# [R] Re-assigning vector elements based on their initial values.

Petr Pikal petr.pikal at precheza.cz
Mon Feb 3 13:05:04 CET 2003

```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

```