R help - Sep 2008 - looping through variables

Hello everyone,

I have the following problem:

My analysis includes many predictor variables (>50) in the form of 
raster maps (asc), but I am trying to avoid having to type all their 
names over and over again in the analysis (e.g. for vectorisation, for 
deletion of NA's, etc.)

So ideally I would like to store them in some way that their names 
only have to be typed once and can always be referred back to.

First step would be to automate the vectorisation of the raster maps:

# these are the raster maps which need to combined somehow ??
variables <- (temperature, precipitation, elevation, vegcover) 

VariablesNew=c()

For (i in 1:length(variables)) {
Varnew <- as.vector(variables[i])
VariablesNew <- cbind(VariablesNew, Varnew)
}

This should return a data frame called VariablesNew with each column 
representing one of the variables. 

So the BIG QUESTION is how to input the variable names that they can 
be referred to easily and, the variable itself can be pulled out and 
not just its name!!
I believe this cant be too difficult??

Thanx in advance,
Katrin

Perhaps what you want is get().

    apple <- rnorm(5)
    orange <- runif(5)

    fruits <- c("apple", "orange")

    fruit.data <- NULL
    for( fruit in fruits ){
       v <- get(fruit)
       fruit.data <- cbind(fruit.data, v)
    }
    colnames(fruit.data) <- fruits

    fruit.data

Here the resulting output is a matrix which works if all of your inputs 
have the same length. If they don't, then you probably want to use a 
list instead. Also have a look at assign().

Regards, Adai




K. Fleischer wrote:
> Hello everyone,
> 
> I have the following problem:
> 
> My analysis includes many predictor variables (>50) in the form of 
> raster maps (asc), but I am trying to avoid having to type all their 
> names over and over again in the analysis (e.g. for vectorisation, for 
> deletion of NA's, etc.)
> 
> So ideally I would like to store them in some way that their names 
> only have to be typed once and can always be referred back to.
> 
> First step would be to automate the vectorisation of the raster maps:
> 
> # these are the raster maps which need to combined somehow ??
> variables <- (temperature, precipitation, elevation, vegcover) 
> 
> VariablesNew=c()
> 
> For (i in 1:length(variables)) {
> Varnew <- as.vector(variables[i])
> VariablesNew <- cbind(VariablesNew, Varnew)
> }
> 
> This should return a data frame called VariablesNew with each column 
> representing one of the variables. 
> 
> So the BIG QUESTION is how to input the variable names that they can 
> be referred to easily and, the variable itself can be pulled out and 
> not just its name!!
> I believe this cant be too difficult??
> 
> Thanx in advance,
> Katrin
> 
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