如何在for循环中访问子数组的长度？

Question

I have a 2D array that I am trying to manipulate/merge similar entries. I push -1 on the 3rd index of the arrays I am marking for delete. Here is a sample of rowArray :

[
   [44,16,0,-1],
   [44,16,1,-1],
   [44,16,0]
]

In my for-loop, I decrement through the array and if rowArray[i][3] exists I am trying to pop this array from the matrix. My problem is that in my if i'm always receiving a problem. When I use:

if( rowArray[i][3] ).. or if( rowArray[i][3] !== 'undefined').. etc. then I receive a TypeError:

Uncaught TypeError: Cannot read property 'length' of undefined

I have also tried using:

if( rowArray[i].length > 3 ) but this gives me the error: Uncaught TypeError: Cannot read property 'length' of undefined

Here is my code:

  rowArray = [ [44, 16, 0, -1], [44, 16, 1, -1], [44, 16, 0] ]; for (var i = 1; i < rowArray.length; i++) { if (rowArray[parseInt(i - 1)][0] == rowArray[i][0] && rowArray[parseInt(i - 1)][1] == rowArray[i][1]) { rowArray[i][2] += parseFloat(rowArray[parseInt(i - 1)][2]); rowArray[parseInt(i - 1)][3] = -1; } } for (var i = rowArray.length; i >= 0; i--) { console.log(rowArray[i].length); if (rowArray[i][3] !== 'undefined') { rowArray.splice(i, 1); } } 

Expected Output:

[[44,16,1]]

I should mention that I am using jQuery 3.4.0. Also open to better solutions to this problem (as always).

Answer 1

The problem is that in the second loop you are starting from the rowArray.length which is not right because the last item is rowArray.length-1 . That's because the array starts from 0. So your second loop would be like this:

for (var i = rowArray.length-1; i >= 0; i--) {
   console.log(rowArray[i].length);
   if (rowArray[i][3] !== 'undefined') {
     rowArray.splice(i, 1);
   }
 }