[R] how to assign a value?

[David Winsemius]

On Dec 11, 2011, at 9:07 PM, Jinsong Zhao wrote:

> On 2011-12-12 0:00, David Winsemius wrote:
>>
>> On Dec 11, 2011, at 10:27 AM, Jinsong Zhao wrote:
>>
>>> Hi there,
>>>
>>> I hope to modify values in a vector or matrix in the following code:
>>>
>>> for (i in 1:9) {
>>> assign(paste("a.", i, sep = ""), 1:i)
>>> get(paste("a.", i, sep = ""))[i] <- i+50
>>> }
>>
>> Just one matrix? Then you seem to have inappropriately borrowed using
>> "." as an indexing operation. In R that is just another character  
>> when
>> used as an object name. "a.1" is notgoing to evaulate to a[1]. Look  
>> at
>> what you would have had after
>>
>> > for (i in 1:9) {
>> + assign(paste("a.", i, sep = ""), 1:i)
>> + }
>> > ls()
>> [1] "a" "a.1" "a.2"
>> [4] "a.3" "a.4" "a.5"
>> [7] "a.6" "a.7" "a.8"
>> [10] "a.9"
>>
>> > a.1
>> [1] 1
>> > a.2
>> [1] 1 2
>>
>> Each of those assign() operations created a single vector of length  
>> i. I
>> doubt that was what you intended,
>
> yes, it was what I intended.

Then you are free to continue banging your head against a wall.

>
>>
>> Better would be to describe your objects and your intentions, rather
>> than expecting us to understand your goals by just looking at code  
>> that
>> doesn't achieve thos goals. (There is no `get<-` function which was  
>> the
>> source of the error.)
>>
>
> The question is why
>
> get(paste("a.", i, sep = ""))[i] <- i+50
>
> give the following error message:

What part of THERE IS NO "get<-" function (much less a `get[<-`  
function)  don't you understand?

>
> Error in get(paste("a.", i, sep = ""))[i] <- i + 50 :
>  target of assignment expands to non-language object
>
> The a.1 to  a.9 was created in the previous step.
>
> if only
>
> get(paste("a.", i, sep = ""))[i]
>
> can give correct output.

Right. They are there and can even be indexed:

 > get(paste("a", 9, sep="."))[9]
[1] 9

You could assign the value of get(paste("a", 9, sep=".")) to an  
intermediate object, which you could then reference using "[" and then  
use `assign` to push that object's value back to an object named "a. 
1", , "a.2", etc. Very clumsy and not an idiom that people want to  
promote.

 > x <- get(paste("a", 9, sep="."))
 > x[9] <- x[9]+50
 > assign(paste("a", 9, sep="."), x)
 > a.9
[1]  1  2  3  4  5  6  7  8 59


> Why I cannot assign values to it?

Using get, you mean? Because that is not the way R is designed. get()  
returns a value. `assign` is used... wait for it ... assignment.

 > get(paste("a", 1, sep="."))
[1] 1

Not a.1 but rather a.1's value. You cannot assign something else to  
the number 1. You are free to complain about the fact that R is is not  
languageX as much as you like, but it won't create new capabilities  
for functions. You've been given advice about how to get to the goal  
you desire by both Dunlap and Burns. The counter-question is why you  
have such trouble accepting advice.

>
> Regards,
> Jinsong

David Winsemius, MD
West Hartford, CT