[英]Get max value per key in a javascript array
说,我有以下数组:
[
{a:1, b:"apples"},
{a:3, b:"apples"},
{a:4, b:"apples"},
{a:1, b:"bananas"},
{a:3, b:"bananas"},
{a:5, b:"bananas"},
{a:6, b:"bananas"},
{a:3, b:"oranges"},
{a:5, b:"oranges"},
{a:6, b:"oranges"},
{a:10, b:"oranges"}
]
我想为每种类型的“ b”有效地获得具有最高a的整个对象,因此我的函数应产生:
[
{a:4, b:"apples"},
{a:6, b:"bananas"},
{a:10, b:"oranges"}
]
现在,我将执行以下操作:
var cache = {};
var resultobj = {};
result = [];
array.forEach(function (r) {
if (cache[r.b] && cache[r.b] > r.a) {
result[r.b] = r;
}
})
for (var key in result) {
result.push(result[key]);
}
看起来很糟糕,效率低下...?
在ES5中是两线,而在ES6中是一线:
ary = [ {a: 0, b:"apples"}, {a:-3, b:"apples"}, {a:-4, b:"apples"}, {a:1, b:"bananas"}, {a:3, b:"bananas"}, {a:5, b:"bananas"}, {a:6, b:"bananas"}, {a:3, b:"oranges"}, {a:5, b:"oranges"}, {a:6, b:"oranges"}, {a:10, b:"oranges"} ] // ES5 maxes = {}; ary.forEach(function(e) { maxes[eb] = eb in maxes ? Math.max(maxes[eb], ea) : ea; }); document.write('<pre>'+JSON.stringify(maxes,0,3)); // ES6 maxes = ary.reduce((m, e) => Object.assign(m, { [eb]: eb in m ? Math.max(m[eb], ea) : ea }), {}); document.write('<pre>'+JSON.stringify(maxes,0,3));
您可以尝试如下操作:
b
排序 )。 var data = [ {a:1, b:"apples"}, {a:3, b:"apples"}, {a:4, b:"apples"}, {a:1, b:"bananas"}, {a:3, b:"bananas"}, {a:5, b:"bananas"}, {a:6, b:"bananas"}, {a:3, b:"oranges"}, {a:5, b:"oranges"}, {a:6, b:"oranges"}, {a:10, b:"oranges"} ] var distinctB = data.slice(0,1); data.forEach(function(o){ if(distinctB[distinctB.length-1] && ob !== distinctB[distinctB.length-1].b){ distinctB.push(o); } else{ distinctB[distinctB.length-1] = o; } }); document.write("<pre>" + JSON.stringify(distinctB,0,4) + "</pre>");
它在对象的索引的帮助下起作用。
var data = [{ a: 1, b: "apples" }, { a: 3, b: "apples" }, { a: 4, b: "apples" }, { a: 1, b: "bananas" }, { a: 3, b: "bananas" }, { a: 5, b: "bananas" }, { a: 6, b: "bananas" }, { a: 3, b: "oranges" }, { a: 5, b: "oranges" }, { a: 6, b: "oranges" }, { a: 10, b: "oranges" }], result = function (array) { var r = [], o = {}; array.forEach(function (a) { if (!(ab in o)) { o[ab] = r.push(a) - 1; return; } if (r[o[ab]].a < aa) { r[o[ab]] = a; } }); return r; }(data); document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');
您接近了,代码中有一些错误的逻辑,您已经声明了cache
,但是没有使用过它。
if (cache[r.b] && cache[r.b] > r.a) //This always will be "false"
参见工作示例
var array = [ { a: 1, b: "apples" }, { a: 3, b: "apples" }, { a: 4, b: "apples" }, { a: 1, b: "bananas" }, { a: 3, b: "bananas" }, { a: 5, b: "bananas" }, { a: 6, b: "bananas" }, { a: 3, b: "oranges" }, { a: 5, b: "oranges" }, { a: 6, b: "oranges" }, { a: 10, b: "oranges" } ]; var cache = {}; array.forEach(function(e) { var t = cache[eb]; if (t) { ta = ta > ea ? ta : ea; } else { cache[eb] = e; } }); var res = Object.keys(cache).map(e => cache[e]); document.write(JSON.stringify(res));
确实,如果您的水果名称将唯一,那么最好的数据结构是对象而不是对象数组。
var out = arr.reduce(function (p, c) {
var key = c.b;
p[key] = p[key] || 0;
if (c.a > p[key]) p[key] = c.a;
return p;
}, {}); // { apples: 4, bananas: 6, oranges: 10 }
Array.foreach
和Math.max
方法的简短解决方案:
var map = {},result = [];
obj.forEach(function(v){
(!(v['b'] in map)) ? map[v['b']] = [v['a']] : map[v['b']].push(v['a']);
});
for (var prop in map) {
result.push({a: Math.max.apply(null, map[prop]), b: prop});
}
console.log(result);
// the output:
[
{a:4, b:"apples"},
{a:6, b:"bananas"},
{a:10, b:"oranges"}
]
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Math/max
您正在结果result[rb] = r;
中设置值result[rb] = r;
但比较中的值cache
通过这样做if (cache[rb] && cache[rb] > ra) {
您需要在cache
而不是result
设置值,然后针对预期结果迭代cache
。
将forEach和for循环替换为
array.forEach(function (r) {
if (cache[r.b] && cache[r.b] > r.a) {
cache[r.b] = r.a;
}
});
for (var key in cache)
{
result.push(result[key]);
}
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.