length of object in repeated measures

in this case you can use ave(), e.g., say 'dat' is the name of your data 
frame, then try this:

dat$NoCaps <- ave(dat$Id, dat$Id, FUN = length)
dat


I hope it helps.

Best,
Dimitris


clion wrote:

Hi there.
I collectad data of several animals (Id) that were caught and measured at
several occasions.
The dataframe looks like this:
Grouped Data: mass ~ age | Id
Id      age       mass
1        1           5.4
1        3           6.2
1       15         10.0
2        3           8.1
2       10         12.8
3        2           5.9
3       10          7.1
3       15         15.4
3       17         16.2

Now, I would like to add a column that shows the number of captures per
Animal (so it'll look like this:) 
Id  ....   NoCaps
1   ....     3
1   ....     3
1   ....     3
2   ....     2
2   ....     2
3   ....     4  
3   ....     4
....
 
I understand that with 
tapply(Id,Id, length)
I can find out how many times each animal was caught, but how do I get
this
information into an extra column? I'm sure, this is an easy question, but
I'm lost, where to find the answer, or especially where to look it up. if
this is not a new question, please give me the key words to look for
it...

thanks in advance

-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus Medical Center

Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014