JS 为什么整数变量被重置为初始值而数组变量却没有？

Question

给定一个 matrix/graph/multidimensional 数组的输入（不太确定它们在编程中是否具有唯一定义），下面的函数应该输出最大岛的大小。土地用 1 表示，水用 0 表示，两个有土地的区域只有在水平或垂直（不是对角线）相邻时才相连。

下面的算法有效，但只有当我将所有岛屿保存在一个数组中时，"islands" 变量然后 return 保存的最大值。

最初，我试图仅跟踪 "biggest" 变量的最大值的大小。但是，由于某种原因，它一直被重置为初始值 0。

为什么会这样？我特别困惑为什么这可能发生在 "biggest" 变量而不是 "islands" 变量上，因为两者似乎具有完全相同的范围。

const bigIsland = matrix => {
  let biggest = 0
  let islands = []
  let visited = matrix.map(row => row.map(node => false))
  for(let y=0; y<matrix.length; y++){
    for(let x=0; x<matrix[y].length; x++){
      if (visited[y][x]) continue
      checkIsland(matrix, visited, y, x, islands, biggest)
    }
  }
  return Math.max(...islands)
}

const checkIsland = (matrix, visited, y, x, islands, biggest) => {
  console.log("biggestStart", biggest)
  console.log("islandsStart", islands)
  let currIsland = 0
  let nodesToCheck = [[y,x]]
  while(nodesToCheck.length){
    const [y, x] = nodesToCheck.pop()
    if (visited[y][x]) continue
    visited[y][x] = true
    if (matrix[y][x]===0) continue
    currIsland++
    console.log("currIsland", currIsland)
    getValidNeighbors(matrix, visited, y, x, nodesToCheck)

  }
  if (currIsland > biggest) biggest = currIsland
  if (currIsland > 0) islands.push(currIsland)
  console.log("biggest", biggest)
  console.log("islands", islands)
}

const getValidNeighbors = (matrix, visited, y, x, nodesToCheck) => {
  if(y>0) nodesToCheck.push([y-1, x])
  if(y<matrix.length-1) nodesToCheck.push([y+1, x])
  if(x>0) nodesToCheck.push([y, x-1])
  if(x<matrix[y].length-1) nodesToCheck.push([y, x+1])
}

bigIsland([ [1, 0, 1], 
            [1, 0, 1], 
            [1, 0, 0] ])

Answer 1

函数bigIsland中的变量biggest独立于checkIsland中的变量biggest。改变一个函数中的变量对另一个函数没有影响。数组是不同的，因为当您传递数组时，您会在变量中获得对同一数组的引用。考虑：

function one() {
  let variable = 0;
  console.log("variable in on before calling two ", variable)

  two(variable)
  console.log("variable in on after calling two ", variable)
}

function two(variable) {
  variable += 200
  console.log("variable in two", variable)
}

one()

如果你想让两个函数都改变同一个变量，你可以把这个变量放在一个范围内，两者都可以看到：

let variable = 0
function one(){
    console.log("variable in on before calling two ", variable)

    two()
    console.log("variable in on after calling two ", variable)
}

function two() {
    variable += 200
    console.log("variable in two", variable)
}

one()

或者您可以将其作为对象传递，这是一种非常标准的传递方式：

function one() {
  let obj = {variable: 0}
  console.log("variable in on before calling two ", obj.variable)

  two(obj)
  console.log("variable in on after calling two ", obj.variable)
}

function two(obj) {
  // notice we are not reassigning the variable obj with obj = 200
  //  but changing the thing obj refers to
  obj.variable += 200
  console.log("variable in two", obj.variable)
}

one()

Answer 2

当您将原始变量传递给函数时（例如 biggest 传递给 checkIsland），函数中的任何内容 都可以将原始引用更改为调用函数中的那个变量。为了在 bigIsland 中更改 biggest，您需要明确 return 从 checkIsland:

中找到的 biggest

const bigIsland = matrix => {
  let biggest = 0
  let islands = []
  let visited = matrix.map(row => row.map(node => false))
  for(let y=0; y<matrix.length; y++){
    for(let x=0; x<matrix[y].length; x++){
      if (visited[y][x]) continue
      biggest = checkIsland(matrix, visited, y, x, islands, biggest)
    }
  }
  return Math.max(...islands)
}

const checkIsland = (matrix, visited, y, x, islands, biggest) => {
  console.log("biggestStart", biggest)
  console.log("islandsStart", islands)
  let currIsland = 0
  let nodesToCheck = [[y,x]]
  while(nodesToCheck.length){
    const [y, x] = nodesToCheck.pop()
    if (visited[y][x]) continue
    visited[y][x] = true
    if (matrix[y][x]===0) continue
    currIsland++
    console.log("currIsland", currIsland)
    getValidNeighbors(matrix, visited, y, x, nodesToCheck)

  }
  if (currIsland > biggest) biggest = currIsland
  if (currIsland > 0) islands.push(currIsland)
  console.log("biggest", biggest)
  console.log("islands", islands)
  return biggest;
}

const getValidNeighbors = (matrix, visited, y, x, nodesToCheck) => {
  if(y>0) nodesToCheck.push([y-1, x])
  if(y<matrix.length-1) nodesToCheck.push([y+1, x])
  if(x>0) nodesToCheck.push([y, x-1])
  if(x<matrix[y].length-1) nodesToCheck.push([y, x+1])
}

bigIsland([ [1, 0, 1], 
            [1, 0, 1], 
            [1, 0, 0] ])

另一方面，当您传递 islands 时，islands 是一个数组，它是一个对象（非原始类型），所以当islands 在 checkIsland 内部发生变异，它的变化也将在其他任何地方可见。引用非基元的变量本质上是对对象在内存中的内存位置的引用 - 对象在传递时不会被复制，除非你明确地这样做。

JS 为什么整数变量被重置为初始值而数组变量却没有？

JS Why is the integer variable being reset to initial value but the array variable not?

javascript

arrays

scope

graph-algorithm

ecmascript-6