r - R data.table 中的分组计数聚合

Question

包含日期、买入值和卖出值的表格。我想计算每天的买卖次数以及买卖的总数。我发现在 data.table 中这样做有点棘手。

   date   buy sell      
2011-01-01  1   0
2011-01-02  0   0
2011-01-03  0   2
2011-01-04  3   0
2011-01-05  0   0
2011-01-06  0   0
2011-01-01  0   0
2011-01-02  0   1
2011-01-03  4   0
2011-01-04  0   0
2011-01-05  0   0
2011-01-06  0   0
2011-01-01  0   0
2011-01-02  0   8
2011-01-03  2   0
2011-01-04  0   0
2011-01-05  0   0
2011-01-06  0   5

上面的 data.table 可以使用以下代码创建:

 DT = data.table(
          date=rep(as.Date('2011-01-01')+0:5,3) , 
          buy=c(1,0,0,3,0,0,0,0,4,0,0,0,0,0,2,0,0,0),
          sell=c(0,0,2,0,0,0,0,1,0,0,0,0,0,8,0,0,0,5));

结果我想要的是:

   date   total_buys   total_sells
2011-01-01    1            0
2011-01-02    0            2
                and so on  

此外，我还想知道买卖的总数:

 total_buys   total_sells
     4            4

我试过了 :

 length(DT[sell > 0 | buy > 0])
 > 3 

这是一个奇怪的答案(想知道为什么)

Answer 1

## by date
DT[, list(total_buys = sum(buy > 0), total_sells = sum(sell > 0)), by = date]
##          date total_buys total_sells
## 1: 2011-01-01          1           0
## 2: 2011-01-02          0           2
## 3: 2011-01-03          2           1
## 4: 2011-01-04          1           0
## 5: 2011-01-05          0           0
## 6: 2011-01-06          0           1

DT[, list(total_buys = sum(buy > 0), total_sells = sum(sell > 0))]
##    total_buys total_sells
## 1:          4           4