How to check values of adjacent items in a list? Javascript-CodePudding

I have a list of list that looks something like this

let pixels = [
 [0,0,0,0]
 [0,0,0,0]
 [0,0,0,0]
]

and I would like to iterate through this list and create a list of the adjacent items for each item. In this example the output I would want from pixels[0][0] is: [undefined, undefined, undefined, undefined, 0, undefined, 0, 0]. But when I run my code I get I get an error

The code cannot read pixels[-1][-1], I just want it to read it as undefined and not crash the whole program.

let pixels = [
  [0, 0, 0, 0],
  [0, 0, 0, 0],
  [0, 0, 0, 0]
]

function adjacent(i1, i2) {
  const yep = [pixels[i1 - 1][i2 - 1], pixels[i1 - 1][i2], pixels[i1 - 1][i2   1], pixels[i1][i2 - 1], pixels[i1][i2   1], pixels[i1   1][i2 - 1], pixels[i1   1][i2], pixels[i1   1][i2   1]]
  return yep
}

console.log(adjacent(1,4))

console.log(adjacent(-1,-1))

CodePudding user response：

When i is out of range, pixel[i] is undefined, and undefined[anything] is an error.

You can use optional chaining to suppress these errors.

let pixels = [
  [0, 0, 0, 0],
  [0, 0, 0, 0],
  [0, 0, 0, 0]
]

function adjacent(i1, i2) {
  const yep = [pixels[i1 - 1]?.[i2 - 1], pixels[i1 - 1]?.[i2], pixels[i1 - 1]?.[i2   1], 
    pixels[i1]?.[i2 - 1], pixels[i1]?.[i2   1], pixels[i1   1]?.[i2 - 1], 
    pixels[i1   1]?.[i2], pixels[i1   1]?.[i2   1]
  ]
  return yep
}

console.log(adjacent(1, 4))

console.log(adjacent(-1, -1))

CodePudding user response：

You can use && to test the first element exists

let pixels = [
  [0, 0, 0, 0],
  [0, 0, 0, 0],
  [0, 0, 0, 0]
]

function adjacent(i1, i2) {
  const yep = [
  pixels[i1 - 1] && pixels[i1 - 1][i2 - 1], 
  pixels[i1 - 1] && pixels[i1 - 1][i2], 
  pixels[i1 - 1] && pixels[i1 - 1][i2   1], 
  pixels[i1]     && pixels[i1][i2 - 1], 
  pixels[i1]     && pixels[i1][i2   1], 
  pixels[i1   1] && pixels[i1   1][i2 - 1], 
  pixels[i1   1] && pixels[i1   1][i2],
  pixels[i1   1] && pixels[i1   1][i2   1]]
  return yep
}

console.log(adjacent(1,4))

console.log(adjacent(-1,-1))

CodePudding user response：

In addition to using optional chaining, you can use two loops to generate the adjacent coordinates rather than manually writing out each possibility, which is easier to maintain.

let pixels = [
  [0, 0, 0, 0],
  [0, 0, 0, 0],
  [0, 0, 0, 0]
]

function adjacent(i1, i2) {
  const res = [];
  for(let deltaY = -1; deltaY <= 1; deltaY  )
    for(let deltaX = -1; deltaX <= 1; deltaX  )
      if(deltaY != 0 || deltaX != 0)
        res.push(pixels[i1   deltaY]?.[i2   deltaX]);
  return res;
}

console.log(...adjacent(1,4))

console.log(...adjacent(-1,-1))