res2.push(i, cur[1]/ids.length, cur[2]/ids.length);
    });
}

edited Mar 26, 2015 at 12:19

Grundy

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answered Mar 26, 2015 at 11:50

vp_arth

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1 Comment

JuniorCompressor Over a year ago

This will eat double memory and it isn't inplace. You just assign res to another array.

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