________________________________________
> From: David L Carlson [dcarl...@tamu.edu]
>This overwrites the data so you might want to create a copy first.
>
> example <- data.frame(V1=c(3, -1), V2=c(-2, 4), V3=c(4, 1))
> tf <- ifelse(example<0, TRUE, FALSE)
> example[tf] <- NA
> apply(example, 1, mean, na.rm=TRUE)
'simpler' to do something like
apply(example, 1, function(x) mean(x[x>=0]))
Also note that this is averaging _rows_; if you want the column (variable)
means, which would be much more usual in a data frame, use apply(example, 2,
function(x) mean(x[x>=0]))
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