Use max on each element of a matrix

Use pmax:

pmax(x,0)#     [,1] [,2] [,3] [,4] [,5]#[1,]    0    0    0    2    6#[2,]    0    0    0    3    7#[3,]    0    0    0    4    8#[4,]    0    0    1    5    9

You can use R's indexing function [ to do this directly:

x <- array(-10:10, dim=c(4,5))x[x < 0] <- 0

This works because x < 0 creates a logical matrix output:

x < 0     [,1] [,2]  [,3]  [,4]  [,5][1,] TRUE TRUE  TRUE FALSE FALSE[2,] TRUE TRUE  TRUE FALSE FALSE[3,] TRUE TRUE FALSE FALSE FALSE[4,] TRUE TRUE FALSE FALSE FALSE

And the resulting matrix is:

     [,1] [,2] [,3] [,4] [,5][1,]    0    0    0    2    6[2,]    0    0    0    3    7[3,]    0    0    0    4    8[4,]    0    0    1    5    9

The timing between the two methods is surprisingly similar. Here's a larger example illustrating the comparable timings:

xbigC <- xbigE <- matrix(sample(-100:100, 1e8, TRUE), ncol = 1e4)system.time(xbigC[xbigC < 0] <- 0) #---   user  system elapsed    4.56    0.37    4.93 system.time(xbigE  <- pmax(xbigE,0))#---   user  system elapsed    4.10    0.51    4.62 all.equal(xbigC, xbigE)#---[1] TRUE

It appears that the order of the arguments to pmax() affects the class of what is returned when the input is a matrix:

pmax(0,x)[1] 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9pmax(x,0)     [,1] [,2] [,3] [,4] [,5][1,]    0    0    0    2    6[2,]    0    0    0    3    7[3,]    0    0    0    4    8[4,]    0    0    1    5    9