Anthony Chung - 6 months ago 39

Javascript Question

when it comes to matrix traversals and any kind of search, I find that I often need to validate a potential item to either add to my queue or recurse over.

say we're looking at this matrix:

`var matrixTest = [`

[1,1,0,0,0],

[1,1,0,0,0],

[0,0,1,0,0],

[0,0,0,1,1]

]

In this case, I am iterating over all items and performing a BFS (marking as visited) when I encounter a 1.

I have a subroutine within my BFS function that, given a coordinate r and c, it validates all possible paths leading from it (r+1, r-1, c+1, c-1).

The problem is with this piece of code

`if(!visitMap[r+1][c] && matrix[r+1][c] === 1) {`

q.push([r+1, c])

}

visitMap is a matrix I created alongside the algorithm to ensure I'm not processing a point twice

matrix is the input matrix

However, it appears that I need to validate inputs before testing because JavaScript throws this error

`TypeError: Cannot read property '3' of undefined`

I presume this means that my "r+1" expression lies outside the bounds of the matrix, and thus processes as undefined.

It seems exceedingly tedious to add another layer of if/then flow to check the bounds of r+1, r-1, c+1, AND c-1.

Is there a particular code pattern you'd recommend using to avoid having to do this many times?

otherwise I think the code block will look like this:

`if (r+1 < matrix.length) {`

if(!visitMap[r+1][c] && matrix[r+1][c] === 1) {

q.push([r+1, c])

}

}

Answer

You can try to encapsulate the validation of bounds and your condition inside a function like this:

```
function pushToQueue(visitMap, matrix, r, c) {
if( typeof(matrix[r]) == "undefined"
|| typeof(matrix[r][c]) == "undefined" ) {
return false;
}
return !visitMap[r][c] && matrix[r][c] === 1;
}
```

And you can call it like this:

```
if(pushToQueue(visitMap, matrix, r + 1, c)) {
q.push([r+1, c]);
}
if(pushToQueue(visitMap, matrix, r - 1, c)) {
q.push([r-1, c]);
}
// etc ...
```