beginner's loop issue

Yes, the short answer is that you need to define out before running
the loop. The most effective way to do so will be to set up a matrix
with the exact right dimensions (if you know them up front); something
like out <- matrix(NA, nrow = length(input), ncol = 9)

Michael

Dear All,

I hope you don't mind helping me with this small issue. I haven't been using
R in years and I'm trying to fill in a matrix
with the output of a function (I'm probably using the Matlab logic here and
it's not working).
Here is my code:

for (i in 1:length(input)){
?out[i,1:3] <- MyFunction(input[i,1],input[i,2], input[i,3])
? ?out[i,4:6] <- MyFunction(input[i,5],input[i,7], input[i,6])
? ? ?out[i,7:9] <- MyFunction(input[i,8],input[i,10], input[i,9])
}

'input' is a matrix

dim(input)

[1] 46 10

and each raw corresponds to a different subject.
The error I get here is

/Error in out[i, 1:3] <- get.vaTer(input[i, 2], input[i, 4], input[i, 3], ?:
?object 'out' not found/

So I wonder, what's wrong in the assignment to the variable out?
Should I define the variable before the loop?

Thanks for your help
Best

Ale

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Dear All,

I hope you don't mind helping me with this small issue. I haven't been using
R in years and I'm trying to fill in a matrix
with the output of a function (I'm probably using the Matlab logic here and
it's not working).
Here is my code:

for (i in 1:length(input)){
?out[i,1:3] <- MyFunction(input[i,1],input[i,2], input[i,3])
? ?out[i,4:6] <- MyFunction(input[i,5],input[i,7], input[i,6])
? ? ?out[i,7:9] <- MyFunction(input[i,8],input[i,10], input[i,9])
}

'input' is a matrix

dim(input)

[1] 46 10

and each raw corresponds to a different subject.
The error I get here is

/Error in out[i, 1:3] <- get.vaTer(input[i, 2], input[i, 4], input[i, 3], ?:
?object 'out' not found/

out has to exist first, as previous commenter said.

Furthermore, suggestions:

Consider making MyFunction accept a vector of 3 arguments, rather than
separate arguments.

Consider making out 3 columns, as in

out <- matrix(0, nrow=N, ncol=3)
for(i ...){
out[i,1:3] <- MyFunction(input[i,1:3])
out[i,1:3] <- MyFunction(input[i,4:6])
out[i,1:3] <- MyFunction(input[i,7:9])
}

If you could re-shape your input "thing" as a list with one element
that needs to go into MyFunction, this could get easier still:

lapply(input, MyFunction)

 or if input were an array with 3 columns, you could revise MyFuntion
to accept a 3-vector.

apply(input, 1, MyFunction)

Hardly ever in R does one need to specify inputs as you have done in
your example.
pj

Paul E. Johnson
Professor, Political Science ? ?Assoc. Director
1541 Lilac Lane, Room 504 ? ? Center for Research Methods
University of Kansas ? ? ? ? ? ? ? University of Kansas
http://pj.freefaculty.org ? ? ? ? ? ?http://quant.ku.edu

Thank you all for your helpful comments!
I solved my problem by creating an empty matrix before the loop and
adjusting the loop itself, this is the code:

size <- dim(input)
out <- matrix('',nrow =size[1], ncol =  9)

for (i in 1:nrow(input)) {
  
  out[i,1:3] <- unlist(MyFunction(input[i,1],input[i,2], input[i,3]) 
    out[i,4:6] <- unlist(MyFunction(input[i,5],input[i,7], input[i,6]) 
      out[i,7:9] <- unlist(MyFunction(input[i,8],input[i,10], input[i,9]) 

}

It's probably not the most efficient way of doing it but it gives me the
desired output matrix... 
Thanks for pointing me to this pdf R inferno pdf, I had an old version of
it..
I'm, starting to use R again after long time and this is a very useful
guide!

All the best

Ale




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