Use Filter() on a particular row of a 2 dimensional array (Javascript)

Question

I would like to have a "unique" array of another multi-dimensional array, the array looks something like this:

var bigArray = Array(
                ["text", "text", "A", 1, 2],
                ["text", "text", "S", 3, 4],
                ["text", "text", "S", 4, 4],
                ["text", "text", "S", 5, 7],
                ["text", "text", "S", 2, 1],
                ["text", "text", "S", 1, 0]
            );

The result should be something like

uniqueArray = [A, S]

And this is the filter function I have atm:

var uniqueArray = bigArray.filter(function(item, i, ar){ return ar.indexOf(item) === i; });

The function gives me ALL the unique values in the big array, but I only need the unique values from row[2]. I guess the parameters must be tweaked a bit from the filter function but I don't understand them well. Thanks for your help

Nina Scholz · Accepted Answer · 2017-03-09 16:40:25Z

4

Array#filter returns items if the array and not parts of the inner array. Therefore, you need a different approach, by mapping only the wanted value and fillter later.

With ES6 you could map the wanted column and use Set for unique values.

var bigArray = [["text", "text", "A", 1, 2], ["text", "text", "S", 3, 4], ["text", "text", "S", 4, 4], ["text", "text", "S", 5, 7], ["text", "text", "S", 2, 1], ["text", "text", "S", 1, 0]],
    result = [...new Set(bigArray.map(a => a[2]))];
    
console.log(result);

ES5 with a hash table.

var bigArray = [["text", "text", "A", 1, 2], ["text", "text", "S", 3, 4], ["text", "text", "S", 4, 4], ["text", "text", "S", 5, 7], ["text", "text", "S", 2, 1], ["text", "text", "S", 1, 0]],
    hash = Object.create(null),
    result;

bigArray.forEach(function (a) {
    hash[a[2]] = true;
});
result = Object.keys(hash);
    
console.log(result);

answered Mar 9, 2017 at 16:40

Nina Scholz

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1 Comment

smirnoff103 Over a year ago

Thanks Nina for your answer, but it's too complex for me I'm sorry :p I will go for the answer here below from @brk it's more readable and I understand what it does. Thanks for your time anyway.

brk · Accepted Answer · 2017-03-09 16:45:24Z

1

You can do like this

var bigArray = Array(
  ["text", "text", "A", 1, 2],
  ["text", "text", "S", 3, 4],
  ["text", "text", "S", 4, 4],
  ["text", "text", "S", 5, 7],
  ["text", "text", "S", 2, 1],
  ["text", "text", "S", 1, 0]
);

var newArray = []
bigArray.forEach(function(item) {
  if (newArray.indexOf(item[2]) == -1) {
    newArray.push(item[2])
  }
})
console.log(newArray)

DEMO

answered Mar 9, 2017 at 16:45

brk

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