data.table Reshaping, stacking and splitting Reshape using `data.table`

Example

data.table extends reshape2's melt & dcast functions

(Reference: Efficient reshaping using data.tables)

library(data.table)

## generate some data
dt <- data.table(
  name = rep(c("firstName", "secondName"), each=4),
  numbers = rep(1:4, 2),
  value = rnorm(8)
)
dt
#          name numbers      value
# 1:  firstName       1 -0.8551881
# 2:  firstName       2 -1.0561946
# 3:  firstName       3  0.2671833
# 4:  firstName       4  1.0662379
# 5: secondName       1 -0.4771341
# 6: secondName       2  1.2830651
# 7: secondName       3 -0.6989682
# 8: secondName       4 -0.6592184

Long to Wide

dcast(data = dt, 
      formula = name ~ numbers, 
      value.var = "value")

#          name          1          2         3         4
# 1:  firstName  0.1836433 -0.8356286 1.5952808 0.3295078
# 2: secondName -0.8204684  0.4874291 0.7383247 0.5757814

On multiple columns (as of data.table 1.9.6)

## add an extra column
dt[, value2 := value * 2]

## cast multiple value columns
dcast(data = dt, 
      formula = name ~ numbers, 
      value.var = c("value", "value2"))

#          name    value_1    value_2   value_3   value_4   value2_1   value2_2 value2_3  value2_4
# 1:  firstName  0.1836433 -0.8356286 1.5952808 0.3295078  0.3672866 -1.6712572 3.190562 0.6590155
# 2: secondName -0.8204684  0.4874291 0.7383247 0.5757814 -1.6409368  0.9748581 1.476649 1.1515627

Wide to Long

## use a wide data.table
dt <- fread("name          1          2         3         4
firstName  0.1836433 -0.8356286 1.5952808 0.3295078
secondName -0.8204684  0.4874291 0.7383247 0.5757814", header = T)
dt
#          name          1          2         3         4
# 1:  firstName  0.1836433 -0.8356286 1.5952808 0.3295078
# 2: secondName -0.8204684  0.4874291 0.7383247 0.5757814

## melt to long, specifying the id column, and the name of the columns 
## in the resulting long data.table
melt(dt, 
     id.vars = "name", 
     variable.name = "numbers",
     value.name = "myValue")
#          name  numbers    myValue
# 1:  firstName        1  0.1836433
# 2: secondName        1 -0.8204684
# 3:  firstName        2 -0.8356286
# 4: secondName        2  0.4874291
# 5:  firstName        3  1.5952808
# 6: secondName        3  0.7383247
# 7:  firstName        4  0.3295078
# 8: secondName        4  0.5757814