R：想要将数据表中的列中的数据复制到特定的行中

Question

I currently have multiple data tables which look like this:

First Quartile

Q1DT = data.table(
  Tree = c("Cherry", "Birch", "Oak"), 
  Height = c("1m", "3m", "5m"), 
  Age = c(10L, 12L, 15L)
)
Q1DT
#      Tree Height Age
# 1: Cherry     1m  10
# 2:  Birch     3m  12
# 3:    Oak     5m  15

Mean

AVGDT = data.table(
  Tree = c("Cherry", "Birch", "Oak"), 
  Height = c("2m", "5m", "7m"), 
  Age = c(13L, 17L, 19L)
)
AVGDT
#      Tree Height Age
# 1: Cherry     1m  10
# 2:  Birch     3m  12
# 3:    Oak     5m  15

Quartile 3

Q3DT = data.table(
  Tree = c("Cherry", "Birch", "Oak"), 
  Height = c("7m", "10m", "25m"), 
  Age = c(17L, 22L, 25L)
)
Q3DT
#      Tree Height Age
# 1: Cherry     1m  10
# 2:  Birch     3m  12
# 3:    Oak     5m  15

I want to combine them so they look like this:

    Tree     Measure  Q1  Q2  Q3
    Cherry   Height    
    Cherry   Age
    Birch    Height
    Birch    Age
    Oak      Height
    Oak      Age  

I used the following to make the bare-bones of the table:

output.complete <- data.table(Tree= rep(trees.quartile.1$Tree, each = 2), measure = c(rep(c("Height", "Age"), times = length(unique(trees.quartile.1$Tree)))))

However, I cannot work out how to populate this table now from the data in the separate quartile data tables. Any help would be massively appreciated!

Answer 1

There are a couple of ways to do this. I'm going to use rbind , melt , and dcast to reshape your data:

stack = rbind(Q1 = Q1DT, AVG = AVGDT, Q3 = Q3DT, idcol = 'stat')

# this will give a warning -- in the input, 'Age' is
#   an integer, and 'Height' is a character, so melt
#   must force 'Age' to be a character to proceed
out = stack[ , melt(.SD, id.vars = c('stat', 'Tree'), variable.name = 'Measure')
             ][ , dcast(.SD, Tree + Measure ~ stat, value.var = 'value')]
#      Tree Measure AVG Q1  Q3
# 1:  Birch  Height  5m 3m 10m
# 2:  Birch     Age  17 12  22
# 3: Cherry  Height  2m 1m  7m
# 4: Cherry     Age  13 10  17
# 5:    Oak  Height  7m 5m 25m
# 6:    Oak     Age  19 15  25

We can either use setcolorder afterwards to get the columns in the desired order, or define stat to be a factor in stack with the right order:

stack[ , stat := factor(stat, levels = c('Q1', 'AVG', 'Q3'))]