dataset - How to create a new vector from grouped data in R? -

possible duplicate on so: computing new variable using conditions of existing data frame

i trying create vector (e.g. z) group means. vector have length equal number of rows of original data frame group mean created , each element of vector z correspond group member.

for example,

a      mean of group c      mean of group c f      mean of group f c      mean of group c 

etc.

any appreciated.

sounds need function ave

try looking at

?ave 

e.g. try

ave(insectsprays$count,insectsprays$spray) 

(insectsprays comes r, should work stands) or

with(insectsprays, ave(count,spray)) 

so equivalent data , assign z

here's (try pasting first 2 lines in , examining insp):

 insp <- insectsprays  insp$ave <- with(insp, ave(count,spray))  head(insp,15)    count spray      ave 1     10     14.50000 2      7     14.50000 3     20     14.50000 4     14     14.50000 5     14     14.50000 6     12     14.50000 7     10     14.50000 8     23     14.50000 9     17     14.50000 10    20     14.50000 11    14     14.50000 12    13     14.50000 13    11     b 15.33333 14    17     b 15.33333 15    21     b 15.33333 

the function ave has many other uses because can supply function argument (and instead of calculating means use whatever other function group wish); particularly subtle use of it, see this answer.