[BioC] ask for some code to remove absent genes

[zhihua li]

hi netters,

i have a gene expression matrix where rows are genes and columns are 
conditions. The values are either "A"(absent) or "P"(present). There are 15 
conditions in total. If a gene is absent in most of these conditions, say 
at least 13 out of the 15 values are "A", then I would like to remove this 
gene from the matrix. So finally I'll get a new matrix with only those 
genes that are present in at least 2 of the conditions. Could anyone give 
me a hint about how to write the code? 

Thanks a lot!