How to get the element which are diagonal to a certain index in an array which represents a rectangle

Question

Consider an array whose length will be always product of two numbers. For below array l is 4 and w is 5.

There is also a given index. I want to get the two arrays containing elements which lie on the diagonal line which passes through that particular index.

[
    0,  1,  2,  3,  4
    5,  6,  7,  8,  9
    10, 11, 12, 13, 14
    15, 16, 17, 18, 19
]  

index = 7 => [3, 7, 11, 15] and [1, 7, 13, 19]
index = 16 => [4, 8, 12, 16] and [10, 16]
index = 0 => [0, 6, 12, 18] and [0]

I have tried following:

[
    0,  1,  2,  3,  4
    5,  6,  7,  8,  9
    10, 11, 12, 13, 14
    15, 16, 17, 18, 19
]  

index = 7 => [3, 7, 11, 15] and [1, 7, 13, 19]
index = 16 => [4, 8, 12, 16] and [10, 16]
index = 0 => [0, 6, 12, 18] and [0]

The code have two problems. Both the arrays in the result are same. And secondly they are not in order.

Note: I don't want to use sort() to reorder the array. And also I don't want loop through all 20 elements. Just want to get the elements of that diagonal row

Answer 1

尝试下面的代码

let arr = Array(20).fill().map((x,i) => i);

function getDias(arr, l, w, ind){
  let arr1 = [];
  let arr2 = [];
  n = l*w;
  lVal = Math.floor(ind/w);
  rVal = ind%w;
  temp1  = lVal;
  temp2  = rVal;
  while(temp1>=0 && temp2>=0){
    val = ((w*temp1) + temp2);
    arr1.unshift(arr[val]);  
    temp1--;
    temp2--;
  }
  temp1  = lVal;
  temp2  = rVal;
  temp1++;
  temp2++;
  while(temp1<l && temp2<w){
    val = ((w*temp1) + temp2);
    arr1.push(arr[val]);  
    temp1++;
    temp2++;
  }

   
  console.log(arr1);
  temp1  = lVal;
  temp2  = rVal;
  while(temp1>=0 && temp2<w){
    val = ((w*temp1) + temp2);
    arr2.unshift(arr[val]);  
    temp1--;
    temp2++;
  }
  
  temp1  = lVal;
  temp2  = rVal;
  temp1++;
  temp2--;
  while(temp1<l && temp2>=0){
    val = ((w*temp1) + temp2);
    arr2.push(arr[val]);  
    temp1++;
    temp2--;
  }

  console.log(arr2);

}
getDias(arr, 4, 5, 7);
getDias(arr, 4, 5, 16);
getDias(arr, 4, 5, 0);

这个想法是计算 l_val 和 r_val。

l_val = index/w
r_val = index%w

现在 arr[l_val][r_val] 标记由 l_val* w+ r_val 找到的矩阵中的位置

接下来是 4 个步骤：

1) 从arr[l_val][r_val]开始迭代。两者都减 1，直到到达终点。将其取消移动到 array_1 （以维持顺序）

2）从[l_val][r_val]开始迭代。两者都加1，直到到达终点。将其推入 array_1。

3) 从arr[l_val][r_val]开始迭代。从 l_val 中减去 1，并添加 1 t r_val 直到到达末尾。将其取消移动到 array_2 （以维持顺序）

4）从[l_val][r_val]开始迭代。l_val加1，r_val减1，直到迭代结束。将其推入 array_2。