[R] standardize columns selectively within a dataframe
David Winsemius
dwinsemius at comcast.net
Wed Sep 1 18:42:18 CEST 2010
On Sep 1, 2010, at 10:42 AM, David Winsemius wrote:
>
> On Sep 1, 2010, at 10:35 AM, Olga Lyashevska wrote:
>
>> Dear all,
>>
>> I have a dataframe:
>> df<-dataframe(a=c(1,2,3),b=c(4,5,6),c=c(7,8,9),d=c(10,11,12))
>>
>> I want to obtain a new dataframe with columns a and b being
>> standardized
>> ((x-mean(x))/sd(x)); the other two columns (c,d) I want to leave
>> unchanged. What is the best way to achieve this? I have been trying
>> to
>> use subscripts but did not succeed so far.
>
> > df[ , 1:2] <- scale(df[ , 1:2])
> > df
> a b c d
> 1 -1 -1 7 10
> 2 0 0 8 11
> 3 1 1 9 12
I suspect you might have tried (df-mean(df))/sd(x) and gotten
unsatisfactory results; I know I did. If you had really wanted to
persist and do it from first principles, so to speak, or perhaps as
"homework", then consider the sweep operation. It takes an object of
lower dimension and applies a function, ("-") by default, with the
third argument repeatedly across the specified (in the second
argument) dimension. You wanted to work on columns, so this would
accomplish the subtraction of means() followed by division by sd():
> sweep(as.matrix(df[ , 1:2]), 2L, colMeans(mm)) # using the default
"-" operator
a b
[1,] -1 -1
[2,] 0 0
[3,] 1 1
> sweep(sweep(df[ , 1:2], 2L, colMeans(mm)), 2, sd(mm), "/")
a b
1 -1 -1
2 0 0
3 1 1
(Your test columns happened to be scaled already and only needed to be
centered. This is how scale() does its work, and their help pages have
links cross-referencing each other.)
This is probably a good time to reference Burns', The R Inferno, which
has an entry for sweep (p 57) as well tips regarding the drop=FALSE
maneuver (p 54) that I tried first for this problem but it "didn't
work".
--
David Winsemius, MD
West Hartford, CT
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