[R] Replacing missing values

[Mark Myatt]

Jean Vidal <jean.vidal at freesurf.fr> writes:
>I'm discovering R (very impressive), and didn't find in the docs a simple 
>method for replacing, in a data frame, missing values (NA) with the 
>column's mean (or any other method for reconstructing missing values when 
>needed).
>Thanks in advance for your help.

There are a series of na.* Functions in R but they are not well
documented (which is my way of saying "I can't work out how to make them
work"!). 

I use indexing to deal with missing values. For example:

        var[var == -99] <- NA

To replace a missing value code (e.g. -99) with NA. Replacing with an
imputed value can be done in the same manner but with the imputation
function on the RHS of the assignment. For example:

        var[is.na(var)] <- mean(var, na.rm = TRUE)

If var is a vector in a data.frame then you need to specify the
data.frame:

        df$var[is.na(df$var)] <- mean(df$var, na.rm = TRUE)

I hope that helps.

Mark

BTW: While I am here, can anyone explain how the na.* functions work?


--
Mark Myatt


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