[R] Substituting numerical values using `apply'

[Olivella]

Hello,

I wish to perform a substitution of certain numerical values in a data
matrix with the corresponding column name. For instance, if I have a data
matrix
V1  V2  V3
2    0    1
0    1    2 
1    5    0
5    0    0 

I want to substitute the `1' and the `2' for the corresponding column name,
and make everything else `NA' like this
V1    V2    V3
V1    NA    V3
NA    V2    V3 
V1    NA    NA
NA    NA    NA 

I have done this using an explicit `for' loop, but it takes a really long
time to finish. Is there any way I can do this using `apply' or some form of
implicit looping?

Thank you for your help,

SO
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