Code bug unresolved involving if condition - R-help

Fri, Apr 25, 2003 8:15 AM #

Hi R lovers!

I am a beginner in coding with R so my question may be very easily solved
but I don't know how.

I have written the following function in a .txt file


ClearDelta <- function(Matrix)
{
ncol<-ncol(Matrix);nrow<-nrow(Matrix);
for (i in 1:nrow) {
                  for (j in 1:(ncol-1))
                        {if (Matrix[i,j]==NA) (NA->Matrix[i,j+1])}
                  }
}

I can charge it with the source() command
But I get the following message when applied to a matrix

Error in if (Matrix[i, j] == NA) (Matrix[i, j + 1] <- NA) :
        missing value where logical needed

MatCor is the following Matrix

[,1]         [,2]       [,3]
[1,]         NA          0.9870676  0.04648933
[2,] 0.98706757  1.0000000 -0.17353590
[3,] 0.04648933 -0.1735359  1.00000000

Do you know why I get such an unpleasant message from so polite a software?

Thanks to anybody who could help.







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Brian Ripley

Fri, Apr 25, 2003 8:34 AM #

The message is both pleasant and accurate!  It indicates that you have 
made an error, and tells you what the error is.

foo == NA is always missing, since NA denotes a missing value.
Use is.na(foo): see ?is.na.

On Fri, 25 Apr 2003 vincent.stoliaroff at socgen.com wrote:

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

Thomas Lumley

Fri, Apr 25, 2003 8:56 AM #

On Fri, 25 Apr 2003 vincent.stoliaroff at socgen.com wrote:

The test (Matrix[i,j] ==NA) returns NA, not TRUE or FALSE as you expected.
Think of NA as meaning "I don't know what this value is", so you are
asking if Matrix[i,j] is equal to a number that you don't know. The answer
is that R doesn't know whether this is TRUE or FALSE, The value NA in a
logical variable means just that "This is TRUE or FALSE but I don't know
which".  You can use is.na() test for NA, ie

  if(is.na(Matrix[i, j]))


I can't resist also pointing out that for large matrices there is a more
efficient answer

for(i in 1:nrow(Matrix))
	Matrix[i,]<- diff(c(0,cumsum(Matrix[i,])))


	-thomas