在减少功能的javascript中找到最大值和最小值

Question

如何在reduce函数中找到最大值和最小值。我可以在数据集中进行“ a”和“ b”的求和，但是我找不到最大值a和最小值b并将其添加到同一列表中

 var liste = [ {"id":1,"a":5,"b":4}, {"id":1,"a":2,"b":2}, {"id":1,"a":7,"b":1}, {"id":1,"a":1,"b":0}, {"id":2,"a":11,"b":5}, {"id":2,"a":32,"b":75}, {"id":3,"a":1,"b":1}, {"id":4,"a":1,"b":1}, {"id":4,"a":213,"b":1}, {"id":5,"a":1,"b":1}, {"id":5,"a":1,"b":1}, ] var max = 0; let map = liste.reduce((prev, next) => { if (next.id in prev) { prev[next.id].a += next.a; prev[next.id].b += next.b; if(next.a > max){ max = next.a; prev[next.id].max = max; } } else { prev[next.id] = next; next.max =max } return prev; }, {}); let result = Object.keys(map).map(id => map[id]); console.log(result); 

Answer 1

您基本上就是在做您要做的事情，只是在要进行汇总的条目中包括a和b最大值。

但是reduce是错误的工具，您需要的只是一个简单的for-of循环。 （您可以将它塞入reduce调用，因为基本上任何数组操作都可以塞入reduce调用，但是除了复杂性之外，它不会给您带来任何好处。）我还将使用Map而不是空白对象进行查找（但是对象也可以工作；为此使用对象时，我通常建议通过Object.create(null)创建没有原型的对象）。

const map = new Map();
for (const entry of liste) {
    const {id, a, b} = entry;
    // Do we have an entry for this ID?
    let known = map.get(id);
    if (!known) {
        // No, use this one (note: I would make a copy rather than modifying the original)
        known = entry;
        map.set(id, entry);
        // Initialize its max with the values from this entry
        entry.max_a = a;
        entry.max_b = b;
    } else {
        // Update our max values
        known.max_a = Math.max(known.max_a, a);
        known.max_b = Math.max(known.max_b, b);
        // Add in the current entries to the sum
        known.a += a;
        known.b += b;
    }
}
// Show the result
console.log([...map.values()]);

现场示例：

 var liste = [ {"id":1,"a":5,"b":4}, {"id":1,"a":2,"b":2}, {"id":1,"a":7,"b":1}, {"id":1,"a":1,"b":0}, {"id":2,"a":11,"b":5}, {"id":2,"a":32,"b":75}, {"id":3,"a":1,"b":1}, {"id":4,"a":1,"b":1}, {"id":4,"a":213,"b":1}, {"id":5,"a":1,"b":1}, {"id":5,"a":1,"b":1}, ] const map = new Map(); for (const entry of liste) { const {id, a, b} = entry; // Do we have an entry for this ID? let known = map.get(id); if (!known) { // No, use this one (note: I would make a copy rather than modifying the original) known = entry; map.set(id, entry); // Initialize its max with the values from this entry entry.max_a = a; entry.max_b = b; } else { // Update our max values known.max_a = Math.max(known.max_a, a); known.max_b = Math.max(known.max_b, b); // Add in the current entries to the sum known.a += a; known.b += b; } } // Show the result console.log([...map.values()]); 

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Answer 2

您可以检查id存在并更新对象，否则使用给定的值以及最小值和最大值创建一个新对象。

 var liste = [{ id: 1, a: 5, b: 4 }, { id: 1, a: 2, b: 2 }, { id: 1, a: 7, b: 1 }, { id: 1, a: 1, b: 0 }, { id: 2, a: 11, b: 5 }, { id: 2, a: 32, b: 75 }, { id: 3, a: 1, b: 1 }, { id: 4, a: 1, b: 1 }, { id: 4, a: 213, b: 1 }, { id: 5, a: 1, b: 1 }, { id: 5, a: 1, b: 1 }], global_min = Infinity, global_max = -Infinity, map = liste.reduce((r, { id, a, b }) => { if (id in r) { r[id].a += a; r[id].b += b; r[id].max = Math.max(r[id].max, a); r[id].min = Math.min(r[id].min, b); } else { r[id] = { id, a, b, max: a, min: b }; } global_max = Math.max(global_max, a); global_min = Math.min(global_min, b); return r; }, {}), result = Object.values(map).map(o => Object.assign(o, { global_max, global_min })); console.log(result); 

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