CODEWITHSUNDEEP
rdata.table
talat
talat

Reputation: 70296

Count combinations of length 2 per id

I have a largish data.table with two columns, id and var:

head(DT)
#    id var
# 1:  1   B
# 2:  1   C
# 3:  1   A
# 4:  1   C
# 5:  2   B
# 6:  2   C

I would like to create a kind of cross-table that would show how many times different length 2-combinations of var occured in the data.

Expected output for the sample data:

out
#    A  B C
# A  0  3 3
# B NA  1 3
# C NA NA 0

Explanation:

(The result could also be given in long-format as long as the relevant information is present.)

I'm sure there's a clever (efficient) way of computing this, but I can't currently wrap my head around it.

Sample data:

DT <- structure(list(id = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 4L), var = c("B", "C", "A", 
"C", "B", "C", "C", "A", "B", "B", "C", "C", "C", "C", "B", "C", 
"B", "A", "C", "B")), .Names = c("id", "var"), row.names = c(NA, 
-20L), class = "data.frame")

library(data.table)
setDT(DT, key = "id")

Upvotes: 12

Views: 197

Answers (1)

eddi
eddi

Reputation: 49448

Since you're ok with long-form results:

DT[, if(all(var == var[1]))
       .(var[1], var[1])
     else
       as.data.table(t(combn(sort(unique(var)), 2))), by = id][
   , .N, by = .(V1, V2)]
#   V1 V2 N
#1:  A  B 3
#2:  A  C 3
#3:  B  C 3
#4:  B  B 1

Or if we call the above output res:

dcast(res[CJ(c(V1,V2), c(V1,V2), unique = T), on = c('V1', 'V2')][
          V1 == V2 & is.na(N), N := 0], V1 ~ V2)
#   V1  A  B C
#1:  A  0  3 3
#2:  B NA  1 3
#3:  C NA NA 0

An alternative to combn is doing:

DT[, if (all(var == var[1]))
       .(var[1], var[1])
     else
       CJ(var, var, unique = T)[V1 < V2], by = id][
   , .N, by = .(V1, V2)]
#    V1 V2 N
# 1:  A  B 3
# 2:  A  C 3
# 3:  B  C 3
# 4:  B  B 1

# or combn with list output (instead of matrix)

unique(DT, by=NULL)[ order(var), if(.N==1L)
       .(var, var)
     else
       transpose(combn(var, 2, simplify=FALSE)), by = id][
   , .N, by = .(V1, V2)]

Upvotes: 10

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