CODEWITHSUNDEEP
r
Ku-trala
Ku-trala

Reputation: 651

Max of row in matrix

I have a matrix containing three columns in R. The matrix could look something like this:

A=matrix(c(1,2,3,4,0.5,1,7,1.2,3,4,2,1),nrow=4, ncol=3)

I want to create a matrix based on A, which in each row of A returns a 1 for the highest value in that row and zero otherwise. So in the specific case above, I need a matrix that looks like this:

B=matrix(c(0,0,0,1,0,0,1,0,1,1,0,0),nrow=4,ncol=3)

I have tried to search the forum, but couldn't find a proper answer.

Thanks.

Upvotes: 3

Views: 197

Answers (2)

JPC
JPC

Reputation: 1919

The below is pretty much the same as Ananda's answer, but the small changes could make a difference in terms of speed if your A is sufficiently large

> A<-matrix(rnorm(1000*1000),nrow=1000)
> system.time(t(apply(A, 1, function(x) as.numeric(x == max(x)))))
   user  system elapsed
  0.117   0.024   0.141
> system.time(1*(A==apply(A,1,max)))
   user  system elapsed
  0.056   0.008   0.065

Upvotes: 5

A5C1D2H2I1M1N2O1R2T1
A5C1D2H2I1M1N2O1R2T1

Reputation: 193677

Perhaps something like this?

t(apply(A, 1, function(x) as.numeric(x == max(x))))
#      [,1] [,2] [,3]
# [1,]    0    0    1
# [2,]    0    0    1
# [3,]    0    1    0
# [4,]    1    0    0

Note that in the case of multiple values matching the maximum in a row, there may be more than one "1" per row.

Upvotes: 5

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