在對數據框中的唯一因子進行計數時，如何保留另一個矢量的值？

Question

從我的數據幀「lotsadates」的數據是這樣的：

>Date 

1 2012-09-26 
2 2012-09-26 
3 2012-09-26 
4 2012-09-27 
5 2012-09-28 
6 2012-09-28 

有相同長度的DAY_OF_WEEK向量：

> day_of_week 

1 3 
2 3 
3 3 
4 4 
5 5 
6 5 

我使用下面的計算數量觀察日期：

ndist <-tapply(1:NROW(lotsadates), 
       lotsadates$Date, 
       function(x) length(unique(x))) 

所以ndist看起來像這樣：

觀測/日期/ ndist
1/2012-09-26/3
2/2012-09-27/1
3/2012-09-28/2

但我想ndist看起來像這樣：

日期/ ndist/DAY_OF_WEEK
1/2012-09-26/3/3
2/2012-09-27/1/4
3/2012-09-28/2/5

我認爲有一個相當簡單的解決方案，但我不能弄清楚。您的建議非常感謝！

Answer 1

library(plyr) 
# assuming lotsadates has 2 columns, Date and day_of_week 
ndist <- ddply(lotsadates, .(Date, day_of_week), summarise, n=length(Date)) 

Answer 2

library(reshape2) 
result <- dcast(lotsadates, Date ~., value.var='day_of_week') 
result$day_of_week <- as.POSIXlt(result$Date)$wday 
names(result)[2] <- "ndist" 
> result 
     Date ndist day_of_week 
1 2012-09-26  3   3 
2 2012-09-27  1   4 
3 2012-09-28  2   5 

Answer 3

編碼優雅

library(data.table) 
# assuming lotsadates has 2 columns, Date and day_of_wee 
DT <- as.data.table(lotsadates) 
DT[, .N, by = list(Date, day_of_week)]