R help - Jan 2010 - calculations on columns with partially matching names

Is there a command for partial matching of character strings? Specifically,
I'd like to be able to calculate the mean of the values in any columns in a
data frame or matrix that have identity in part of their column names.  For
example, columns labeled "mpw06a" and "mpw06b" match on the
first five
characters; their mean would be taken whereas any columns beginning with
other than "mpw06" would be excluded.  I need to compare every pair of
columns in the frame, and in some cases, possibly three at a time. 

Thanks in advance for any ideas.




Jim Bouldin
Research Ecologist
Department of Plant Sciences, UC Davis
Davis CA, 95616
530-554-1740

On Jan 3, 2010, at 6:09 PM, Jim Bouldin wrote:
>
> Is there a command for partial matching of character strings?  
> Specifically,
> I'd like to be able to calculate the mean of the values in any  
> columns in a
> data frame or matrix that have identity in part of their column  
> names.  For
> example, columns labeled "mpw06a" and "mpw06b" match on
the first five
> characters; their mean would be taken whereas any columns beginning  
> with
> other than "mpw06" would be excluded.
?grep
?"["

 > tdf <- data.frame(mpw06a=rnorm(10), mpw06b=rnorm(10), abc=rnorm(10))

 > lapply(tdf[ , grep("mpw06", names(tdf)) ], mean)
$mpw06a
[1] -0.1825447

$mpw06b
[1] -0.2386772
> I need to compare every pair of
> columns in the frame, and in some cases, possibly three at a time.
?combn
>
> Thanks in advance for any ideas.


>
>
>
>
> Jim Bouldin
> Research Ecologist
> Department of Plant Sciences, UC Davis
> Davis CA, 95616
> 530-554-1740
>
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Heritage Laboratories
West Hartford, CT