Dear R User, 
I was wondering how I can unstack my data. For example I have following data set

data<-structure(list(siteS = structure(c(3L, 3L, 3L, 4L, 4L, 5L, 5L, 
5L, 5L, 5L, 1L, 1L, 1L, 2L), .Label = c("11a", "12d", "1a", "2v", 
"6a"), class = "factor"), species = structure(c(1L, 2L, 3L, 1L, 
4L, 1L, 4L, 5L, 2L, 3L, 1L, 6L, 3L, 1L), .Label = c("sa", "sb", 
"sc", "sd", "se", "sg"), class = "factor"), abundance = c(31L, 
55L, 62L, 42L, 40L, 30L, 84L, 10L, 23L, 74L, 11L, 51L, 37L, 15L
)), .Names = c("siteS", "species", "abundance"), class = "data.frame", row.names = c(NA, 
-14L))

I wanted to have this data into following format

newData<-structure(list(siteS = structure(c(3L, 4L, 5L, 1L, 2L), .Label = c("11a", 
"12d", "1a", "2v", "6a"), class = "factor"), sa = c(31L, 42L, 
30L, 11L, 15L), sb = c(55L, NA, 84L, NA, NA), sc = c(62L, NA, 
10L, 37L, NA), sd = c(NA, 40L, 23L, NA, NA), se = c(NA, NA, 74L, 
NA, NA), sg = c(NA, NA, NA, 51L, NA)), .Names = c("siteS", "sa", 
"sb", "sc", "sd", "se", "sg"), class = "data.frame", row.names = c(NA, 
-5L))

I tried several ways such as:
> data.frame(unstack(data, species~siteS))
Error in data.frame(`11a` = c("sa", "sg", "sc"), `12d` = "sa", `1a` = c("sa",  : 
  arguments imply differing number of rows: 3, 1, 2, 5
 

 		 	   		  
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