R help - Feb 2003 - to modify a vector

Hi All.

I am quite a newbie to R.
This is a basic question.

I like to modify elements of a vector.
For Example:
a1 <- c(1,2,3,4,3,5)

TThe following program sentence does not work but the intention is;
  
  if (a1==3) a1*3 .

3 in the vector should be changed to 9, and
the resulted vector is (1,2,9,4,9,5).

How can I get the result?

Thanks in advance for help.
-------========--------
mitsu5
mitsu5 at ruby.famille.ne.jp

On Saturday 08 February 2003 12:41 am, Mitsuo Igarashi
wrote:
> Hi All.
>
> I am quite a newbie to R.
> This is a basic question.
>
> I like to modify elements of a vector.
> For Example:
> a1 <- c(1,2,3,4,3,5)
>
> TThe following program sentence does not work but the intention is;
>
>   if (a1==3) a1*3 .
>
> 3 in the vector should be changed to 9, and
> the resulted vector is (1,2,9,4,9,5).
>
> How can I get the result?
if you want to store the result in a1 itself,

a1[a1==3] <- a1[a1==3] * 3

Alternately, 

ifelse(a1 == 3, a1 * 3, a1)

On 8 Feb 2003 at 15:41, Mitsuo Igarashi wrote:

You should realy read "An Introduction to R", which comes
with your R installation.
> Hi All.
> 
> I am quite a newbie to R.
> This is a basic question.
> 
> I like to modify elements of a vector.
> For Example:
> a1 <- c(1,2,3,4,3,5)
> 
> TThe following program sentence does not work but the intention is;
>   
>   if (a1==3) a1*3 .
> 
> 3 in the vector should be changed to 9, and
> the resulted vector is (1,2,9,4,9,5).
> 
a1[a1==3] <- 9

Kjetil Halvorsen
> How can I get the result?
> 
> Thanks in advance for help.
> -------========--------
> mitsu5
> mitsu5 at ruby.famille.ne.jp
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> http://www.stat.math.ethz.ch/mailman/listinfo/r-help

Hi, if I understand you correctly, you would like to change the value of
all elements with current value of 3 to 9. This is how you do it:

   a1 <- c(1,2,3,4,3,5)

   idx <- (a1 == 3)
   a1[idx] <- 9

or in one line

   a1[a1 == 3] <- 9

Look at 'idx' above. It is a logical vector of the same length as
'a1',
i.e.

   idx:   FALSE FALSE  TRUE FALSE  TRUE FALSE

When you then do 'a1[idx]' you select only those elements for which idx
is TRUE, here it is the third and the fifth. 'a1[idx] <- 9' selects
those elements and then assign the value 9 to them. Details: You could
also have done

   idx <- which(a1 == 3)

where 'idx' then becomes equal to c(3,5), giving 'a1[c(3,5)] <-
9',
which gives identical results (of course).

Hope this helps

Henrik Bengtsson

> -----Original Message-----
> From: r-help-admin at stat.math.ethz.ch 
> [mailto:r-help-admin at stat.math.ethz.ch] On Behalf Of Mitsuo Igarashi
> Sent: den 8 februari 2003 17:42
> To: r-help at stat.math.ethz.ch
> Subject: [R] to modify a vector
> 
> 
> Hi All.
> 
> I am quite a newbie to R.
> This is a basic question.
> 
> I like to modify elements of a vector.
> For Example:
> a1 <- c(1,2,3,4,3,5)
> 
> TThe following program sentence does not work but the intention is;
>   
>   if (a1==3) a1*3 .
> 
> 3 in the vector should be changed to 9, and
> the resulted vector is (1,2,9,4,9,5).
> 
> How can I get the result?
> 
> Thanks in advance for help.
> -------========--------
> mitsu5
> mitsu5 at ruby.famille.ne.jp
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list 
> http://www.stat.math.ethz.ch/mailman/listinfo/> r-help
> 
>