JavaScript趣题：统计降水量

气象台的工作人员委托你对一组数据进行分析，这里有全球一些城市从一月到十二月份的降水量，每个城市的数据都由"\n"分割开来，每个城市名后面紧跟着一个":"冒号。

如下所示：

	
		var data = "Rome:Jan 81.2,Feb 63.2,Mar 70.3,Apr 55.7,May 53.0,Jun 36.4,Jul 17.5,Aug 27.5,Sep 60.9,Oct 117.7,Nov 111.0,Dec 97.9"
				+ "\n"
				+ "London:Jan 48.0,Feb 38.9,Mar 39.9,Apr 42.2,May 47.3,Jun 52.1,Jul 59.5,Aug 57.2,Sep 55.4,Oct 62.0,Nov 59.0,Dec 52.9"
				+ "\n"
				+ "Paris:Jan 182.3,Feb 120.6,Mar 158.1,Apr 204.9,May 323.1,Jun 300.5,Jul 236.8,Aug 192.9,Sep 66.3,Oct 63.3,Nov 83.2,Dec 154.7"
				+ "\n"
				+ "NY:Jan 108.7,Feb 101.8,Mar 131.9,Apr 93.5,May 98.8,Jun 93.6,Jul 102.2,Aug 131.8,Sep 92.0,Oct 82.3,Nov 107.8,Dec 94.2"
				+ "\n"
				+ "Vancouver:Jan 145.7,Feb 121.4,Mar 102.3,Apr 69.2,May 55.8,Jun 47.1,Jul 31.3,Aug 37.0,Sep 59.6,Oct 116.3,Nov 154.6,Dec 171.5"
				+ "\n"
				+ "Sydney:Jan 103.4,Feb 111.0,Mar 131.3,Apr 129.7,May 123.0,Jun 129.2,Jul 102.8,Aug 80.3,Sep 69.3,Oct 82.6,Nov 81.4,Dec 78.2"
				+ "\n"
				+ "Bangkok:Jan 10.6,Feb 28.2,Mar 30.7,Apr 71.8,May 189.4,Jun 151.7,Jul 158.2,Aug 187.0,Sep 319.9,Oct 230.8,Nov 57.3,Dec 9.4"
				+ "\n"
				+ "Tokyo:Jan 49.9,Feb 71.5,Mar 106.4,Apr 129.2,May 144.0,Jun 176.0,Jul 135.6,Aug 148.5,Sep 216.4,Oct 194.1,Nov 95.6,Dec 54.4"
				+ "\n"
				+ "Beijing:Jan 3.9,Feb 4.7,Mar 8.2,Apr 18.4,May 33.0,Jun 78.1,Jul 224.3,Aug 170.0,Sep 58.4,Oct 18.0,Nov 9.3,Dec 2.7"
				+ "\n"
				+ "Lima:Jan 1.2,Feb 0.9,Mar 0.7,Apr 0.4,May 0.6,Jun 1.8,Jul 4.4,Aug 3.1,Sep 3.3,Oct 1.7,Nov 0.5,Dec 0.7";

现在任务是这样的，你得提供两个函数，一个计算指定城市的各月降水量平均数，另一个计算降水量方差。

函数原型如下：

function mean(town, string)

function variance(town, string)

town参数指代城市名，string则是降水量数据

用法举例如下：

mean("London", data) \\ 51.19(9999999999996) 
variance("London", data) \\ 57.42(833333333374)

思路比较简单：

1.要计算方差必须先算出平均数来

2.要计算平均数，要先根据城市名取出降水量数据

3.要取降水量数据必须先遍历所有数据，找到了再分割字符串

应该注意的问题：

1.有可能找不到指定的城市，它不存在，那么直接返回-1

2.浮点数运算有可能出现精度问题，忽略不计，不必取整或截断

		
		//扩展一个startWith方法
		if (typeof String.prototype.startWith !== "function") {
			String.prototype.startWith = function(str) {
				return this.indexOf(str) === 0;
			};
		}

		//查询指定城市的降水量数据
		function searchData(town, strng) {
			var datas = strng.split("\n");
			var num = datas.length;
			for (var i = 0; i < num; i++) {
				if (datas[i].startWith(town)) {
					return datas[i];
				}
			}
			return null;
		}
		
		//求降水量平均数
		function mean(town, strng) {
			var total = 0;
			var current = searchData(town, strng);
			if (current) {
				var pair = current.split(":");
				if (pair[0] !== town) {
					return -1;
				}
				var allMonths = pair[1].split(",");
				for (var j = 0; j < allMonths.length; j++) {
					var perMonth = allMonths[j];
					total += parseFloat(perMonth.split(" ")[1]);
				}
				return total / 12;
			}
			return -1;
		}
		
		//求方差
		function variance(town, strng) {
			var average = mean(town, strng);
			var total = 0;
			var current = searchData(town, strng);
			if (current) {
				var pair = current.split(":");
				if (pair[0] !== town) {
					return -1;
				}
				var allMonths = pair[1].split(",");
				for (var j = 0; j < allMonths.length; j++) {
					var perMonth = allMonths[j];
					var rainfall = parseFloat(perMonth.split(" ")[1]);
					total += Math.pow(average - rainfall, 2);
				}
				return total / 12;
			}
			return -1;
		}		

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