R help - Jan 2008 - add/subtract matrices, ignoring NA or missing values

Hi,

For example, given two 2x2 matrices m1 and m2. I would like to add/subtract
element by element
> m1
         [,1] [,2]
[1,]   NA   NA
[2,]    1    2
> m2
        [,1] [,2]
[1,]    1   NA
[2,]   NA    2
> m1 + m2
        [,1] [,2]
[1,]   NA   NA
[2,]   NA    4

How can I ignore the NA, and get this ? Hope the solution can be extended to
subtract and modulo also.

        [,1] [,2]
[1,]   1   NA
[2,]   1    4

	[[alternative HTML version deleted]]

On 29/01/2008, at 3:34 PM, Ng Stanley wrote:
> Hi,
>
> For example, given two 2x2 matrices m1 and m2. I would like to add/ 
> subtract
> element by element
>
>> m1
>          [,1] [,2]
> [1,]   NA   NA
> [2,]    1    2
>
>> m2
>         [,1] [,2]
> [1,]    1   NA
> [2,]   NA    2
>
>> m1 + m2
>         [,1] [,2]
> [1,]   NA   NA
> [2,]   NA    4
>
> How can I ignore the NA, and get this ? Hope the solution can be  
> extended to
> subtract and modulo also.
>
>         [,1] [,2]
> [1,]   1   NA
> [2,]   1    4
In a word ***DON'T***.  A missing value is missing.  It is not the  
same as zero!
If it really should be zero it would've been coded as zero to start  
with.

It appears from your example that you want NAs to be treated as being  
zero, except
when you are adding two of them, in which case you want the sum to be  
NA.  I may
be suffering from poverty of imagination, but I cannot imagine a  
situation in which
this would be a sensible thing to do.

I could easily give you code to arrange for what you want to happen,  
but I'm not going
to, since I do not wish to encourage misguided behaviour.

	cheers,

		Rolf Turner


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Perhaps you could explain the motivation behind this.  At any rate here
are three different solutions:

ifelse(is.na(m1), ifelse(is.na(m2), NA, m2), ifelse(is.na(m2), m1, m1 + m2))

apply(array(c(m1, m2), c(2,2,2)), 1:2, function(x) sum(c(na.omit(x), NA)[1]))

na.m1 <- is.na(m1)
na.m2 <- is.na(m2)
ifelse(na.m1 & na.m2, NA, ifelse(na.m1, 0, m1) + ifelse(na.m2, 0, m2))


On Jan 28, 2008 9:34 PM, Ng Stanley <stanleyngkl at gmail.com>
wrote:
> Hi,
>
> For example, given two 2x2 matrices m1 and m2. I would like to add/subtract
> element by element
>
> > m1
>         [,1] [,2]
> [1,]   NA   NA
> [2,]    1    2
>
> > m2
>        [,1] [,2]
> [1,]    1   NA
> [2,]   NA    2
>
> > m1 + m2
>        [,1] [,2]
> [1,]   NA   NA
> [2,]   NA    4
>
> How can I ignore the NA, and get this ? Hope the solution can be extended
to
> subtract and modulo also.
>
>        [,1] [,2]
> [1,]   1   NA
> [2,]   1    4
>
>        [[alternative HTML version deleted]]
>
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