R help - Jun 2009 - Efficient lookup on a two-dimensional table

Dear R-Users,
I need to lookup values from a 2-d table using the row names and column
names as indices. I was wondering if there's a way to do this without an
explicit loop.

Example:
#x is the 2-d table that holds the values

x <- matrix(rnorm(26*12),nrow=26)

rownames(x) <- letters

colnames(x) <- month.name


#y is a data frame that has the "keys" I want to use as indices into x

y <- data.frame(ltrs=sample(letters,5),mnths=sample(month.name,5),values=0)


#I want to fill in the "values" column using the "ltrs" and
"mnths" columns
as keys to look up

# the associated value from x

#One way to do this is with a FOR loop

for (i in 1:nrow(y)) {y$val[i] <- x[y$ltrs[i],y$mnths[i]]}


My question: Is there a more efficient way (e.g., one without using an
explicit loop) to do this?


Thanks in advance!


-Rama Ramakrishnan

	[[alternative HTML version deleted]]

Resending after fixing a mistake in the earlier email ... sorry for the
confusion.

******
Dear R-Users,
I need to lookup values from a 2-d table using the row names and column
names as indices. I was wondering if there's a way to do this without an
explicit loop.

Example:
#x is the 2-d table that holds the values

x <- matrix(rnorm(26*12),nrow=26)

rownames(x) <- letters

colnames(x) <- month.name


#y is a data frame that has the "keys" I want to use as indices into x

y <- data.frame(ltrs=sample(letters,5),mnths=sample(month.name,5),values=0)


#I want to fill in the "values" column using the "ltrs" and
"mnths" columns
as keys to look up

# the associated value from x

#One way to do this is with a FOR loop


for (i in 1:nrow(y)) {y$values[i] <- x[as.character(y$ltrs[i]),as.character(
y$mnths[i])]}


My question: Is there a more efficient way (e.g., one without using an
explicit loop) to do this?


Thanks in advance!


-Rama Ramakrishnan

	[[alternative HTML version deleted]]

Try this:

y$values <- diag(x[y$ltrs, y$mnths])

On Thu, Jun 25, 2009 at 11:02 AM, Rama Ramakrishnan
<rama@alum.mit.edu>wrote:
> Dear R-Users,
> I need to lookup values from a 2-d table using the row names and column
> names as indices. I was wondering if there's a way to do this without
an
> explicit loop.
>
> Example:
> #x is the 2-d table that holds the values
>
> x <- matrix(rnorm(26*12),nrow=26)
>
> rownames(x) <- letters
>
> colnames(x) <- month.name
>
>
> #y is a data frame that has the "keys" I want to use as indices
into x
>
> y <- data.frame(ltrs=sample(letters,5),mnths=sample(month.name
> ,5),values=0)
>
>
> #I want to fill in the "values" column using the "ltrs"
and "mnths" columns
> as keys to look up
>
> # the associated value from x
>
> #One way to do this is with a FOR loop
>
> for (i in 1:nrow(y)) {y$val[i] <- x[y$ltrs[i],y$mnths[i]]}
>
>
> My question: Is there a more efficient way (e.g., one without using an
> explicit loop) to do this?
>
>
> Thanks in advance!
>
>
> -Rama Ramakrishnan
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

	[[alternative HTML version deleted]]

On Jun 25, 2009, at 10:02 AM, Rama Ramakrishnan wrote:
> Dear R-Users,
> I need to lookup values from a 2-d table using the row names and  
> column
> names as indices. I was wondering if there's a way to do this  
> without an
> explicit loop.
>
> Example:
> #x is the 2-d table that holds the values
>
> x <- matrix(rnorm(26*12),nrow=26)
>
> rownames(x) <- letters
>
> colnames(x) <- month.name
>
>
> #y is a data frame that has the "keys" I want to use as indices
into x
>
> y <- data.frame(ltrs=sample(letters,5),mnths=sample(month.name, 
> 5),values=0)
>
>
> #I want to fill in the "values" column using the "ltrs"
and "mnths"
> columns
> as keys to look up
>
> # the associated value from x
>
?apply

x$value <- apply(y, 1, function(z) x[ z['ltrs'], z['mnths']
])
> #One way to do this is with a FOR loop
>
> for (i in 1:nrow(y)) {y$val[i] <- x[y$ltrs[i],y$mnths[i]]}
>
>
> My question: Is there a more efficient way (e.g., one without using an
> explicit loop) to do this?
>
>
> Thanks in advance!
>
>
> -Rama Ramakrishnan
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
Heritage Laboratories
West Hartford, CT