Working with Missing Values

Hadley Wickham — written Dec 10, 2012 — source

If you’re working with missing values, you need to know two things: - What happens when you put missing values in scalars (e.g. double1) - How to get and set missing values in vectors (e.g. NumericVector)

Scalars

The following code explores what happens when you take one of R’s missing values, coerce it into a scalar, and then coerce back to an R vector.

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
List scalarMissings() {
  int int_s = NA_INTEGER;
  String chr_s = NA_STRING;
  bool lgl_s = NA_LOGICAL;
  double num_s = NA_REAL;

  return List::create(int_s, chr_s, lgl_s, num_s);
}

Which as expected yields missing values in R:

str(scalarMissings())

List of 4
 $ : int NA
 $ : chr NA
 $ : logi TRUE
 $ : num NA

So:

IntegerVector -> int: stored as the smallest integer
CharacterVector -> String: the string “NA”
LogicalVector -> bool: TRUE. To work with missing values in logical vectors, use an int instead of a bool.
NumericVector -> double: stored as an NaN, and preserved. Most numerical operations will behave as you expect, but as discussed below logical comparison will not.

If you’re working with doubles, you may be able to get away with ignoring missing values and working with NaN (not a number). R’s missing values are a special type of the IEEE 754 floating point number NaN. That means if you coerce them to double in your C++ code, they will behave like regular NaN’s. That means, in a logical context they always evaluate to FALSE.

Vectors

To set a missing value in a vector, you need to use a missing value specific to the type of vector:

// [[Rcpp::export]]
List missingSampler() {
  return(List::create(NumericVector::create(NA_REAL), 
                      IntegerVector::create(NA_INTEGER),
                      LogicalVector::create(NA_LOGICAL), 
                      CharacterVector::create(NA_STRING)));
}

Now let’s confirm that these values do in fact appear missing in R:

str(missingSampler())

List of 4
 $ : num NA
 $ : int NA
 $ : logi NA
 $ : chr NA

To check if a value in a vector is missing, use the class method is_na:

// [[Rcpp::export]]
LogicalVector isNA(NumericVector x) {
  int n = x.size();
  LogicalVector out(n);
  
  for (int i = 0; i < n; ++i) {
    out[i] = NumericVector::is_na(x[i]);
  }
  return out;
}

Here we test with some missing and non-missing values:

isNA(c(NA, 5.4, 3.2, NA))

[1]  TRUE FALSE FALSE  TRUE

Equivalent behavior to the isNA function can be obtained by calling the is_na sugar function, which takes a vector and returns a logical vector.

tags: basics

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