[R] How to Populate List

[Rolf Turner]

On 10/07/2009, at 5:25 PM, Gaurav Kumar wrote:

> Hi Rolf,
>
> My apologies for the inconvenience caused  due to  my reply @ "R- 
> help diges".
> Thanks for providing me a clue to solve the problem .
>
> I modify a bit to  populate a list. This is throwing an error
> ##### Error in `*tmp*`[[j]] : subscript out of bounds"
>
> myData <- read.table("table.data",
>                  header=T,
>                  sep="\t",
>                  comment.char = "#"
>                  );
>  inp <- scan("table.data", what=list(comp=" ", A="", B="", C=""));
>  n <- c(0:length(inp$comp));
>  myList=list();
>  for(i in n-1)
>  {
>     if(i < length(inp$comp))
>     {
>         j=i+1;
>         myList[[j]][1] <-c(as.numeric(myData$A[i]));
>         myList[[j]][2] <-c(as.numeric(myData$B[i]));
>         myList[[j]][2] <-c(as.numeric(myData$C[i]));
>     }
>
>  }
>  print(myList);
>
> Any idea where i'm doing a mistake which is causing this error.

When you're having difficulty, construct a very simple example that  
reproduces
the problem.  E.g.

	myList <- list()
	for(i in 1:10) {
		myList[[i]][1] <- 42
	}

You'll get the same ``subscript out of bounds'' error.  Now  
***think*** a bit!!!

Strangely enough ( :-) ) the error message means ***exactly*** what  
it says.
The subscript is out of bounds!  You are trying to assign a value to  
the 1st,
and 2nd, and ... 2nd, but I guess you mean the 3rd, components of  
myList[[j]].

But at this stage myList[[j]] ***DOESN'T HAVE ANY COMPONENTS***.  It  
doesn't exist
yet.  How could it have components?

There are a couple of ways you could go to do it right.

(1) (Sticking with your original syntax, more or less.)

if(i < length(inp$comp))
     {
         j=i+1;
	myList[[j]] <- numeric(3) # Now myList[[j]] ***does*** have  
components, and all is well.
         myList[[j]][1] <- as.numeric(myData$A[i])
         myList[[j]][2] <- as.numeric(myData$B[i])
         myList[[j]][2] <- as.numeric(myData$C[i])
     }

NOTE: The ``c()'' construction you were using is unnecessary and  
confusing.  What on earth
do you think ``c()'' means, anyway? Also the semicolons are  
unnecessary and confusing.
This is ***R*** not SAS or C!!!

(2) (Better.)

if(i < length(inp$comp))
     {
         j=i+1;
	myList[[j]] <- c(as.numeric(myData$A[i]),
                          as.numeric(myData$B[i]),
                          as.numeric(myData$C[i]))
     }

NOTE: The foregoing is an ***appropriate*** use of ``c()''!!!

	cheers,

		Rolf

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