Extract column names for each value in column in matrix in R

I have following matrix:

r1 <- c("M","A","T","D","T","Y")
r2 <- c("M","A","G","G","T", "J")
r3 <- c("M","B","H","G","T", "Y")
r4 <- c("M","B","G","G","X", "Y")
r5<- c("F","A","H","D","T", "Y")
n.mat <- rbind(r1,r2,r3,r4,r5)

I would like to extract a column name for each unique value in a column as a vector. At the moment the column names are: V1 V2 V3 V4 V5 V6
The output should look like this:

V1_M V1_F V2_A V2_B V3_T V3_G V3_H V4_D V4_G V5_T V5_X V6_Y V6_J

The order should be in the order as the unique values occur in the column from top to bottom.

My code:

apply(n.mat,2,FUN = function(x) paste(unique(x), seq_along(x), sep = '_'))

But this converts only the values in the matrix and not the columnnames and neither outputs it as a vector.

>Solution :

We may use paste in a vectorized way by replicating the column names with col index and then get the unique after pasteing with the values of the n.mat (it is a data.frame after as.data.frame – so we used unlist)

unique(paste(colnames(n.mat)[col(n.mat)], unlist(n.mat), sep="_"))


[1] "V1_M" "V1_F" "V2_A" "V2_B" "V3_T" "V3_G" "V3_H" "V4_D"
[9] "V4_G" "V5_T" "V5_X" "V6_Y" "V6_J"

Or if need a loop, use Map

unlist(Map(function(x, y) unique(paste(y, x, sep = "_")), 
     unname(n.mat), names(n.mat)))

The apply wouldn’t work here as the names for each column will be the row.names attribute (even if change the OP’s code to names)

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