[R] ifelse statement

[Duncan Murdoch]

On 07/07/2010 7:32 PM, Gabor Grothendieck wrote:
> On Wed, Jul 7, 2010 at 7:22 PM, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:
>   
>> On 07/07/2010 5:58 PM, karena wrote:
>>     
>>> Hi, I am a newbie of R, and playing with the "ifelse" statement.
>>>
>>> I have the following codes:
>>> ## first,
>>>
>>> for(i in 1:3) {
>>> for(j in 2:4) {
>>> cor.temp <- cor(iris.allnum[,i], iris.allnum[,j])
>>> if(i==1 & j==2) corr.iris <- cor.temp
>>> else corr.iris <- c(corr.iris, cor.temp)
>>> }
>>> }
>>>
>>> this code is working fine.
>>>
>>> I also tried to perform the same thing in another way with "ifelse":
>>> for(i in 1:3) {
>>> for(j in 2:4) {
>>> cor.temp <- cor(iris.allnum[,i], iris.allnum[,j])
>>> corr.iris <- ifelse(i==1 & j==2, cor.temp, c(corr.iris, cor.temp))
>>> }
>>> }
>>>
>>> This is not working. Seems the value of "c(corr.iris, cor.temp)" has not
>>> been assigned to corr.iris, even  when the (i==1 & j==2) is not satisfied.
>>>
>>> what's the problem here?
>>>       
>> See ?ifelse.  It computes something the same shape as the test object.  In
>> your case the test is the result of
>>
>> i==1 & j==2
>>
>> and is a scalar that is either TRUE or FALSE, so the result of ifelse() will
>> be a scalar too.
>>
>> To do what you want in one line, you can use
>>
>> corr.iris <- if (i==1 && j==2) cor.temp else c(corr.iris, cor.temp)
>>
>> but to most people this looks unnatural, and your original code is what I'd
>> recommend using.  In this case it makes no difference whether you use & or
>> && in the test, but in other cases only && makes sense with if, and only &
>> makes sense with ifelse().
>>     
>
> Just to quibble I find the
>
>    corr.iris <- if ...
>
> construct easier to understand than
>
>    if (...) corr.iris <- ... else corr.iris <- ...
>
> because in the first case you immediately see that the purpose of the
> construct is to set corr.iris whereas setting it separately in each
> leg requires that you must examine more code, i.e. both legs, to make
> such a determination adding to the mental load.
>   

Well, I did say "most people", not "all people".  The reason I think 
most people prefer the separate statement is that they don't realize 
that "if (test) value1 else value2" is just a different way to write the 
function call "`if`(test, value1, value2)", they think of it as a 
flow-of-control statement, as it is in languages like C.

Duncan Murdoch