[R] Logical subset of the columns in a dataframe

[Prof Brian Ripley]

On Wed, 28 Jan 2009, Mark Na wrote:

> Hi R-helpers,
>
> I've been struggling with a problem for most of the day (!) so am finally
> resorting to R-help.
>
> I would like to subset the columns of my dataframe based on the frequency
> with which the columns contain non-zero values. For example, let's say that
> I want to retain only those columns which contain non-zero values in at
> least 1% of their rows.
>
> In Excel I would calculate a row at the bottom of my data sheet and use the
> following function
>
> =countif(range,">0")
>
> to identify the number of non-zero cells in each column. Then, I would
> divide that by the number of rows to obtain the frequency of non-zero values
> in each column. Then, I would delete those columns with frequencies < 0.01.
>
> But, I'd like to do this in R. I think the missing link is an analog to
> Excel's countif function. Any ideas?

Use something like

     DF[sapply(DF, function(x) mean(x) >= 0.01)]

Since logical values are converted to 0/1, mean() gives the frequency 
(and sum() the count).

>
> Thanks! Mark
>
> 	[[alternative HTML version deleted]]
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595