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

[R. Michael Weylandt]

# Changing to variable Z since array() is a function
apply(Z.temp <- Z[,,,a:b],1:3,sum)/dim(Z.temp)[4]
# Should work, though it may be more clear to define Z.temp in its own line

M

On Wed, Oct 5, 2011 at 5:10 PM, Martin Batholdy <batholdy at googlemail.com> wrote:
> 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!
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>