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I'm going nuts since two days with this problem.

I have an array of arrays, like this ( the number of the arrays can change, also the count of their elements, but to get the thing behind let's assume this example):

array = [ ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18"], ["19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36"], ["37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48", "49", "50", "51", "52", "53", "54"] ];

All I want is to create a new array with some arrays within which look like that:

result = [ ["1", "19", "37"], ["2", "20", 38"], ... ];

I got this working with some foreach-loops, but I want to know how to do this with map() and reduce(). I tried and got some results, but far away from what I need.

Can someone please give me a helping hand for that?

Thanks in advance, Cheers

sagerobert
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1 Answers1

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You could iterate the arrays and change the outer i and inner j index for assigning the actual value.

//                                                v--------v------v
result = array.reduce((r, a, i) => (a.forEach((v, j) => (r[j] = r[j] || [])[i] = v), r), []);
//                           ^----------------------------------------------^


var array = [["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18"], ["19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36"], ["37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48", "49", "50", "51", "52", "53", "54"]],
    result = array.reduce((r, a, i) => (a.forEach((v, j) => (r[j] = r[j] || [])[i] = v), r), []);
    
console.log(result);
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With explicit check and assignment of an empty array.

var array = [["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18"], ["19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36"], ["37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48", "49", "50", "51", "52", "53", "54"]],
    result = array.reduce((r, a, i) => {
        a.forEach((v, j) => {
            if (!(j in r)) {
                r[j] = [];
            }
            r[j][i] = v;
        });
        return r;
    }, []);
    
console.log(result);
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Nina Scholz
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  • Great! It works and explanation included! Thank you so much! Dank Dir ;-) – sagerobert Jun 14 '18 at 20:29
  • One little problem in Typescript: `(r[j] = r[j] || [])[i]` Die Indexsignatur in Typ "string | undefined[]" lässt nur Lesevorgänge zu. The index signature in Type Typ "string | undefined[]" only allows reading operations. The type "string | undefined[]" cannot be assigned to the type "string" . Der Typ "undefined[]" kann dem Typ "string" nicht zugewiesen werden But in Code snippet here, it works... – sagerobert Jun 15 '18 at 05:00