Subtract all vector pairs-CodePudding

I have a matrix of size 2000 X 700. I want to subtract all possible pairs of rows. If x_i represent a row then I want to compute: x_1-x_2, x_1-x_3, ..., x_2-x_3,...

Example:

My output should be

x_1 - x_2: -4 -1  1
x_1 - x_3:  0  1 -3
x_2 - x_3:  4  2 -4

I tried using a loop, but it takes too long. Is there an efficient way to compute this?

CodePudding user response：

Perhaps use combn

combn(row.names(m1), 2, function(x) m1[x[1],] - m1[x[2],])

CodePudding user response：

A relatively fast approach would be to pre-define an index list and then use it on the data, set to data.table.

library(data.table)

setDT(mat)

rows <- nrow(mat)
idx <- as.matrix(rbindlist(lapply(1:(rows - 1), function(x) 
  rbindlist(lapply((x   1):rows, function(y) list(x, y)))))) # takes approx. 6 secs on my crappy system for 2000 x 2000 combinations
idx
     V1 V2
[1,]  1  2
[2,]  1  3
[3,]  2  3

mat[idx[, 1], ] - mat[idx[, 2], ] # takes approx. 0.2 - 1 secs
   V1 V2 V3
1: -4 -1  1
2:  0  1 -3
3:  4  2 -4

Data

mat <- structure(list(V1 = c(1L, 5L, 1L), V2 = c(2L, 3L, 1L), V3 = c(3L, 
2L, 6L)), class = "data.frame", row.names = c(NA, -3L))

CodePudding user response：

Another use of combn (along with asplit)

> t(combn(asplit(mat, 1), 2, function(x) do.call(`-`, x)))
     [,1] [,2] [,3]
[1,]   -4   -1    1
[2,]    0    1   -3
[3,]    4    2   -4