Array.prototype.filter() 的就地替代方法是什么 [英] What is the in-place alternative to Array.prototype.filter()

问题描述

我有一个数组，我想从中删除一些元素.我不能使用 Array.prototype.filter()，因为我想就地修改数组(因为它节省了内存分配，而且对我来说更重要的是，使代码在我的用例).是否有我可以使用的 filter 的就地替代方案，可能类似于 Array.prototype.forEach() 可以用作 Array.prototype.map()?

I've got an array that I would like to remove some elements from. I can't use Array.prototype.filter(), because I want to modify the array in place (because it saves a memory allocation and, more important for me, makes the code more simple in my use case). Is there an in-place alternative to filter that I can use, maybe analogously to how Array.prototype.forEach() can be used as an in-place variant to Array.prototype.map()?

应要求提供的最小示例:

Minimum example upon request:

function someCallback(array) {
  // do some stuff
  array.filterInPlace(function(elem) {
    var result = /* some logic */
    return result;
  })
  // do some more stuff
}

推荐答案

是否有过滤器的就地替代方案

Is there an in-place alternative to filter

不，但自己编写并不难.这是一种挤出所有不符合条件的值的方法.

No, but it's not hard to write your own. Here is an approach which squeezes out all the values which fail a condition.

function filterInPlace(a, condition) {
  let i = 0, j = 0;

  while (i < a.length) {
    const val = a[i];
    if (condition(val, i, a)) a[j++] = val;
    i++;
  }

  a.length = j;
  return a;
}

condition 被设计为与传递给 Array#filter 的回调具有相同的签名，即 (value, index, array).为了与 Array#filter 完全兼容，您还可以接受第四个 thisArg 参数.

condition is designed to have the same signature as the callback passed to Array#filter, namely (value, index, array). For complete compatibility with Array#filter, you could also accept a fourth thisArg parameter.

使用 forEach 有一个小优势，它会跳过空槽.这个版本:

Using forEach has the minor advantage that it will skip empty slots. This version:

• 用空槽压缩数组
• 实现thisArg
• 跳过分配，如果我们还没有遇到失败的元素

function filterInPlace(a, condition, thisArg) {
  let j = 0;

  a.forEach((e, i) => { 
    if (condition.call(thisArg, e, i, a)) {
      if (i!==j) a[j] = e; 
      j++;
    }
  });

  a.length = j;
  return a;
}

a = [ 1,, 3 ];
document.write('<br>[',a,']');

filterInPlace(a, x=>true);
document.write('<br>[',a,'] compaction when nothing changed');

b = [ 1,,3,,5 ];
document.write('<br>[',b,']');

filterInPlace(b, x=>x!==5);
document.write('<br>[',b,'] with 5 removed');

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