R help - Sep 2002 - how to use if statement in function correctly

Dear all

I try to persuade if statement in my function to work as I want (but I am not
very
successful:(

My question is why my function with if statement is evaluated correctly in case
of
atomic variables and incorrectly in case of vector variables.

Here is an example:

#function with if statement

fff <- function(otac,sklon)  
{
test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
if(test<65)
{otac/sklon * 141.76}
else
{otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon}
}
> fff(20,70)
[1] 40.50286
# correct
> fff(50,70)
[1] 80.42371
#correct
> fff(seq(20,50,5),70)
[1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
#wrong
-----------------------------------------------------------------------------------------------
It does not work because if statement is obviously evaluated as TRUE in all 
instances and computes wrongly some items in vector according to the first part 
of the fff function

# see direct computation
> seq(20,50,5)/70*141.76
[1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
> seq(20,50,5) * 3.69683+286.6*(1/70)-0.03100345*seq(20,50,5)*70
[1] 34.62606 42.25900 49.89194 57.52488 65.15783 72.79077 80.42371

On the other hand ifelse works
******************************
fff <- function(otac,sklon)  
{
test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
ifelse(test<65, (otac/sklon * 141.76),  (otac * 3.69683+286.6*(1/sklon)-
0.03100345*otac*sklon))
}
> fff(seq(20,50,5),70)
[1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371
#correct

Can anybody help?

Thank you in advance
Petr Pikal
petr.pikal at precheza.cz
p.pik at volny.cz


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Dear Petr,

The test in if isn't vectorized: When the condition is a vector, the first 
element controls the result. That's why ifelse exists -- to provide a 
vectorized conditional.

John

At 11:46 AM 9/19/2002 +0200, Petr Pikal wrote:
>Dear all
>
>I try to persuade if statement in my function to work as I want (but I am 
>not very
>successful:(
>
>My question is why my function with if statement is evaluated correctly in 
>case of
>atomic variables and incorrectly in case of vector variables.
>
>Here is an example:
>
>#function with if statement
>
>fff <- function(otac,sklon)
>{
>test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
>if(test<65)
>{otac/sklon * 141.76}
>else
>{otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon}
>}
>
> > fff(20,70)
>[1] 40.50286
># correct
>
> > fff(50,70)
>[1] 80.42371
>#correct
>
> > fff(seq(20,50,5),70)
>[1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
>#wrong
>-----------------------------------------------------------------------------------------------
>It does not work because if statement is obviously evaluated as TRUE in all
>instances and computes wrongly some items in vector according to the first 
>part
>of the fff function
>
># see direct computation
> > seq(20,50,5)/70*141.76
>[1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
>
> > seq(20,50,5) * 3.69683+286.6*(1/70)-0.03100345*seq(20,50,5)*70
>[1] 34.62606 42.25900 49.89194 57.52488 65.15783 72.79077 80.42371
>
>On the other hand ifelse works
>******************************
>fff <- function(otac,sklon)
>{
>test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
>ifelse(test<65, (otac/sklon * 141.76),  (otac * 3.69683+286.6*(1/sklon)-
>0.03100345*otac*sklon))
>}
>
> > fff(seq(20,50,5),70)
>[1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371
>#correct
>
>Can anybody help?
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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You need the vectorized version of if() ... else..., which is ifelse().
Try:

fff <- function(otac,sklon)  
{
  test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
  ifelse(test<65, otac/sklon * 141.76,
         otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
}

which gives:
> fff(seq(20,50,5),70)
[1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371

Andy
> From: Petr Pikal [mailto:petr.pikal at precheza.cz]
> 
> Dear all
> 
> I try to persuade if statement in my function to work as I 
> want (but I am not very 
> successful:(
> 
> My question is why my function with if statement is evaluated 
> correctly in case of 
> atomic variables and incorrectly in case of vector variables.
> 
> Here is an example:
> 
> #function with if statement
> 
> fff <- function(otac,sklon)  
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> if(test<65)
> {otac/sklon * 141.76}
> else
> {otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon}
> }
> 
> > fff(20,70)
> [1] 40.50286
> # correct
> 
> > fff(50,70)
> [1] 80.42371
> #correct
> 
> > fff(seq(20,50,5),70)
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  
> 91.13143 101.25714
> #wrong
> --------------------------------------------------------------
> ---------------------------------
> It does not work because if statement is obviously evaluated 
> as TRUE in all 
> instances and computes wrongly some items in vector according 
> to the first part 
> of the fff function
> 
> # see direct computation
> > seq(20,50,5)/70*141.76
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  
> 91.13143 101.25714
> 
> > seq(20,50,5) * 3.69683+286.6*(1/70)-0.03100345*seq(20,50,5)*70
> [1] 34.62606 42.25900 49.89194 57.52488 65.15783 72.79077 80.42371
> 
> On the other hand ifelse works
> ******************************
> fff <- function(otac,sklon)  
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> ifelse(test<65, (otac/sklon * 141.76),  (otac * 
> 3.69683+286.6*(1/sklon)-
> 0.03100345*otac*sklon))
> }
> 
> > fff(seq(20,50,5),70)
> [1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371
> #correct
> 
> Can anybody help?
> 
> Thank you in advance
> Petr Pikal
> petr.pikal at precheza.cz
> p.pik at volny.cz
> 
> 
> -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
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> _._._._._._._._._
> 

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Petr Pikal wrote:
> 
> Dear all
> 
> I try to persuade if statement in my function to work as I want (but I am
not very
> successful:(
> 
> My question is why my function with if statement is evaluated correctly in
case of
> atomic variables and incorrectly in case of vector variables.
> 
> Here is an example:
> 
> #function with if statement
> 
> fff <- function(otac,sklon)
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> if(test<65)
> {otac/sklon * 141.76}
> else
> {otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon}
> }
> 
> > fff(20,70)
> [1] 40.50286
> # correct
> 
> > fff(50,70)
> [1] 80.42371
> #correct
> 
> > fff(seq(20,50,5),70)
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
> #wrong
>
-----------------------------------------------------------------------------------------------
> It does not work because if statement is obviously evaluated as TRUE in all
> instances and computes wrongly some items in vector according to the first
part
> of the fff function
> 
> # see direct computation
> > seq(20,50,5)/70*141.76
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
> 
> > seq(20,50,5) * 3.69683+286.6*(1/70)-0.03100345*seq(20,50,5)*70
> [1] 34.62606 42.25900 49.89194 57.52488 65.15783 72.79077 80.42371
> 
> On the other hand ifelse works
> ******************************
> fff <- function(otac,sklon)
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> ifelse(test<65, (otac/sklon * 141.76),  (otac * 3.69683+286.6*(1/sklon)-
> 0.03100345*otac*sklon))
> }
> 
> > fff(seq(20,50,5),70)
> [1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371
> #correct
> 
> Can anybody help?
For the if(){} else{} behaviour see the manual "R Language
Definition",
section 3.2.1:

if(stat1){stat2} else{stat3} is expected to use only  eval(stat1)[1]  to
decide whether  stat2  or  stat3  should be evaluated next, while
ifelse() is expected to work vectorized.

Uwe Ligges
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Thanks to all for prompt responses (all of them same or similar). 

The difference is between vectorised 
ifelse() form
and non vectorised
if() form of statement.

Therefore the problem was on my improper usage if instead of 
ifelse.

Thanks again
Petr Pikal
petr.pikal at precheza.cz
p.pik at volny.cz


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if is not vectorized, it is for control flow in functions. Called with a
vector of truth values , it only chooses  "if" part or
"else" part
according to first value in vectos:
> x <- -1:1
> x
[1] -1  0  1
> if (x < 0) "A" else "B"
[1] "A"


For vectorized computations, you need ifelse, as you have found out.

I be,.leave all this is in the help pages.

Kjetil Halvorsen


Petr Pikal wrote:
> 
> Dear all
> 
> I try to persuade if statement in my function to work as I want (but I am
not very
> successful:(
> 
> My question is why my function with if statement is evaluated correctly in
case of
> atomic variables and incorrectly in case of vector variables.
> 
> Here is an example:
> 
> #function with if statement
> 
> fff <- function(otac,sklon)
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> if(test<65)
> {otac/sklon * 141.76}
> else
> {otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon}
> }
> 
> > fff(20,70)
> [1] 40.50286
> # correct
> 
> > fff(50,70)
> [1] 80.42371
> #correct
> 
> > fff(seq(20,50,5),70)
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
> #wrong
>
-----------------------------------------------------------------------------------------------
> It does not work because if statement is obviously evaluated as TRUE in all
> instances and computes wrongly some items in vector according to the first
part
> of the fff function
> 
> # see direct computation
> > seq(20,50,5)/70*141.76
> [1]  40.50286  50.62857  60.75429  70.88000  81.00571  91.13143 101.25714
> 
> > seq(20,50,5) * 3.69683+286.6*(1/70)-0.03100345*seq(20,50,5)*70
> [1] 34.62606 42.25900 49.89194 57.52488 65.15783 72.79077 80.42371
> 
> On the other hand ifelse works
> ******************************
> fff <- function(otac,sklon)
> {
> test <- (otac * 3.69683+286.6*(1/sklon)-0.03100345*otac*sklon)
> ifelse(test<65, (otac/sklon * 141.76),  (otac * 3.69683+286.6*(1/sklon)-
> 0.03100345*otac*sklon))
> }
> 
> > fff(seq(20,50,5),70)
> [1] 40.50286 50.62857 60.75429 70.88000 65.15783 72.79077 80.42371
> #correct
> 
> Can anybody help?
> 
> Thank you in advance
> Petr Pikal
> petr.pikal at precheza.cz
> p.pik at volny.cz
> 
>
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> (in the "body", not the subject !)  To: r-help-request at
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>
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