javascript 数组的快速分组
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Fast grouping of a javascript array
提问by Joe
I have an array of a couple thousand strings
我有几千个字符串的数组
['7/21/2011', '7/21/2011', '7/21/2011', '7/20/2011', etc]
I am currently, running this code to group by the string and get the max group value:
我目前正在运行此代码以按字符串分组并获取最大组值：
var max = 0;
var group = {};
arr.map(function (value) {
if (group[value]) {
group[value]++;
} else {
group[value] = 1;
}
max = Math.max(max, group[value]);
});
Are there any improvements to make this code run faster?
是否有任何改进可以使此代码运行得更快？
EDIT:The results are in: http://jsperf.com/javascript-array-grouping2
编辑：结果在：http: //jsperf.com/javascript-array-grouping2
EDIT EDIT: that test was flawed. Mike Samuel's code was the fastest.
编辑 编辑：该测试有缺陷。Mike Samuel的代码是最快的。
6000 entries test -> http://jsperf.com/javascript-array-grouping2
6000 个条目测试 -> http://jsperf.com/javascript-array-grouping2
10K entries test -> http://jsperf.com/javascript-array-grouping
10K 条目测试 -> http://jsperf.com/javascript-array-grouping
采纳答案by Mike Samuel
If you're sure this is a hotspot and speed is really important, I would try to cut out several thousand function calls by inlining maxand map.
如果您确定这是一个热点并且速度非常重要，我会尝试通过内联max和map.
You can also make the body of your function faster by cutting out a comparison.
您还可以通过删除比较来使函数体更快。
var max = 0;
var group = {};
for (var i = arr.length; --i >= 0;) {
var value = arr[i];
var n = group[value] = 1 - -(group[value] | 0);
if (n > max) { max = n; }
}
The best thing to do is measure on the browsers you care about.
最好的办法是衡量您关心的浏览器。
回答by orlp
Yes certainly. I would calculate the max last, instead of every iteration, and not use an if:
是的，当然了。我会最后计算最大值，而不是每次迭代，并且不使用 if：
var group = {};
arr.map(function (value) {
group[value] = (group[value] || 0) + 1;
});
var max = 0;
for (key in group) {
if (group[key] > max) max = group[key];
}
EDIT: As Mike Samuel says you mightget faster by using an index instead of map:
编辑：正如 Mike Samuel 所说，使用索引而不是地图可能会更快：
var group = {};
var max = 0;
for (var i = arr.length; --i >= 0;) {
group[value] = (group[value] || 0) + 1;
}
for (key in group) {
if (group[key] > max) max = group[key];
}
回答by 6502
I think it really depends on the JS engine that you will run this code on. An alternative I think it's worth a shot is using
我认为这真的取决于您将运行此代码的 JS 引擎。我认为值得一试的另一种选择是使用
n = group[value] = (group[value]||0) + 1;
if (n > max) max = n;
for each element.
对于每个元素。
I also think that may be using a regular loop can be faster because the variables you will use will be just locals and not closed-over variables of a closure (that are normally slower) and you will also save a function call per element. Both those problems are non-issues if the implementation can inline this closure, but I don't know if there are JS implementations smart enough for that.
我还认为使用常规循环可能会更快，因为您将使用的变量只是局部变量，而不是闭包的封闭变量（通常较慢），并且您还将为每个元素保存一个函数调用。如果实现可以内联这个闭包，那么这两个问题都不是问题，但我不知道是否有足够智能的 JS 实现。
