测试值是奇数还是偶数 [英] Testing whether a value is odd or even

问题描述

我决定用一个非常简单的算法创建简单的 isEven 和 isOdd 函数：

I decided to create simple isEven and isOdd function with a very simple algorithm:

function isEven(n) {
  n = Number(n);
  return n === 0 || !!(n && !(n%2));
}

function isOdd(n) {
  return isEven(Number(n) + 1);
}

如果n具有某些参数，则可以，但在许多情况下都会失败。所以我开始创建强大的函数，为尽可能多的场景提供正确的结果，这样只测试javascript数量限制内的整数，其他一切都返回false（包括+和 - 无穷大）。注意零是偶数。

That is OK if n is with certain parameters, but fails for many scenarios. So I set out to create robust functions that deliver correct results for as many scenarios as I could, so that only integers within the limits of javascript numbers are tested, everything else returns false (including + and - infinity). Note that zero is even.

// Returns true if:
//
//    n is an integer that is evenly divisible by 2
//
// Zero (+/-0) is even
// Returns false if n is not an integer, not even or NaN
// Guard against empty string

(function (global) {

  function basicTests(n) {

    // Deal with empty string
    if (n === '') 
      return false;

    // Convert n to Number (may set to NaN)
    n = Number(n);

    // Deal with NaN
    if (isNaN(n)) 
      return false;

    // Deal with infinity - 
    if (n === Number.NEGATIVE_INFINITY || n === Number.POSITIVE_INFINITY)
      return false;

    // Return n as a number
    return n;
  }

  function isEven(n) {

    // Do basic tests
    if (basicTests(n) === false)
      return false;

    // Convert to Number and proceed
    n = Number(n);

    // Return true/false
    return n === 0 || !!(n && !(n%2));
  }
  global.isEven = isEven;

  // Returns true if n is an integer and (n+1) is even
  // Returns false if n is not an integer or (n+1) is not even
  // Empty string evaluates to zero so returns false (zero is even)
  function isOdd(n) {

    // Do basic tests
    if (basicTests(n) === false)
      return false;

    // Return true/false
    return n === 0 || !!(n && (n%2));
  }
  global.isOdd = isOdd;

}(this));

任何人都可以看到上述问题吗？是否有更好的（即更准确，更快或更简洁而不被混淆）版本？

Can anyone see any issues with the above? Is there a better (i.e. more accurate, faster or more concise without being obfuscated) version?

有各种与其他语言相关的帖子，但我似乎无法找到ECMAScript的最终版本。

There are various posts relating to other languages, but I can't seem to find a definitive version for ECMAScript.

添加了代码审查标记，但我不是在代码审查之后。我简单地发布了代码，以便其他人可以看到我在哪里，而不是要审查的东西。到目前为止发布的答案似乎得到了。

A code review tag was added, but I'm not after a code review. I posted the code simply so others could see where I was up to, not as something to review. Answers posted so far seem to get that.

根据史蒂夫的回答，最终函数：

A final function, based on Steve's answer:

// Use abstract equality == for "is number" test
function isEven(n) {
  return n == parseFloat(n)? !(n%2) : void 0;
}

// Use strict equality === for "is number" test
function isEvenStrict(n) {
  return n === parseFloat(n)? !(n%2) : void 0;
}

任何不是数字的东西都会返回 undefined ，数字返回 true 或 false 。它可能是一个带有严格标志的函数，但我认为严格的比较并不是真正需要的。

Anything that is not a number returns undefined, numbers return either true or false. It could be one function with a strict flag, but I think the strict comparison is not really required.

推荐答案

使用模数：

function isEven(n) {
   return n % 2 == 0;
}

function isOdd(n) {
   return Math.abs(n % 2) == 1;
}

您可以检查Javascript中的任何值是否都可以强制转换为以下数字：

You can check that any value in Javascript can be coerced to a number with:

Number.isFinite(parseFloat(n))

此检查最好在 isEven 和 isOdd 函数之外完成，因此您不必在两个函数中复制错误处理。

This check should preferably be done outside the isEven and isOdd functions, so you don't have to duplicate error handling in both functions.

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