[R] minimum from matrix

[oarabile at stams.strath.ac.uk]

I have a very large matrix with columns that have some of their
entries as zero


A small example if a=

         [,1]  [,2]  [,3]  [,4]
     [,1] 0     2     0     0
     [,2] 1     3     0     3
     [,3] 2     0     3     5
     [,4] 0     4     0     0

and what to get the minimum number from each column but that number
should not be zero. If I use apply (a,2,min) I will get a vector of
zeros as the minimum but what I want it for example from column 1 I
should get 1 i.e for all the matrix I should get a vector (1,2,3,3). I
wonder if someone can give an idea on how to go about it.

thanks in advance for your help.

Oarabile