在javascript中创建累积和数组

Question

这是我需要做的一个例子:

var myarray = [5, 10, 3, 2];

var result1 = myarray[0];
var result2 = myarray[1] + myarray[0];
var result3 = myarray[2] + myarray[1] + myarray[0];
var result4 = myarray[3] + myarray[2] + myarray[1] + myarray[0];

所有这些都会输出5,15,18,20

但不是像这样写出所有的变量,我想要它说:

var result = arrayitem + the sum of any previous items 

那有意义吗？那可能吗？我怎么做？

Answer 1

Javascript reduce提供了当前索引,这在这里很有用:

var myarray = [5, 10, 3, 2];
var new_array = [];
myarray.reduce(function(a,b,i) { return new_array[i] = a+b; },0);
new_array // [5, 15, 18, 20]

Answer 2

reduce避免制作新数组的替代方法:

var result = myarray.reduce(function(r, a) {
  r.push((r.length && r[r.length - 1] || 0) + a);
  return r;
}, []);

没有必要为每个结果重新求和子数组.

编辑同样的东西的丑陋版本:

var result = myarray.reduce(function(r, a) {
  if (r.length > 0)
    a += r[r.length - 1];
  r.push(a);
  return r;
}, []);

Answer 3

从Nina Scholz复制的一种优雅的解决方案，使用currying访问先前的值。

const cumulativeSum = (sum => value => sum += value)(0);

console.log([5, 10, 3, 2].map(cumulativeSum));

cumulativeSum是函数value => sum += value，且sum初始化为零。每次调用时，sum都会进行更新，并且下次调用时（与input [n]相同）将等于前一个值（output [n-1]）。

Answer 4

ES6阵列传播还有更多选择

[1, 2, 3].reduce((a, x, i) => [...a, x + (a[i-1] || 0)], []); //[1, 3, 6]

要么

[3, 2, 1].reduce((a, x, i) => [...a, a.length > 0 ? x + a[i-1] : x], []); //[3, 5, 6]

Answer 5

使用 ES6 的简单解决方案

let myarray = [5, 10, 3, 2];
    let new_array = [];  
    myarray.reduce( (prev, curr,i) =>  new_array[i] = prev + curr , 0 )
    console.log(new_array);

有关更多信息Array.reduce()

箭头函数

Answer 6

我需要保留结果并添加一个运行总计属性。我有一个带有日期和收入的 json 对象，并且还想显示运行总计。

//i'm calculating a running total of revenue, here's some sample data
let a = [
  {"date":  "\/Date(1604552400000)\/","revenue":  100000 },
  {"date":  "\/Date(1604203200000)\/","revenue":  200000 },
  {"date":  "\/Date(1604466000000)\/","revenue":  125000 },
  {"date":  "\/Date(1604293200000)\/","revenue":  400000 },
  {"date":  "\/Date(1604379600000)\/","revenue":  150000 }
];

//outside accumulator to hold the running total
let c = 0;

//new obj to hold results with running total
let b = a
  .map( x => ({...x,"rtotal":c+=x.revenue}) )
  
//show results, use console.table if in a browser console
console.log(b)

Answer 7

这个解决方案怎么样

var new_array = myarray.concat(); //Copy initial array

for (var i = 1; i < myarray.length; i++) {
  new_array[i] = new_array[i-1] + myarray[i];
}

console.log(new_array);

PS：你也可以使用原始数组。我只是复制它以防我们不想污染它。

Answer 8

更通用（且高效）的解决方案：

Array.prototype.accumulate = function(fn) {
    var r = [this[0]];
    for (var i = 1; i < this.length; i++)
        r.push(fn(r[i - 1], this[i]));
    return r;
}

或者

Array.prototype.accumulate = function(fn) {
    var r = [this[0]];
    this.reduce(function(a, b) {
        return r[r.length] = fn(a, b);
    });
    return r;
}

进而

r = [5, 10, 3, 2].accumulate(function(x, y) { return x + y })