dcast without ID variables

Question

In the "An Introduction to reshape2" package Sean C. Anderson presents the following example.

He uses the airquality data and renames the column names

names(airquality) <- tolower(names(airquality))

The data look like

#   ozone solar.r wind temp month day
# 1    41     190  7.4   67     5   1
# 2    36     118  8.0   72     5   2
# 3    12     149 12.6   74     5   3
# 4    18     313 11.5   62     5   4
# 5    NA      NA 14.3   56     5   5
# 6    28      NA 14.9   66     5   6

Then he melts them by

aql <- melt(airquality, id.vars = c("month", "day"))

to get

#   month day variable value
# 1     5   1    ozone    41
# 2     5   2    ozone    36
# 3     5   3    ozone    12
# 4     5   4    ozone    18
# 5     5   5    ozone    NA
# 6     5   6    ozone    28

Finally he gets the original one (different column order) by

aqw <- dcast(aql, month + day ~ variable)

My Quesiton

Assume now that we do not have ID variables (i.e. month and day) and have melted the data as follows

aql <- melt(airquality)

which look like

#   variable value
# 1    ozone    41
# 2    ozone    36
# 3    ozone    12
# 4    ozone    18
# 5    ozone    NA
# 6    ozone    28

My question is how can I get the original ones? The original ones would correspond to

#   ozone solar.r wind temp 
# 1    41     190  7.4   67 
# 2    36     118  8.0   72 
# 3    12     149 12.6   74
# 4    18     313 11.5   62 
# 5    NA      NA 14.3   56
# 6    28      NA 14.9   66

Answer 1

Another option is unstack

out <- unstack(aql,value~variable)
head(out)
#   ozone solar.r wind temp month day
#1    41     190  7.4   67     5   1
#2    36     118  8.0   72     5   2
#3    12     149 12.6   74     5   3
#4    18     313 11.5   62     5   4
#5    NA      NA 14.3   56     5   5
#6    28      NA 14.9   66     5   6

As the question is about dcast, we can create a sequence column and then use dcast

aql$indx <- with(aql, ave(seq_along(variable), variable, FUN=seq_along))
out1 <- dcast(aql, indx~variable, value.var='value')[,-1]
head(out1)
#   ozone solar.r wind temp month day
#1    41     190  7.4   67     5   1
#2    36     118  8.0   72     5   2
#3    12     149 12.6   74     5   3
#4    18     313 11.5   62     5   4
#5    NA      NA 14.3   56     5   5
#6    28      NA 14.9   66     5   6

If you are using data.table, the devel version of data.table ie. v1.9.5 also has dcast function. Instructions to install the devel version are here

 library(data.table)#v1.9.5+
 setDT(aql)[, indx:=1:.N, variable]
 dcast(aql, indx~variable, value.var='value')[,-1]

Answer 2

One option using split,

out <- data.frame(sapply(split(aql, aql$variable), `[[`, 2))

Here, the data is split by the variable column, then the second column of each group is combined back into a data frame (the [[ function with the argument 2 is passed to sapply)

head(out)
#   Ozone Solar.R Wind Temp Month Day
# 1    41     190  7.4   67     5   1
# 2    36     118  8.0   72     5   2
# 3    12     149 12.6   74     5   3
# 4    18     313 11.5   62     5   4
# 5    NA      NA 14.3   56     5   5
# 6    28      NA 14.9   66     5   6