R help - Jul 2010 - eval and assign in loop problem

deaR useRs,

I am trying to assign different values to different objects in a for loop.
The following is a toy example of the part that has been giving me a hard
time.

The first "for loop" generates four objects, b0, b1, b2, b3 with
random
numbers.
And, the second "for loop" is equivalent to
b1 = b0
b2 = b1
b3 = b2
b4 = b3

But, when I run this code, the result is equivalent to
b1 = b0
b2 = b0
b3 = b0
b4 = b0

So, the increment does not seem to be properly working for the second part
of the "assign" function.
Why would this be?

for (i in 0:3)
       {
           r = runif(1)
           assign(paste('b',i,sep=''),r)
       }


for (i in 1:4)
       {

assign(paste('b',i,sep=''),eval(parse(text=paste('b',i-1,sep=''))))
       }

Thank you.

Nic

	[[alternative HTML version deleted]]

> 
> for (i in 0:3)
>        {
>            r = runif(1)
>            assign(paste('b',i,sep=''),r)
>        }
> 
> 
> for (i in 1:4)
>        {
> 
>
assign(paste('b',i,sep=''),eval(parse(text=paste('b',i-1,sep=''))))
>        }
> 
First, b1 is set to b0
Then,  b2 is set to b1 (which was just set to b0)
Then,  b3 is set to b2 (which was just set to b1, which was just set to b0)

...

All b[i] are set to b0

If this is all you're trying to accomplish, it's much better to keep the
common objects in a vector (or list if they are more complicated) and 
use sapply/lapply and vectorized functions to perform the manipulations. 
You've managed to use three seldom-needed methods in 2 lines of R code 
:) (for loops, assign, and eval(parse(text=)))

On Fri, 09-Jul-2010 at 08:25AM -0700, S.Nicholas wrote:

|> deaR useRs,
|> 
|> I am trying to assign different values to different objects in a for loop.
|> The following is a toy example of the part that has been giving me a hard
|> time.
|> 
|> The first "for loop" generates four objects, b0, b1, b2, b3 with
random
|> numbers.
|> And, the second "for loop" is equivalent to
|> b1 = b0
|> b2 = b1
|> b3 = b2
|> b4 = b3

I don't quite get the idea, but would you be able to simply reverse
the order of those last 4 (for i in 3:0)?  That way, they won't all be
overwritten with b0.

HTH



|> 
|> But, when I run this code, the result is equivalent to
|> b1 = b0
|> b2 = b0
|> b3 = b0
|> b4 = b0
|> 
|> So, the increment does not seem to be properly working for the second part
|> of the "assign" function.
|> Why would this be?
|> 
|> for (i in 0:3)
|>        {
|>            r = runif(1)
|>            assign(paste('b',i,sep=''),r)
|>        }
|> 
|> 
|> for (i in 1:4)
|>        {
|> 
|>
assign(paste('b',i,sep=''),eval(parse(text=paste('b',i-1,sep=''))))
|>        }
|> 
|> Thank you.
|> 
|> Nic
|> 
|> 	[[alternative HTML version deleted]]
|> 
|> ______________________________________________
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|> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
|> and provide commented, minimal, self-contained, reproducible code.

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