通过商索引数组

Question

在下面的代码中，i 总是一个偶数，所以商 i / 2 应该总是一个整数。为了安全起见，我还是应该使用 Math.floor(i / 2) 吗？我问是因为 JavaScript 确实将所有数字都视为浮点数，所以我担心舍入误差。

for (var i = 0; i < data.length; i = i + 2) {
    var name = names[i / 2];
    ...
}

Answer 1

没有。在这种情况下您不必使用 Math.floor()。

因为 i 总是偶数而且 names[1.00] 等价于 names[1].

要检查，请在 javascript 控制台中尝试以下操作。

数组的长度为 20，将打印前 10 个数组项

var names = ["nums1", "nums2", "nums3","nums4", "nums5", "nums6","nums7", 
    "nums8", "nums9","nums10", "nums11","nums12", "nums13","nums14", "nums15", 
    "nums16","nums17", "nums18", "nums19","nums20"];

for (var i = 0; i < names.length; i = i + 2) {
    console.log(names[i/2]);
}

Answer 2

偶数除以 2 总是 returns 整数只要不溢出

Number.MAX_SAFE_INTEGER

在那之后，整数以 2 的幂存储，因此它们失去了精度。
然而，大于这个大小的数组在访问它们的元素时会遇到问题（因为索引变得不精确），你可以放入数组的最大元素数被限制为这个最大安全整数。所以基本上你的代码将始终有效，如果它不起作用，那是因为数组溢出而不是因为索引错误。但是，我更推荐你这样做：

  var array = ["one", "two", "three","four", "five", "six","seven", 
    "eight", "nine","ten", "eleven","twelve", "thirteen","fourteen", "fifteen", 
    "sixteen","seventeen", "eighteen", "nineteen","twenty"];
    
  for(var i=0,l=array.length/2;i<l;i++){
      console.log(array[i]);
    }

因为它省去了这些不必要的数学运算...

Answer 3

根据JavaScript(ECMAScript^®) Language Specification 如果

i <= Number.MAX_SAFE_INTEGER + 1

那么 i 就可以准确表示了，除法应该没问题。

The value of Number.MAX_SAFE_INTEGER is the largest integer n such that n and n + 1 are both exactly representable as a Number value.

The value of Number.MAX_SAFE_INTEGER is 9007199254740991 (2⁵³−1).

Section 12.7.3.2 Applying the / Operator

In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.

通过商索引数组

Indexing an array by a quotient

javascript

rounding-error

rounding