Fill a column with values from a table (or matrix) given certain conditions

Question

I'm not sure exactly what this would be called, so I'm struggling to search for it. Sorry if this is a repeated question, that would be why.

I have a data 43x17 data matrix,lets call it A, with four levels in the "site" column. I also have another 4x3 matrix containing coordinates for each site, lets call it B. I'd like to create a column in A where each site is assigned the coordinate listed in B.

Using the data below as an example, I'd like to make a new column in A and populate it with values from Coordinate1 (in B) with the associated "Site" value.

A <- matrix(c(1:4), nrow = 12, ncol = 3, byrow = TRUE,
               dimnames = list(c(1:12),
                               c("Site", "D.2", "D.3")))
A
B<- matrix(c(1:4), nrow = 4, ncol = 3, byrow = TRUE,
           dimnames = list(c(1:4),
                           c("Site", "Coordinate1", "Coordinate2")))
B

I'm sure this is simple, but I'm blanking on what to call this and have apparently forgotten how to do it!

Answer 1

There are several options.

Without conversion to data.frame and using merge() as suggested in most of the comments, you may try to use match() and cbind() :

cbind(A, B[match(A[, "Site"], B[, "Site"]), "Coordinate1", drop = FALSE])

  Site D.2 D.3 Coordinate1 1 1 2 3 2 2 4 1 2 1 3 3 4 1 4 4 2 3 4 3 5 1 2 3 2 6 4 1 2 1 7 3 4 1 4 8 2 3 4 3 9 1 2 3 2 10 4 1 2 1 11 3 4 1 4 12 2 3 4 3 

or, to cbind all columns of B

cbind(A, B[match(A[, "Site"], B[, "Site"]), ])

  Site D.2 D.3 Site Coordinate1 Coordinate2 1 1 2 3 1 2 3 2 4 1 2 4 1 2 3 3 4 1 3 4 1 4 2 3 4 2 3 4 5 1 2 3 1 2 3 6 4 1 2 4 1 2 7 3 4 1 3 4 1 8 2 3 4 2 3 4 9 1 2 3 1 2 3 10 4 1 2 4 1 2 11 3 4 1 3 4 1 12 2 3 4 2 3 4 

---

Conversion to data.frame and merge()

 as.matrix(merge(data.frame(A), data.frame(B), by = "Site", sort = FALSE))

distorts the original order of rows:

  Site D.2 D.3 Coordinate1 Coordinate2 [1,] 1 2 3 2 3 [2,] 1 2 3 2 3 [3,] 1 2 3 2 3 [4,] 4 1 2 1 2 [5,] 4 1 2 1 2 [6,] 4 1 2 1 2 [7,] 3 4 1 4 1 [8,] 3 4 1 4 1 [9,] 3 4 1 4 1 [10,] 2 3 4 3 4 [11,] 2 3 4 3 4 [12,] 2 3 4 3 4 

---

Using right join from the data.table package maintains the order of rows

library(data.table)
as.matrix(data.table(B)[data.table(A), on = "Site"])

  Site Coordinate1 Coordinate2 D.2 D.3 [1,] 1 2 3 2 3 [2,] 4 1 2 1 2 [3,] 3 4 1 4 1 [4,] 2 3 4 3 4 [5,] 1 2 3 2 3 [6,] 4 1 2 1 2 [7,] 3 4 1 4 1 [8,] 2 3 4 3 4 [9,] 1 2 3 2 3 [10,] 4 1 2 1 2 [11,] 3 4 1 4 1 [12,] 2 3 4 3 4