表达式`p[i&1]+=v,p` 是什么意思？

Question

我正在用 CodeFights 练习我的 javascript，在我完成一个练习后，我看到了这个函数：

// Subject : 
// Several people are standing in a row and need to be divided into two teams. 
// The first person goes into team 1, the second goes into team 2, 
// the third goes into team 1 again, the fourth into team 2, and so on.
// You are given an array of positive integers - the weights of the people. 
// Return an array of two integers, where the first element is the total weight of 
// team 1, and the second element is the total weight of team 2 
// after the division is complete.

// Example :
// For a = [50, 60, 60, 45, 70], the output should be
// alternatingSums(a) = [180, 105].

// answer
alternatingSums = a => a.reduce((p,v,i) => (p[i&1]+=v,p), [0,0])

我不明白什么p[i&1]+=v,p意思。

Answer 1

该&符号是一个按位二元运算符。

要了解会发生什么，您必须将每个项目转换为二进制。

   | i (decimal) | i (binary) | i & 1 |
   |-------------|------------|-------|
   |           0 |          0 |     0 |
   |           1 |          1 |     1 |
   |           2 |         10 |     0 |
   |           3 |         11 |     1 |
   |           4 |        100 |     0 |
   |           5 |        101 |     1 |

实际上，每个偶数都将转换为 0，每个奇数都将转换为 1。

如果我试图达到那个结果，我个人会使用模数运算符 ( %)

 p[i%2] += v;

但这只是我。

另一部分是用逗号分隔的两个语句：

 (p[i&1]+=v,p)

这就是说“执行此操作，然后返回p。它的简写为：

alternatingSums = a => a.reduce((p,v,i) => { 
                                              p[i&1]+=v;
                                              return p;
                                           }, 
                                [0,0])