Simple Matrix Rotation

Question

I needed to rotate a non-square matrix in my program to transpose some spreadsheet data and decided to make a function to rotate 90, 180, or 270 degrees.

It doesn't rotate the matrix in place, but instead generates a copy and returns that. I want to go for processing efficiency, let me know what I can do better:

//Rotates the provided matrix by provided degrees counterclockwise
function RotateMatrix(matrix, degrees){  
  if(degrees == 90){
    //For 90 degrees swap the height/width and swap the location of each element
    var output = GenerateMatrix(matrix[0].length, matrix.length, 0); //Swapping the width and height for non square matrices
    for(var i = 0; i < matrix[0].length; i++){
      for(var j = 0; j < matrix.length; j++){
        output[i][j] = matrix[j][i];
      }
    }    
  } else if(degrees == 180) {
    //For 180 degrees, rebuild array backwards
    var output = GenerateMatrix(matrix.length, matrix[0].length, 0);
    for(var i = matrix.length - 1; i >= 0; i--){
      for(var j = matrix[0].length - 1; j >=0; j--){
        output[matrix.length - 1 - i][matrix[0].length - 1 - j] = matrix[i][j];
      }
    }    
  } else if(degrees == 270) {
    //For 270 degrees, not sure how to make short description
    var output = GenerateMatrix(matrix[0].length, matrix.length, 0); //Swapping the width and height for non square matrices
    for(var i = 0; i < matrix[0].length; i++){
      for(var j = matrix.length - 1; j >=0; j--){
        output[i][matrix.length - 1 - j] = matrix[j][i];
      }
    }      
  }   
  return output;
}

//Generates a matrix with the requested length and width and value
function GenerateMatrix(length, width, value){
  var output = [];
  for(var i = 0; i < length; i++){
    width > 0 ? output.push([]) : output.push(value); //If matrix has 0 width
    for(var j = 0; j < width; j++){
      output[i].push(value)
    }
  }
  return output;
}

SirPython · Accepted Answer · 2016-04-11 00:02:12Z

2

Unnecessary matrix generation

In JavaScript, Arrays don't have a fixed size. For example, you could do this:

var arr = [];
arr[324] = 91;

with no problem.

Your GenerateMatrix function is really over-doing it. All you need to do is create the arrays for the rows/columns then change their values.

And, you can do that with a neat trick:

Array.apply(null, Array(rowCount)).map(function(){return []});

edited Apr 11, 2016 at 0:02

answered Apr 10, 2016 at 23:20

SirPython

13.5k3 gold badges38 silver badges93 bronze badges

\$\begingroup\$ Thanks for the reply. I've typically programmed in C#, so JS is pretty new to me. That's good to know that I don't need to generate the "blank" array first. I initially avoided map due to the performance hit, looping ended up being more performance friendly when this is being executed in a cloud environment with very limited processing time. \$\endgroup\$

Douglas Gaskell
– Douglas Gaskell

2016-04-10 23:55:47 +00:00
Commented Apr 10, 2016 at 23:55

Add a comment |

tkellehe · Accepted Answer · 2016-04-12 12:42:01Z

Here's a version to where it does not flip, but instead reads or writes into the correct location. Fastest way I know to "rotate" it, but does add some overhead when reading and writing.

function addC(row, i) {
    Object.defineProperty(row.self, i, {
        get: function() { 
             if(!(row.matrix.degrees % 360)) {
                 return row.data[i];
             }
             else if(!(row.matrix.degrees % 270)) {
             }
             else if(!(row.matrix.degrees % 180)) {
             }
             else if(!(row.matrix.degrees % 90)) {
                 return row.matrix.rows[i].data[row.matrix.column];
             }
        },
        set: function(v) { 
             if(!(row.matrix.degrees % 360)) {
                 row.data[i] = v;
             }
             else if(!(row.matrix.degrees % 270)) {
             }
             else if(!(row.matrix.degrees % 180)) {
             }
             else if(!(row.matrix.degrees % 90)) {
                 row.matrix.rows[i].data[row.matrix.column] = v;
             }
        },
        enumerable: true
    });
}

function Row(matrix) {
    var members = {
        self: this,
        matrix: matrix,
        data: {}
    }
    for(var i = matrix.width; i--;) {
        addC(members, i);
    }
    Object.defineProperty(members.self, "data", { value: members.data });
}

function addR(matrix, i) {
    matrix.rows[i] = new Row(matrix);
    Object.defineProperty(matrix.self, i, {
        get: function() { matrix.column = i; return matrix.rows[i]; },
        enumerable: true
    });
}

function Matrix(width, height) {
    var members = {
        self: this,
        width: width,
        height: height,
        degrees: 0,
        column: -1,
        rows: {}
    };

    for(var i = height; i--;) {
        addR(members, i);
    }
    Object.defineProperty(members.self, "degrees", { value: function(v) {
        if(v === undefined) return members.degrees;
        // Can add more checks if need be.
        if(!(v % 90)) members.degrees = v;
        return members.self;
    }});
}

You probably know more about the rotations than I do so I did the ones that I was 100% sure were correct. It is pretty straight forward, when the matrix object is accessed it returns a row object that then figures out what data you actually meant.

var matrix = new Matrix(2, 2);
matrix[0][1] = 12;
matrix.degrees(90);
console.log(matrix[1][0]); // => 12

Hope this helps!

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