[R] extracting a percentage of data by random

[Bill.Venables at csiro.au]

You don't need any explicit loops at all.  Here is a demo of one way to
do it:

> set.seed(23)  # on Windows
> dat <- data.frame(age = factor(sample(1:4, 200, rep = T)), y =
runif(200))
> head(dat)	 # ages are in random order 
  age          y
1   3 0.64275524
2   1 0.56125314
3   2 0.82418228
4   3 0.97050933
5   4 0.02827508
6   2 0.72291636
> with(dat, table(age))   # how many in each age group
age
 1  2  3  4 
37 55 44 64 
> ind <- lapply(split(1:nrow(dat), dat$age),
              function(x) sample(x, round(length(x)/10))) # the trick
> ind
$`1`
[1] 135   2 188 133

$`2`
[1] 124  33 140 162  25  13

$`3`
[1] 115  79  27  44

$`4`
[1]  58 129  84 198  72 109

> sample_dat <- dat[sort(unlist(ind)), ]  # with indices, select data
> sample_dat
    age         y
2     1 0.5612531
13    2 0.7339141
25    2 0.9548750
27    3 0.7419931
33    2 0.6965722
44    3 0.5363812
58    4 0.5464051
72    4 0.2785669
79    3 0.6453164
84    4 0.1203811
109   4 0.9154706
115   3 0.2118767
124   2 0.3056171
129   4 0.7635097
133   1 0.6474702
135   1 0.2466226
140   2 0.6292326
162   2 0.5338671
188   1 0.9882631
198   4 0.1983350
>  


Bill Venables
CSIRO Laboratories
PO Box 120, Cleveland, 4163
AUSTRALIA
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mailto:Bill.Venables at csiro.au
http://www.cmis.csiro.au/bill.venables/ 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Chang Liu
Sent: Thursday, 6 March 2008 10:50 AM
To: r-help at r-project.org
Subject: [R] extracting a percentage of data by random


Hello Gurus:
 
If I have a dataframe with one of the variables called "age" for
example, and I want to extract a random 10% of the observations from
each "age" group of the entire data frame. Do I have to double loop to
split the data and then loop again to assign random numbers? Or is there
a better way to do this?
 
Thanks!
Karen
 
 
 
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