分配给数组无法正常工作

Question

I am trying to rotate array clockwise for example from:

[[1,2,3], 
 [4,5,6], 
 [7,8,9]]

make:

   [[7,4,1], 
    [8,5,2], 
    [9,6,3]]

so I written a function for that its looks as below:

function rotateImage(a) {
    const arr = Array(a.length).fill(Array(a.length).fill(0));
    for(let i =0; i< a.length; i++) {
        for(let j = 0; j < a[i].length;j++ ){
            console.log(i,j);
            arr[j][a[i].length-1 - i] = a[i][j];
            console.log(arr)
        }
    }
    return arr;
}

here are logs i get:

0 0
[ [ 0, 0, 1 ], [ 0, 0, 1 ], [ 0, 0, 1 ] ]  
0 1
[ [ 0, 0, 2 ], [ 0, 0, 2 ], [ 0, 0, 2 ] ] 
0 2
[ [ 0, 0, 3 ], [ 0, 0, 3 ], [ 0, 0, 3 ] ]
1 0
[ [ 0, 4, 3 ], [ 0, 4, 3 ], [ 0, 4, 3 ] ]
1 1
[ [ 0, 5, 3 ], [ 0, 5, 3 ], [ 0, 5, 3 ] ] 
1 2
[ [ 0, 6, 3 ], [ 0, 6, 3 ], [ 0, 6, 3 ] ]
2 0
[ [ 7, 6, 3 ], [ 7, 6, 3 ], [ 7, 6, 3 ] ]
2 1
[ [ 8, 6, 3 ], [ 8, 6, 3 ], [ 8, 6, 3 ] ]
2 2
[ [ 9, 6, 3 ], [ 9, 6, 3 ], [ 9, 6, 3 ] ]

so when i have i=0 and j=0 , My code should turn to something like this:

arr[0][2]= a[0][0];
// so I should get 
[ [ 0, 0, 1 ], [ 0, 0, 0 ], [ 0, 0, 0 ] ]
// insted of 
[ [ 0, 0, 1 ], [ 0, 0, 1 ], [ 0, 0, 1 ] ]

can somebody explain for me what is going on here?

Answer 1

The problem is the way you created your output array:

const arr = Array(a.length).fill(Array(a.length).fill(0));

This says "Every element of arr should equal the result of Array(a.length).fill(0) . That is to say, array arr has three elements, each of which is the same object .

Therefore, when you assign a value to arr[0][2] , you are also assigning it to arr[1][2] , etc.

You could use a loop like this to create your new matrix:

var arr = Array(a[0].length);
for (var i = 0; i < arr.length; i++)
    arr[i] = Array(a.length).fill(0);

Answer 2

the problem is the output array

Solution :

function rotateImage(a) {
    let arr = Array(a.length);

    for(let i =0; i< 3; i++) {
        for(let j = 0; j < a[i].length;j++ ){
                if(!arr[j]) {
                    arr[j] = Array(a[j].length)
                }
                arr[j][a[i].length-1 - i] =  a[i][j];
        }
    }
    return arr;
}

Answer 3

This function can be written as three maps, in a one-liner:

 const rotateImage = a => a[0].map((_, i) => a.map(r => r[i])).map(r => r.reverse()) console.log(rotateImage([ [1, 2, 3], [4, 5, 6], [7, 8, 9], ])) 

Note that this works with non-square grids as well.