[R] For loop by factor.

[Bill.Venables at csiro.au]

If I understand you correctly, you are trying to find the cumulative maximum from the end within each level of the factor.  If this is what you are trying to do, then here is one way you might like to do it.  First, define the function:

> cumMax <- function(x) Reduce(max, x, right = TRUE, accumulate = TRUE)

Here is how you might use it:

> test
  A B
1 a 3
2 a 2
3 a 1
4 b 3
5 b 2
6 c 2
7 c 3
8 c 1
9 c 1
> (test <- within(test, B <- ave(B, A, FUN = cumMax))
  A B
1 a 3
2 a 2
3 a 1
4 b 3
5 b 2
6 c 3
7 c 3
8 c 1
9 c 1 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Christopher Peters
Sent: Monday, 20 June 2011 6:21 AM
To: r-help at r-project.org
Subject: [R] For loop by factor.

I have a data.frame as follows:

a  3
a  2
a  1
b  3
b  2
c  2
c  3
c  1
c  1

Each factor (a, b, c) should be monotonically decreasing, notice that factor
'c' is not.

I could use some help to figure out how to form a logical structure (mostly
just syntax), that will check each 'next value' for each factor to see if it
is less than the previous value.  If it is less than the previous value, do
nothing, else subtract 'next value' from 'current value', add that amount to
the starting value and each previous value to the 'next value' is greater
than 'previous value'.

So basically the data.frame would look like:

a 3
a 2
a 1
b 3
b 2
c 3
c 3
c 1
c 1

Thanks for your help!
__________________________________
Christopher P. Peters
Research Associate
Center for Energy Studies
http://www.enrg.lsu.edu/staff/peters
Energy, Coast & Environment Building
Louisiana State University
Baton Rouge, LA 70803
Telephone: 225-578-4400
Fax: 225-578-4541

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