do calculations as defined by a string / expand mathematical statements in R

Thanks for all the suggestions!



Perhaps my post was not clear enough.

apply(array,1:2,sum)/dim(array)[3]

and

# reproducible example
x <- 1:1000
dim(x)<-rep(10,3)
# code
apply(x,1:2,sum)


would give me the mean over one whole dimension, right?
The problem with that is, that I just want to calculate the mean over a subset of t (where t is the 4th dimension of the array).
And the range of this subset should be easily changeable.


So for example I have 4D array:

x <- 1:10000
dim(x)<-rep(10,4)

Now I would like to average the 3D array(x,y,z) in the 4th dimension (t) from t_start = a to t_end = b.
I don't want to average the whole 3D array.



On 05.10.2011, at 22:21, William Dunlap wrote:

Avoid parsing strings to make expressions. ?It is easy
to do, but hard to do safely and readably.

In your case you could make a short loop out of it
? result <- x[,,,1]
? for(i in seq_len(dim(x)[4])[-1]) {
? ? ?result <- result + x[,,,i]
? }
? result <- result / dim(x)[4]

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

Wouldn't that be much slower than define a string and evaluate it as an expression since I would have to use a for-loop?




thanks again!
You helped me a lot today ;)





On 05.10.2011, at 22:21, William Dunlap wrote:

Avoid parsing strings to make expressions. ?It is easy
to do, but hard to do safely and readably.

In your case you could make a short loop out of it
? result <- x[,,,1]
? for(i in seq_len(dim(x)[4])[-1]) {
? ? ?result <- result + x[,,,i]
? }
? result <- result / dim(x)[4]

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Martin Batholdy
Sent: Wednesday, October 05, 2011 1:14 PM
To: R Help
Subject: [R] do calculations as defined by a string / expand mathematical statements in R

Dear R-group,


is there a way to perform calculations that are defined in a string format?


for example I have different variables:

x1 <- 3
x2 <- 1
x4 <- 1

and a string-variable:

do <- 'x1 + x2 + x3'


Is there any way to perform what the variable 'do'-describes
(just like the formula-element but more elemental)?



Perhaps my idea to solve my problem is a little bit strange.


My general problem is, that I have to do arithmetics for which there seems to be no function available
that I can apply in order to be more flexible.


To be precise, I have to add up three dimensional arrays.

I can do that like this (as someone suggested on this help-list - thanks for that!):

(array[,,1] + array[,,2] + array[,,3]) / 3


However in my case it can happen that at some point, I don't have to add 3 but 8 'array-slices'
(or 10 or x).

And I don't want to manually expand the above statement to:

(array[,,1] + array[,,2] + array[,,3] + array[,,4] + array[,,5] + array[,,6] + array[,,7] +
array[,,8]) / 8

(ok, now I have done it ;)



So, my thinking was that I can easily expand and change a string (with the paste-function / repeat-
function etc.).
But how can I expand a mathematical statement?


thanks for any suggestions!

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