Javascript - Converting columns into rows

Question

relatively new to JS, trying to understand this code which takes an array of arrays and converts its columns into arrays.

grid= 
[[".",".",".","1","4",".",".","2","."], 
[".",".","6",".",".",".",".",".","."], 
[".",".",".",".",".",".",".",".","."], 
[".",".","1",".",".",".",".",".","."], 
[".","6","7",".",".",".",".",".","9"], 
[".",".",".",".",".",".","8","1","."], 
[".","3",".",".",".",".",".",".","6"], 
[".",".",".",".",".","7",".",".","."], 
[".",".",".","5",".",".",".","7","."]]

//Turn columns into rows
var transpose = grid =>
    grid[0].map(
        (_,c) => grid.map(
            row => row[c]
        )
    )

How would this look using regular functions?

What do you mean by “regular” functions?

Sebastian Simon
– Sebastian Simon

2017-10-29 23:13:03 +00:00
Commented Oct 29, 2017 at 23:13 — Sebastian Simon
– Sebastian Simon, Commented Oct 29, 2017 at 23:13
add an explanation on what you should want as result.

user8556290
– user8556290

2017-10-29 23:13:28 +00:00
Commented Oct 29, 2017 at 23:13 — user8556290
– user8556290, Commented Oct 29, 2017 at 23:13
Do you mean using "for" loops?

Dust_In_The_Wind
– Dust_In_The_Wind

2017-10-29 23:15:04 +00:00
Commented Oct 29, 2017 at 23:15 — Dust_In_The_Wind
– Dust_In_The_Wind, Commented Oct 29, 2017 at 23:15

Martin Adámek · Accepted Answer · 2017-10-29 23:24:46Z

1

var transpose = grid =>     // defining `transpose` as function with parameter `grid`, returning the following expression
    grid[0].map(            // return values from first row mapped via following function
        (_,c) => grid.map(  // using second parameter which is an index to do column mapping
            row => row[c]   // returning the value with index of row but from column
        )
    )

So without this ES6 map function, it would be 2 nested for cycles:

var grid = 
  [[".",".",".","1","4",".",".","2","."], 
  [".",".","6",".",".",".",".",".","."], 
  [".",".",".",".",".",".",".",".","."], 
  [".",".","1",".",".",".",".",".","."], 
  [".","6","7",".",".",".",".",".","9"], 
  [".",".",".",".",".",".","8","1","."], 
  [".","3",".",".",".",".",".",".","6"], 
  [".",".",".",".",".","7",".",".","."], 
  [".",".",".","5",".",".",".","7","."]];

function transpose(grid) {
    var ret = [];
    for (var i = 0; i < grid[0].length; i++) {
        ret[i] = [];
        for (var j = 0; j < grid.length; j++) {
            ret[i][j] = grid[j][i];
        }
    }

    return ret;
}

// use map for pretty printing the matrix
console.log(grid.map(row => row.join(' ')));
console.log(transpose(grid).map(row => row.join(' ')));

edited Oct 29, 2017 at 23:24

answered Oct 29, 2017 at 23:18

Martin Adámek

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1 Comment

ninety nine Over a year ago

Thank you, extremely precise and clear response.. Appreciate the expertise and help very much.

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