[R] how to skip a specific value when using apply() function to a matrix?

[Shuhua Zhan]

Hello R experts,
I'd like to studentize a matrix (tmp1) by column using apply() function and skip some specific values such as zeros in the example below to tmp2 but not tmp3. I used the script below and only can get a matrix tmp3. Could you please help me to studentize the matrix (tmp1) without changing the zeros and generate a new matrix tmp2?
Thanks,
Joshua

tmp1
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]   15    7    9   49   60    3    2    0
[2,]   13    7   10   56   21    4    3    0
[3,]   10    2   12   50   19    1    0    2
[4,]   15    5   30   52  120    2    4    0
[5,]   12    9   71   76  211    4    3    1
[6,]   11   12    2   69   18    5    2    0

tmp2
[1,]  1.1296201  0.0000000 -0.5207910 -0.8630035 -0.1913482 -0.1132277 -0.9561829  0.0000000
[2,]  0.1613743  0.0000000 -0.4817316 -0.2380699 -0.6944435  0.5661385  0.2390457  0.0000000
[3,] -1.2909944 -1.4680505 -0.4036130 -0.7737273 -0.7202433 -1.4719601  0.0000000  0.7071068
[4,]  1.1296201 -0.5872202  0.2994548 -0.5951748  0.5826446 -0.7925939  1.4342743  0.0000000
[5,] -0.3227486  0.5872202  1.9008870  1.5474546  1.7565336  0.5661385  0.2390457 -0.7071068
[6,] -0.8068715  1.4680505 -0.7942062  0.9225210 -0.7331431  1.2455047 -0.9561829  0.0000000

tmp3
          [,1]       [,2]       [,3]       [,4]       [,5]       [,6]      [,7]       [,8]
[1,]  1.1296201  0.0000000 -0.5207910 -0.8630035 -0.1913482 -0.1132277 -0.243975 -0.5976143
[2,]  0.1613743  0.0000000 -0.4817316 -0.2380699 -0.6944435  0.5661385  0.487950 -0.5976143
[3,] -1.2909944 -1.4680505 -0.4036130 -0.7737273 -0.7202433 -1.4719601 -1.707825  1.7928429
[4,]  1.1296201 -0.5872202  0.2994548 -0.5951748  0.5826446 -0.7925939  1.219875 -0.5976143
[5,] -0.3227486  0.5872202  1.9008870  1.5474546  1.7565336  0.5661385  0.487950  0.5976143
[6,] -0.8068715  1.4680505 -0.7942062  0.9225210 -0.7331431  1.2455047 -0.243975 -0.5976143

Here is my script:
stud<- function(x){
    x<-(x-mean(x))/sd(x)
    return (x)
}
tmp3<-apply(tmp1,2,stud)