data.table扩展reshape2的melt＆dcast功能
（参考：使用data.tables进行有效的重塑）
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长到宽
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
在多列上（自data.table1.9.6开始）
## 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宽到长
## 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