[R] question regarding vector ops

[Uwe Ligges]

Rajarshi Guha wrote:
> On Fri, 2004-04-09 at 12:03, Rajarshi Guha wrote:
> 
>>Hi,
>>  I was playing with some code and came upon this situation.
>>
>>
>>>x <- c(1, rep(0,9))
>>>x
>>
>> [1] 1 0 0 0 0 0 0 0 0 0
>>
>>>idx <- 1:10
>>>x[idx] <- x[idx] + 1
>>>x
>>
>> [1] 2 1 1 1 1 1 1 1 1 1
>>
>>This is expected. But if I do:
>>
>>
>>>x <- c(1, rep(0,9))
>>>x
>>
>> [1] 1 0 0 0 0 0 0 0 0 0
>>
>>>idx <- rep(0,10)
>>>idx
>>
>> [1] 0 0 0 0 0 0 0 0 0 0
>>
>>>x[idx] <- x[idx] +1
>>>x
>>
>> [1] 1 0 0 0 0 0 0 0 0 0
> 
> 
> Aah, I realize that the second code snippet is not supposed to work.
> What I meant was:
> 
> 
>>x <- c(1, rep(0,9))
>>x
> 
>  [1] 1 0 0 0 0 0 0 0 0 0
> 
>>idx <- rep(1,10)
>>idx
> 
>  [1] 1 1 1 1 1 1 1 1 1 1
> 
>>x[idx] <- x[idx] + 1
> 
>  [1] 2 0 0 0 0 0 0 0 0 0
> 
> I expected that x would be
> 
> [1] 11 0 0 0 0 0 0 0 0 0
> 
> Or am I thinking about it wrong?

Yes, you are wrong: You are indexing 10 times the first value. Writing 
10 times the same value 10 times into the same location does not sum it up.

Consider
  x[idx] <- 1:10
Then you are writing 1 into x[1], 2 into x[1], 3 into x[1], ... and 
finally 10 into x[1], hence at the end x[1] is 10.

Uwe Ligges





> Thanks,
> 
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> Rajarshi Guha <rxg218 at psu.edu> <http://jijo.cjb.net>
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