Javascript 嵌套循环答案

【问题标题】：Javascript nested loopsJavascript 嵌套循环
【发布时间】：2017-11-08 19:38:08
【问题描述】：

您好，我是 javascript 新手，谁能告诉我，既然我已经定义了所有月份，我将如何显示奇数或偶数月份？

谢谢。

<html>
<head>
  <title>Show Odd Months</title>
    <script>
      function demo(){
    var month = new Array(5);
     month[1] = "January";
     month[2] = "February";
     month[3] = "March";
     month[4] = "April";
     month[5] = "May";      
     month[6] = "June"; 
     month[7] = "July"; 
     month[8] = "August"; 
     month[9] = "September";
     month[10] = "October";
     month[11] = "November";   
     month[12] = "December";
        
        
      for (var i in month)
      {
        document.write("month[" + i + "]  = " + month[i] + "<br/>");        
      }
      
      }
      
    </script>
</head>
<body>
  <p>Click the button below</p>
  <button onclick="demo()">Show Odd Months</button>

</body>
</html>

【问题讨论】：

for (var i = 0; i < month.length; i += 2)?如果你从 0 开始，你会得到偶数，如果你从 1 开始，你会得到赔率。（或者，如果您像往常一样从索引 0 开始填充数组）
另外，你为什么把5放在这里？ var month = new Array(5); 你显然不想要一个有 5 个东西的数组，你想要一个有 12 个东西的数组。
这是求奇数和偶数的基本数学运算

标签： javascript html for-loop

【解决方案1】：

使用for loop，步长为 2，从 1 开始表示偶数，从 0 开始表示奇数（因为数组是从 0 开始的，而月份是从 1 开始的）。由于奇数和偶数的概念相似，我们可以创建一个函数，通过更改几个参数来生成相关（奇数、偶数）函数。

// the array is created using an array literal
var months = ["January","February","March","April","May","June","July","August","September","October","November","December"];

// showMonths takes a start and end values, and returns a functions that uses those values when invoked
function showMonths(start, step) {
  start = start || 0; // if start is not stated, it's 0
  step = step || 1; // if step is not stated or is 0, it's 1

  return function() {
    // the loop uses start and step to iterate
    for (var i = start; i < months.length; i += step) {
      document.write("month[" + i + "]  = " + months[i] + "<br/>");
    }
  }
}


var showOdd = showMonths(0, 2); // create showOdd by using start 0, and step 2
var showEven = showMonths(1, 2); // create showEven by using start 1, and step 2

<button onclick="showOdd()">Show Odd Months</button>
<button onclick="showEven()">Show Even Months</button>

【讨论】：

【解决方案2】：

您可以使用模除法来判断索引是否为奇数。模除法与普通除法一样，只是答案是余数。这里的运营商是%。

for (var i in month)
{
    if (i % 2 === 1) {
        document.write("month[" + i + "]  = " + month[i] + "<br/>");        
    }
}

【讨论】：

【解决方案3】：

检查 i 是奇数还是偶数。

<html>
<head>
  <title>Show Odd Months</title>
    <script>
      function demo(){
    var month = new Array(5);
     month[1] = "January";
     month[2] = "February";
     month[3] = "March";
     month[4] = "April";
     month[5] = "May";      
     month[6] = "June"; 
     month[7] = "July"; 
     month[8] = "August"; 
     month[9] = "September";
     month[10] = "October";
     month[11] = "November";   
     month[12] = "December";
        
        
      for (var i in month)
      {if (i%2 ==0){
        document.write("month[" + i + "]  = " + month[i] + "<br/>");        
      }
}
      
      }
      
    </script>
</head>
<body>
  <p>Click the button below</p>
  <button onclick="demo()">Show Odd Months</button>

</body>
</html>

【讨论】：