R: Eliminating observed values with zero variance

March 8, 2010
By

(This article was first published on CYBAEA Data and Analysis, and kindly contributed to R-bloggers)

I needed a fast way of eliminating observed values with zero variance from large data sets using the R statistical computing and analysis platform. In other words, I want to find the columns in a data frame that has zero variance. And as fast as possible, because my data sets are large, many, and changing fast. The final result surprised me a little.

I use the KDD Cup 2009 data sets as my reference for this experiment. (You will need to register to download the data.) It is a realistic example of the type of customer data that I usually work with. It has 50,000 observations of 15,000 variables. To load it into R you'll need a reasonably beefy machine. My workstation has 16GB of memory; if yours have less then use a sample of the data.

We load the data into R and propose a few ways in which we may identify the columns we need:

#!/usr/bin/Rscript
## zero-var.R - find the fastest way of eliminating observations with zero variance
## © 2010 Allan Engelhardt, http://www.cybaea.net

## Read the data file.
## We have already converted it to R format and saved it, so we can do
load("train.RData")
## instead of something like
# train <- read.delim(file="../orange_large_train.data.bz2")

## Some suggestions for zero variance functions:
zv.1 <- function(x) {
    ## The literal approach
    y <- var(x, na.rm = TRUE)
    return(is.na(y) || y == 0)
}
zv.2 <- function(x) {
    ## As before, but avoiding direct comparison with zero
    y <- var(x, na.rm = TRUE)
    return(is.na(y) || y < .Machine$double.eps ^ 0.5)
}
zv.3 <- function(x) {
    ## Maybe it is faster to check for equality than to compute?
    y <- x[!is.na(x)]
    return(all(y == y[1]))
}
zv.4 <- function(x) {
    ## Taking out the special case may speed things up?
    ## (At least for this data set where this case is common.)
    z <- is.na(x)
    if ( all(z) ) return(TRUE);
    y <- x[!z]
    return(all(y == y[1]))
}

Now we just have to load the very useful rbenchmark package and let the machine figure it out:

library("rbenchmark")

cat("Running benchmarks:\n")
benchmark(
          zv1 = { sapply(train, zv.1) },
          zv2 = { sapply(train, zv.2) },
          zv3 = { sapply(train, zv.3) },
          zv4 = { sapply(train, zv.4) },
          replications = 5,
          columns = c("test", "elapsed", "relative", "sys.self"),
          order = "elapsed"
          )

The answer (on my machine) is that it is faster to calculate than to check for equality:

Running benchmarks:
  test elapsed relative sys.self
1  zv1  78.619 1.000000    6.395
2  zv2  79.276 1.008357    6.586
3  zv3 113.024 1.437617    1.735
4  zv4 118.579 1.508274    1.716

The two functions based on the core variance function are easily the fastest (despite having to do arithmetic) while taking out the special case in the equality functions is a Bad Idea.

Can you think of an even faster way to do it?

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