将值添加到数组的最有效方法 [英] Most efficient way to prepend a value to an array

问题描述

假设我的数组大小为 N （其中 N> 0 ），是否有一个更有效的方法是在数组之前不需要O（N + 1）步骤？

Assuming I have an array that has a size of N (where N > 0), is there a more efficient way of prepending to the array that would not require O(N + 1) steps?

在代码中，基本上，我目前正在做的是

In code, essentially, what I currently am doing is

function prependArray(value, oldArray) {
  var newArray = new Array(value);

  for(var i = 0; i < oldArray.length; ++i) {
    newArray.push(oldArray[i]);
  }

  return newArray;
}

推荐答案

我不确定在big-O方面效率更高，但肯定使用 unshift 方法更简洁：

I'm not sure about more efficient in terms of big-O but certainly using the unshift method is more concise:

var a = [1, 2, 3, 4];
a.unshift(0);
a; // => [0, 1, 2, 3, 4]

此 jsPerf基准显示至少在几个浏览器中， unshift 速度要快得多，无论可能不同的大O性能如果你可以修改数组-地点。如果你真的不能改变原始数组，那么你会做类似下面的代码片段，它似乎没有你的解决方案快得多：

This jsPerf benchmark shows that unshift is decently faster in at least a couple of browsers, regardless of possibly different big-O performance if you are ok with modifying the array in-place. If you really can't mutate the original array then you would do something like the below snippet, which doesn't seem to be appreciably faster than your solution:

a.slice().unshift(0); // Use "slice" to avoid mutating "a".

为了完整性，可以使用以下函数代替OP的示例 prependArray（...）来利用数组 unshift （...）方法：

For completeness, the following function can be used instead of OP's example prependArray(...) to take advantage of the Array unshift(...) method:

function prepend(value, array) {
  var newArray = array.slice();
  newArray.unshift(value);
  return newArray;
}

var x = [1, 2, 3];
var y = prepend(0, x);
y; // => [0, 1, 2, 3];
x; // => [1, 2, 3];

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