[R] formula substitution by its calculated value

[Gabor Grothendieck]

Try this:

> a <- 1
> b <- 2
> mat <- matrix(c("a", "a*b", "a+b", "b"), 2)
> mat
     [,1]  [,2]
[1,] "a"   "a+b"
[2,] "a*b" "b"
> matrix(sapply(mat, function(x) eval(parse(text = x))), nrow(mat))
     [,1] [,2]
[1,]    1    3
[2,]    2    2


On Nov 30, 2007 10:06 AM, BOISSON, Pascal <Pascal.BOISSON at veolia.com> wrote:
> Dear R_Users,
>
> I am confronted to a technical issue that I wouldn't even know how to start to handle and where to look at. It is about data manipulation in a dataframe and value substitution
> Imagine I have a matrix describing a set of chemical equations.
> Colnames are Components names and Rownames are Reaction names
>
> Each cell in that matrix corresponds to the stoechiometric coefficient to the pair (Reaction, Component). It would be easy to deal with this if these cells where filled with numbers (int or real) but often, it refers to some parameters name or even to formulae.
>
> I'd like to transform these formulae into their calculated values.
> eg:
> if I have existing R objects :
> f_ox <- 0.2
> f_inox <- 0.5
>
> and in my matrix :
> cell (Oxydation, substrate) = f_ox*(1-f_inox)
>
> I want to replace f_ox*(1-f_inox) by 0.1
>
> Further more, I can not manage to understand the formula type, which I guess would correspond to the kind of things I am dealing with.
>
> Would you have any clue or insights on this?
>
> Pascal Boisson
>
> Assainissement - Traitement Biologique
> Veolia Environnement - Centre de Recherche sur l'Eau
> Anjou Recherche
> Chemin de la Digue - BP 76
> 78603 Maisons Laffitte Cedex
> Tel. : +33 (0)1 34 93 81 44
> Fax. : +33 (0)1 34 93 31 10
> pascal.boisson at veolia.com
>
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