CODEWITHSUNDEEP
javascriptarraysforeachmapreduceecmascript-6
kevin ternet
kevin ternet

Reputation: 4612

Can't figure out a mapreduce algorithm

I have this input array already sorted on the key:

var sortedArray = [ [ 'de', [ 1 ] ],
                    [ 'elle', [ 1 ] ],
                    [ 'elle', [ 1 ] ],
                    [ 'la', [ 1 ] ],
                    [ 'la', [ 1 ] ],
                    [ 'la', [ 1 ] ],
                    [ 'le', [ 1 ] ],
                    [ 'maison', [ 1 ] ],
                    [ 'voiture', [ 1 ] ],
                    [ 'voiture', [ 1 ] ] 
];

I want to obtain this reduced Array : 

[ [ 'de', [ 1 ] ],
  [ 'elle', [ 1, 1 ] ],
  [ 'la', [ 1, 1, 1 ] ],
  [ 'le', [ 1 ] ],
  [ 'maison', [ 1 ] ],
  [ 'voiture', [ 1, 1 ] ] 
];

I proceed like that :

sortedArray.forEach((elem, index, arr) => {
   if (elem[0] === arr[index + 1][0]){
      arr[index][1].push(1);
      arr.splice(index + 1, 1);
   }
});

console.log(sortedArray);

But I can't understand why I obtain this result:

 [ [ 'de', [ 1 ] ],
   [ 'elle', [ 1, 1 ] ],
   [ 'la', [ 1, 1 ] ],
   [ 'la', [ 1 ] ],
   [ 'le', [ 1 ] ],
   [ 'maison', [ 1 ] ],
   [ 'voiture', [ 1, 1 ] ] 
 ]

Help would be apreciated.

Upvotes: 0

Views: 42

Answers (2)

Ori Drori
Ori Drori

Reputation: 192252

Use Array.prototype.reduce to create a new array. Because the original array is sorted, you only need to push 1 to the last item in the array, as long as it's the same as the current item, and add a new item whenever that's not true:

var sortedArray = [
  ['de', [1]],
  ['elle', [1]],
  ['elle', [1]],
  ['la', [1]],
  ['la', [1]],
  ['la', [1]],
  ['le', [1]],
  ['maison', [1]],
  ['voiture', [1]],
  ['voiture', [1]]
];

var result = sortedArray.reduce(function(result, item) {
  if (!result.length || item[0] !== result[result.length - 1][0]) { // check if 1st results array is empty or if current item 'key' doesn't match the last item it result
    result.push([item[0], []]); // push a new 'key' item with an empty array of 1s
  }

  result[result.length - 1][1].push(1); // push 1 to last item in result
  
  return result;
}, []);

console.log(result);

Upvotes: 2

winhowes
winhowes

Reputation: 8065

The issue is that you're splicing your array while iterating over it without resetting your current index. One way to get the desired result while using splice is to do something like this:

sortedArray.forEach((elem, index, arr) => {
   while (arr[index + 1] && elem[0] === arr[index + 1][0]){
      arr[index][1].push(1);
      arr.splice(index + 1, 1);
   }
});

Basically we're changing the if statement to a while loop and adding an extra check.

Upvotes: 2

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