[R] Replace values in data.frame conditional on another data.frame

[Carson Farmer]

Dear List,

I am looking for an efficient method for replacing values in a
data.frame conditional on the values of a separate data.frame. Here is
my scenario:
I have a data.frame (A) with say 1000 columns, and 365 rows. Each cell
in the data.frame has either valid value, or NA. I have an additional
data.frame (B) with the same number of rows and columns, with valid
values in all cells. What I would like to do, is replace the cells in B
with NA if and only if the corresponding cell in A is NA.
I have search extensively for a method to do this, and I'm sure that in
the end the solution will be embarrassingly simple, however, any help is
greatly appreciated!

Cheers,

Carson

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