R help - Feb 2003 - Re-assigning vector elements based on their initial values.

Is there an eloquent solution to re-assign vector element values?
I have a vector which contains chemical data, some of them are
"flagged" as
non-detected values by their negative values.
I can find the statistics on the positive values in vector "v"  simply
by
typing:
>v<- c(5,5,-3,-3,7,8,10)
> v[(v>0)]
[1] 5  5  7  8  10

I can also convert to positive values by
>asb(v)
[1]  5  5  3  3  7  9  10

I then can do mean() and var() etc.

But when I want to come up with a way to change a negative value to a
postive 1/2 value I couldn't come up with a
nice way using "vectoral syntax" to do it.  In the method I came up
with , I
discovered that I have to pre-define a new vector "v.new" or I get an
error.
>v.new <-v
then I have to loop through all of the elements!
>for(i in 1:length(v)){ if (v[i] < 0) v.new[i] <-  -1*v[i]/2 else v[i]
<-
v[i]}
> v.new
[1]  5.0  5.0  -1.5  -1.5  7.0  9.0  10.0

This works but seems labored given the previous crisp vector operations.


REX

Dear Rex,

You can use if.else, which is vectorized:

         > v.new <- ifelse(v < 0, -v/2, v)
         > v.new
         [1]  5.0  5.0  1.5  1.5  7.0  8.0 10.0

(By the way, the solution that you suggested using a loop works, but for 
some reason the sign is wrong in the result that you printed; notice that 
your "else" clause does nothing -- you probably meant to assign to
v.new
not v -- and is unnecessary in any event.)

I hope that this helps,
  John



At 07:38 PM 1/31/2003 -0700, Rex_Bryan at urscorp.com
wrote:
>Is there an eloquent solution to re-assign vector element values?
>I have a vector which contains chemical data, some of them are
"flagged" as
>non-detected values by their negative values.
>I can find the statistics on the positive values in vector "v" 
simply by
>typing:
> >v<- c(5,5,-3,-3,7,8,10)
>
> > v[(v>0)]
>[1] 5  5  7  8  10
>
>I can also convert to positive values by
> >asb(v)
>[1]  5  5  3  3  7  9  10
>
>I then can do mean() and var() etc.
>
>But when I want to come up with a way to change a negative value to a
>postive 1/2 value I couldn't come up with a
>nice way using "vectoral syntax" to do it.  In the method I came
up with , I
>discovered that I have to pre-define a new vector "v.new" or I get
an error.
>
> >v.new <-v
>
>then I have to loop through all of the elements!
>
> >for(i in 1:length(v)){ if (v[i] < 0) v.new[i] <-  -1*v[i]/2 else
v[i] <-
>v[i]}
> > v.new
>[1]  5.0  5.0  -1.5  -1.5  7.0  9.0  10.0
>
>This works but seems labored given the previous crisp vector operations.
-----------------------------------------------------
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
-----------------------------------------------------

Hi

On 1 Feb 2003 at 7:17, John Fox wrote:
> Dear Rex,
> 
> You can use if.else, which is vectorized:
> 
>          > v.new <- ifelse(v < 0, -v/2, v)
>          > v.new
>          [1]  5.0  5.0  1.5  1.5  7.0  8.0 10.0
or simply

abs(v/((v<0)+1))

it should be a little bit quicker

> 
> (By the way, the solution that you suggested using a loop works, but
> for some reason the sign is wrong in the result that you printed;
> notice that your "else" clause does nothing -- you probably meant
to
> assign to v.new not v -- and is unnecessary in any event.)
> 
> I hope that this helps,
>   John
> 
> 
> 
> At 07:38 PM 1/31/2003 -0700, Rex_Bryan at urscorp.com wrote:
> >Is there an eloquent solution to re-assign vector element values? I
> >have a vector which contains chemical data, some of them are
> >"flagged" as non-detected values by their negative values. I
can find
> >the statistics on the positive values in vector "v"  simply
by
> >typing:
> > >v<- c(5,5,-3,-3,7,8,10)
> >
> > > v[(v>0)]
> >[1] 5  5  7  8  10
> >
> >I can also convert to positive values by
> > >asb(v)
> >[1]  5  5  3  3  7  9  10
> >
> >I then can do mean() and var() etc.
> >
> >But when I want to come up with a way to change a negative value to a
> >postive 1/2 value I couldn't come up with a nice way using
"vectoral
> >syntax" to do it.  In the method I came up with , I discovered
that I
> >have to pre-define a new vector "v.new" or I get an error.
> >
> > >v.new <-v
> >
> >then I have to loop through all of the elements!
> >
> > >for(i in 1:length(v)){ if (v[i] < 0) v.new[i] <-  -1*v[i]/2
else
> > >v[i] <-
> >v[i]}
> > > v.new
> >[1]  5.0  5.0  -1.5  -1.5  7.0  9.0  10.0
> >
> >This works but seems labored given the previous crisp vector
> >operations.
> 
> -----------------------------------------------------
> 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
> -----------------------------------------------------
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> http://www.stat.math.ethz.ch/mailman/listinfo/r-help
Cheers
Petr
petr.pikal at precheza.cz
p.pik at volny.cz