R help - Dec 2006 - ifelse question

Dear R-helpers,
How come that in the following code the rnorm() function is evaluated only once
for each branch of the 'ifelse' ?

x <- rnorm(10)
y1 <- ifelse(x > 0, rnorm(1) ,  rnorm(1))


What is the right way to make it called/evaluated for each row, apart from a
'for' loop ?

Thanks, 

Jacques.

Dear R-helpers,
How come that in the following code the rnorm() function is evaluated 
only once for each branch of the 'ifelse' ?

x <- rnorm(10)
y1 <- ifelse(x > 0, rnorm(1) ,  rnorm(1))

What is the right way to make it called/evaluated for each row, apart 
from a 'for' loop ?

Thanks,
Jacques.

ifelse is vectorized but there is no way you could know what's
happening with that command
 because you have rnorm(1) for both conditions. I think you mean to have
something different in one of them ?

Whewn I run your code in my R session, I get 10 values for y1, so there
isn't anything wrong except
That you have the same statement for both cases.




-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of
jropers at freesurf.fr
Sent: Tuesday, December 12, 2006 3:13 PM
To: r-help at stat.math.ethz.ch
Subject: [R] ifelse question

Dear R-helpers,
How come that in the following code the rnorm() function is evaluated
only once for each branch of the 'ifelse' ? 

x <- rnorm(10)
y1 <- ifelse(x > 0, rnorm(1) ,  rnorm(1))


What is the right way to make it called/evaluated for each row, apart
from a 'for' loop ?

Thanks, 

Jacques.

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R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
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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This is not an offer (or solicitation of an offer) to buy/se...{{dropped}}

What is puzzling me is that rnorm(1) is only evaluated *twice*, one time 
for each branch, with only 2 different random deviates, instead of 
giving ten different random deviates. y1 has indeed 10 values but with 
only 2 different ones.
I would like to have rnorm be evaluated for each row and collect ten 
*different* random deviates.

y1
 [1] 0.4087172 0.7707796 0.4087172 0.4087172 0.7707796 0.4087172 0.4087172
 [8] 0.7707796 0.7707796 0.4087172


Thanks.

Jacques.

jropers at freesurf.fr wrote:
> What is puzzling me is that rnorm(1) is only evaluated *twice*, one time 
> for each branch, with only 2 different random deviates, instead of 
> giving ten different random deviates. y1 has indeed 10 values but with 
> only 2 different ones.
>   
I find it more puzzling why you expect that ifelse,  a function of three 
vector arguments, would cause its input arguments to be reevaluated for  
every element of the result.
 
> I would like to have rnorm be evaluated for each row and collect ten 
> *different* random deviates.
>
> y1
>  [1] 0.4087172 0.7707796 0.4087172 0.4087172 0.7707796 0.4087172 0.4087172
>  [8] 0.7707796 0.7707796 0.4087172
>
>
> Thanks.
>
> Jacques.
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

...ifelse,  a function of three **vector** arguments....

Yes !!
I misunderstood the functioning of ifelse.
Thanks
Jacques.


Peter Dalgaard wrote:
> jropers at freesurf.fr wrote:
>> What is puzzling me is that rnorm(1) is only evaluated *twice*, one 
>> time for each branch, with only 2 different random deviates, instead 
>> of giving ten different random deviates. y1 has indeed 10 values but 
>> with only 2 different ones.
>>   
> I find it more puzzling why you expect that ifelse,  a function of 
> three vector arguments, would cause its input arguments to be 
> reevaluated for  every element of the result.
>
>> I would like to have rnorm be evaluated for each row and collect ten 
>> *different* random deviates.
>>
>> y1
>>  [1] 0.4087172 0.7707796 0.4087172 0.4087172 0.7707796 0.4087172 
>> 0.4087172
>>  [8] 0.7707796 0.7707796 0.4087172
>>
>>
>> Thanks.
>>
>> Jacques.
>>
>> ______________________________________________
>> R-help at stat.math.ethz.ch mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide 
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>   
>
>

>>>>> "jropers at freesurf" == jropers at freesurf fr
<jropers at freesurf.Fr>
>>>>>     on Tue, 12 Dec 2006 22:24:33 +0100 writes:
    jropers at freesurf> ...ifelse, a function of three **vector**
    jropers at freesurf> arguments....  Yes !!  I misunderstood the
    jropers at freesurf> functioning of ifelse.  

Seems to happen more an more often.
When I teach "R programming" I nowadays usually emphasize that people
should often *NOT* use ifelse().
In other words, I think ifelse() is much over-used in situations
where something else would be both clearer and more efficient.

Is there a document / book around which lures people into
misusing ifelse() so frequently?

Martin Maechler, ETH Zurich

    jropers at freesurf> Peter Dalgaard wrote:
    >> jropers at freesurf.fr wrote:
    >>> What is puzzling me is that rnorm(1) is only evaluated
    >>> *twice*, one time for each branch, with only 2 different
    >>> random deviates, instead of giving ten different random
    >>> deviates. y1 has indeed 10 values but with only 2
    >>> different ones.
    >>> 
    >> I find it more puzzling why you expect that ifelse, a
    >> function of three vector arguments, would cause its input
    >> arguments to be reevaluated for every element of the
    >> result.