R help - Jun 2008 - help understanding why #function(x, y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})# doesn't work like I think it should

Hello R users and developers,
I am trying to write several functions (fairly new at this) in order
to avoid using loops on large data frames (because they are slow).  I
wrote the function below and it seems to get hung on the first part of
the if statement and then applies that condition to rest of the
function.  So if (x-y) is greater than 0 the function uses the true
statement for the calculations.   Could someone please offer some
advise on how to write these functions a little better or a type
"apply" that I may use with two (or more) different vectors of data
required by a single functions.
#######################  Examples
#######################################################
## example 1 ###
x<-c(5,6,4,3,5,3,1)
y<-c(1,6,2,1,7,1,9)
folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
z<-folds(x,y)
check<-cbind(x,y,z)
View(check)

Warning message:
In if ((x - y) >= 0) { :
  the condition has length > 1 and only the first element will be used
 ### why will it only use the first element and how to I get around
this ####

## example 2 making the fist comparison negative ###
x1<-c(5,6,4,3,5,3,1)
y1<-c(11,6,2,1,7,1,9)
folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
z1<-folds(x1,y1)
check2<-cbind(x1,y1,z1)
View(check2)
Warning message:
In if ((x - y) >= 0) { :
  the condition has length > 1 and only the first element will be used
################################################################################
#### loop I am trying to avoid writing many many times #####
folds2<-NULL
xy<-as.data.frame(cbind(x,y))
for (i in 1:nrow(xy)) {
diff<-xy$x[i]-xy$y[i]
folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
}
xyz<-cbind(xy,folds2)
View(xyz)
#################

Thank you,
Alan

P.S.  why does ?function not work

This is an FAQ (although it doesn't seem to be listed in the official  
FAQs.
The ``if'' command compares scalars (only, in effect).

See ?"if"

You need to use the ifelse() function:

folds <- function(x,y) ifelse(x-y>=0,2^(x-y),-2^(y-x))

This if for illustrative purposes only.  The foregoing would be  
better done as:

	folds <- function(x,y) sign(x-y)*2^abs(x-y)

	cheers,

		Rolf Turner

On 4/06/2008, at 1:47 PM, ALAN SMITH wrote:
> Hello R users and developers,
> I am trying to write several functions (fairly new at this) in order
> to avoid using loops on large data frames (because they are slow).  I
> wrote the function below and it seems to get hung on the first part of
> the if statement and then applies that condition to rest of the
> function.  So if (x-y) is greater than 0 the function uses the true
> statement for the calculations.   Could someone please offer some
> advise on how to write these functions a little better or a type
> "apply" that I may use with two (or more) different vectors of
data
> required by a single functions.
> #######################  Examples
> #######################################################
> ## example 1 ###
> x<-c(5,6,4,3,5,3,1)
> y<-c(1,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z<-folds(x,y)
> check<-cbind(x,y,z)
> View(check)
>
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
>  ### why will it only use the first element and how to I get around
> this ####
>
> ## example 2 making the fist comparison negative ###
> x1<-c(5,6,4,3,5,3,1)
> y1<-c(11,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z1<-folds(x1,y1)
> check2<-cbind(x1,y1,z1)
> View(check2)
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
> ###################################################################### 
> ##########
> #### loop I am trying to avoid writing many many times #####
> folds2<-NULL
> xy<-as.data.frame(cbind(x,y))
> for (i in 1:nrow(xy)) {
> diff<-xy$x[i]-xy$y[i]
> folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
> }
> xyz<-cbind(xy,folds2)
> View(xyz)
> #################
>
> Thank you,
> Alan
>
> P.S.  why does ?function not work
>
> ______________________________________________
> R-help at r-project.org 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.

######################################################################
Attention:\ This e-mail message is privileged and confid...{{dropped:9}}

Use ifelse() rather than if () {} else {}. It's vectorized and very
useful for applying to elements along vectors. The latter idiom is much
better for control of flow in programs and functions.

 folds <- function (x, y) ifelse(x-y >= 0, 2^(x-y), -(2^abs(x-y)))

 z <- folds(x, y)
> z
[1]   16    1    4    4   -4    4 -256
> 
?function works for me. Here's my setup:

R version 2.7.0 (2008-04-22) 
x86_64-pc-linux-gnu 

Cheers,

Simon.


On Tue, 2008-06-03 at 20:47 -0500, ALAN SMITH wrote:
> Hello R users and developers,
> I am trying to write several functions (fairly new at this) in order
> to avoid using loops on large data frames (because they are slow).  I
> wrote the function below and it seems to get hung on the first part of
> the if statement and then applies that condition to rest of the
> function.  So if (x-y) is greater than 0 the function uses the true
> statement for the calculations.   Could someone please offer some
> advise on how to write these functions a little better or a type
> "apply" that I may use with two (or more) different vectors of
data
> required by a single functions.
> #######################  Examples
> #######################################################
> ## example 1 ###
> x<-c(5,6,4,3,5,3,1)
> y<-c(1,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z<-folds(x,y)
> check<-cbind(x,y,z)
> View(check)
> 
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
>  ### why will it only use the first element and how to I get around
> this ####
> 
> ## example 2 making the fist comparison negative ###
> x1<-c(5,6,4,3,5,3,1)
> y1<-c(11,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z1<-folds(x1,y1)
> check2<-cbind(x1,y1,z1)
> View(check2)
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
>
################################################################################
> #### loop I am trying to avoid writing many many times #####
> folds2<-NULL
> xy<-as.data.frame(cbind(x,y))
> for (i in 1:nrow(xy)) {
> diff<-xy$x[i]-xy$y[i]
> folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
> }
> xyz<-cbind(xy,folds2)
> View(xyz)
> #################
> 
> Thank you,
> Alan
> 
> P.S.  why does ?function not work
> 
> ______________________________________________
> R-help at r-project.org 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.
-- 
Simon Blomberg, BSc (Hons), PhD, MAppStat. 
Lecturer and Consultant Statistician 
Faculty of Biological and Chemical Sciences 
The University of Queensland 
St. Lucia Queensland 4072 
Australia
Room 320 Goddard Building (8)
T: +61 7 3365 2506
http://www.uq.edu.au/~uqsblomb
email: S.Blomberg1_at_uq.edu.au

Policies:
1.  I will NOT analyse your data for you.
2.  Your deadline is your problem.

The combination of some data and an aching desire for 
an answer does not ensure that a reasonable answer can 
be extracted from a given body of data. - John Tukey.

Alan -

ALAN SMITH wrote:
> Hello R users and developers,
> I am trying to write several functions (fairly new at this) in order
> to avoid using loops on large data frames (because they are slow).  I
> wrote the function below and it seems to get hung on the first part of
> the if statement and then applies that condition to rest of the
> function.  So if (x-y) is greater than 0 the function uses the true
> statement for the calculations.   Could someone please offer some
> advise on how to write these functions a little better or a type
> "apply" that I may use with two (or more) different vectors of
data
> required by a single functions.
> #######################  Examples
> #######################################################
> ## example 1 ###
> x<-c(5,6,4,3,5,3,1)
> y<-c(1,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z<-folds(x,y)
> check<-cbind(x,y,z)
> View(check)
> 
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
>  ### why will it only use the first element and how to I get around
> this ####
When learning R, you have the power of the interpreter at all times. 
You have defined x and y above to be numeric vectors of length 7 each. 
Try typign x - y at the interpreter, and you'll see you get another 
numeric vector of length 7.  Now try (x - y) >= 0 , and you'll get a 
logical vector of length 7.  However, if does not want a logical vector 
of length 7, it wants a logical vector of length 1, see the 'cond' 
argument of ?if.  However, the ifelse function should do what you want, 
see ?ifelse.

Here's an example using it

folds2 <- function(x, y) {
   ifelse(x - y >= 0, 2^(x - y), -(2^abs(x - y)))
}

folds2(x, y) should give you your answer...

> 
> ## example 2 making the fist comparison negative ###
> x1<-c(5,6,4,3,5,3,1)
> y1<-c(11,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z1<-folds(x1,y1)
> check2<-cbind(x1,y1,z1)
> View(check2)
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
Same idea as above here...

>
################################################################################
> #### loop I am trying to avoid writing many many times #####
> folds2<-NULL
> xy<-as.data.frame(cbind(x,y))
> for (i in 1:nrow(xy)) {
> diff<-xy$x[i]-xy$y[i]
> folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
> }
> xyz<-cbind(xy,folds2)
> View(xyz)
> #################
I'm not exactly sure what you'd like here, but hopefully with the above 
you can figure it out.  Let me know if you have any questions!

Best,
Erik Iverson
> 
> Thank you,
> Alan
> 
> P.S.  why does ?function not work
What do you expect it to do?  It shows the help page for 'function'
here.
> 
> ______________________________________________
> R-help at r-project.org 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.

Hi Alan,
> -----Original Message-----
> From: r-help-bounces at r-project.org on behalf of ALAN SMITH
> Sent: Tue 6/3/2008 6:47 PM
> To: r-help at r-project.org
> Subject: [R] help understanding why #function(x,y) (if((x-y)>=0)
{2^(x-y)} else{-(2^abs(x-y))})# doesn't work likeI think it should
>  
> Hello R users and developers,
> I am trying to write several functions (fairly new at this) in order
> to avoid using loops on large data frames (because they are slow).  I
> wrote the function below and it seems to get hung on the first part of
> the if statement and then applies that condition to rest of the
> function.  So if (x-y) is greater than 0 the function uses the true
> statement for the calculations.   Could someone please offer some
> advise on how to write these functions a little better or a type
> "apply" that I may use with two (or more) different vectors of
data
> required by a single functions.
> #######################  Examples
> #######################################################
> ## example 1 ###
> x<-c(5,6,4,3,5,3,1)
> y<-c(1,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z<-folds(x,y)
> check<-cbind(x,y,z)
> View(check)
> 
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be used
>  ### why will it only use the first element and how to I get around
> this ####

You probably want to use ifelse() instead of if()

folds <- function(x, y) (ifelse( ((x - y) >= 0), 2^(x - y), -(2^abs(x -
y))))

x <- c(5,6,4,3,5,3,1)
y <- c(1,6,2,1,7,1,9)
folds <- function(x, y) (ifelse( ((x - y) >= 0), 2^(x - y), -(2^abs(x -
y))))
z <- folds(x, y)
check <- cbind(x, y, z)
check

R> x <- c(5,6,4,3,5,3,1)
R> y <- c(1,6,2,1,7,1,9)
R> folds <- function(x, y) (ifelse( ((x - y) >= 0), 2^(x - y),
-(2^abs(x - y))))
R> z <- folds(x, y)
R> check <- cbind(x, y, z)
R> check
     x y    z
[1,] 5 1   16
[2,] 6 6    1
[3,] 4 2    4
[4,] 3 1    4
[5,] 5 7   -4
[6,] 3 1    4
[7,] 1 9 -256

                         
> 
> ## example 2 making the fist comparison negative ###
> x1<-c(5,6,4,3,5,3,1)
> y1<-c(11,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z1<-folds(x1,y1)
> check2<-cbind(x1,y1,z1)
> View(check2)
> Warning message:
> In if ((x - y) >= 0) { :
>   the condition has length > 1 and only the first element will be
> used
                         
x1 <- c(5,6,4,3,5,3,1)
y1 <- c(11,6,2,1,7,1,9)
folds <- function(x, y) {ifelse( ((x - y) >= 0), {2^(x - y)}, {-(2^abs(x -
y))} ) }
z1<-folds(x1,y1)
check2<-cbind(x1,y1,z1)
check2
                         
R> x1 <- c(5,6,4,3,5,3,1)
R> y1 <- c(11,6,2,1,7,1,9)
R> folds <- function(x, y) {ifelse( ((x - y) >= 0), {2^(x - y)},
{-(2^abs(x - y))} ) }
R> z1<-folds(x1,y1)
R> check2<-cbind(x1,y1,z1)
R> check2
     x1 y1   z1
[1,]  5 11  -64
[2,]  6  6    1
[3,]  4  2    4
[4,]  3  1    4
[5,]  5  7   -4
[6,]  3  1    4
[7,]  1  9 -256
>
################################################################################
> #### loop I am trying to avoid writing many many times #####
> folds2<-NULL
> xy<-as.data.frame(cbind(x,y))
> for (i in 1:nrow(xy)) {
> diff<-xy$x[i]-xy$y[i]
> folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
> }
> xyz<-cbind(xy,folds2)
> View(xyz)
> #################
> 
> Thank you,
> Alan
> 
> P.S.  why does ?function not work
It does work in my ESS R session buffer, so
it depends on your setup.

try
R> ?"function"

?function

might not work as the R parser doesn't think you have 
finished typing in the expression (function objects
are typically terminated by the final '}' brace).  
> 
> ______________________________________________
> R-help at r-project.org 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.
> 
> 
Best

Steve McKinney

Try this:

data.frame(x, y, folds = (x == y) + sign(x - y) * 2 ^ abs(x - y))

or replace data.frame with cbind if you prefer a matrix result.

On Tue, Jun 3, 2008 at 9:47 PM, ALAN SMITH <alansmith2 at gmail.com>
wrote:
> Hello R users and developers,
> I am trying to write several functions (fairly new at this) in order
> to avoid using loops on large data frames (because they are slow).  I
> wrote the function below and it seems to get hung on the first part of
> the if statement and then applies that condition to rest of the
> function.  So if (x-y) is greater than 0 the function uses the true
> statement for the calculations.   Could someone please offer some
> advise on how to write these functions a little better or a type
> "apply" that I may use with two (or more) different vectors of
data
> required by a single functions.
> #######################  Examples
> #######################################################
> ## example 1 ###
> x<-c(5,6,4,3,5,3,1)
> y<-c(1,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z<-folds(x,y)
> check<-cbind(x,y,z)
> View(check)
>
> Warning message:
> In if ((x - y) >= 0) { :
>  the condition has length > 1 and only the first element will be used
>  ### why will it only use the first element and how to I get around
> this ####
>
> ## example 2 making the fist comparison negative ###
> x1<-c(5,6,4,3,5,3,1)
> y1<-c(11,6,2,1,7,1,9)
> folds<-function(x,y) (if((x-y)>=0) {2^(x-y)} else{-(2^abs(x-y))})
> z1<-folds(x1,y1)
> check2<-cbind(x1,y1,z1)
> View(check2)
> Warning message:
> In if ((x - y) >= 0) { :
>  the condition has length > 1 and only the first element will be used
>
################################################################################
> #### loop I am trying to avoid writing many many times #####
> folds2<-NULL
> xy<-as.data.frame(cbind(x,y))
> for (i in 1:nrow(xy)) {
> diff<-xy$x[i]-xy$y[i]
> folds2[i]<-if(diff>=0) {2^diff} else{-(2^abs(diff))}
> }
> xyz<-cbind(xy,folds2)
> View(xyz)
> #################
>
> Thank you,
> Alan
>
> P.S.  why does ?function not work
>
> ______________________________________________
> R-help at r-project.org 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.
>