select dataframes from a list based on column values

Question

This question has been adapted from a previous different one.

Using R, I am trying to create a new list of dataframes by selecting dataframes from an existing list only when a particular value combination exists in one of the columns. Let me explain the first steps which work fine. This is my original data in a dataframe called df:

                              Taxon      C     N    func.group  trophic.grp
1  Chrysomelidae.Phylotreta.exclamationis -30.23  5.06     grazer   herbivore
2        Chrysomelidae.Neocrepidodera.sp. -27.29  5.55     grazer   herbivore
3        Chrysomelidae.Neocrepidodera.sp. -27.84  5.54     grazer   herbivore
4        Chrysomelidae.Neocrepidodera.sp. -27.69  4.59     grazer   herbivore
5              Mitidulidae.Meligethes.sp. -26.99  5.30     grazer   herbivore
6           Chrysomelidae.Phylotreta.sp.2 -28.50  2.40     grazer   herbivore
7           Chrysomelidae.Phylotreta.sp.2 -28.36  4.17     grazer   herbivore
8           Chrysomelidae.Phylotreta.sp.2 -29.50  3.15     grazer   herbivore
9           Chrysomelidae.Phylotreta.sp.2 -27.69  3.72     grazer   herbivore
10          Chrysomelidae.Phylotreta.sp.2 -28.22  3.26     grazer   herbivore
11                  Gastropoda.snail.sp.1 -26.21  3.54     grazer   herbivore
12                  Gastropoda.snail.sp.1 -27.59  2.61     grazer   herbivore
13                  Gastropoda.snail.sp.1 -25.10  2.66     grazer   herbivore
14                  Gastropoda.snail.sp.2 -26.49  2.55     grazer   herbivore
15                  Gastropoda.snail.sp.4 -27.46 -0.38     grazer   herbivore
16       Lepidoptera.Arctidae.Ermine.moth -28.51  2.44     grazer   herbivore
17       Curculionidae.Ischapterapion.sp. -29.06  2.19     weevil   herbivore
18       Curculionidae.Ischapterapion.sp. -29.27  1.60     weevil   herbivore
19       Curculionidae.Ischapterapion.sp. -29.94  2.08     weevil   herbivore
20       Curculionidae.Ischapterapion.sp. -29.71  2.16     weevil   herbivore
21            Curculionidae.Protapion.sp. -28.45  1.91     weevil   herbivore
22            Curculionidae.Protapion.sp. -25.99  0.55     weevil   herbivore
23            Curculionidae.Protapion.sp. -28.27  1.52     weevil   herbivore
24            Curculionidae.Protapion.sp. -28.01  1.74     weevil   herbivore
25            Curculionidae.Protapion.sp. -27.06  0.54     weevil   herbivore
26             Curculionidae.Hypera.meles -25.41  3.38     weevil   herbivore
27               Curculionidae.Sitona.sp. -27.05  2.01     weevil   herbivore
28               Curculionidae.Sitona.sp. -26.70  3.07     weevil   herbivore
29               Curculionidae.Sitona.sp. -27.64  2.13     weevil   herbivore
30               Curculionidae.Sitona.sp. -27.50  1.47     weevil   herbivore
31            Curculionidae.Phylobius.sp. -28.27  2.66     weevil   herbivore
32      Curculionidae.Hypera.nigrorostris -25.52  2.43     weevil   herbivore

This dataframe (df) contains 14 different "Taxon" some of which have multiple samples, so that there are 32 samples in all. Each Taxon is also classified by the column "func.group" as either "grazer" or "weevil".

Firstly, I want to select 6 Taxon at random from my 14, for all possible combinations of 6. Thus there are some 3003 combinations of 6 taxon that can be made from 14 (sampled at random without replacement and order is not important). For each Taxon selected, I want to include all samples of that Taxon. I use this code, which works well:

combos<-combn(unique(as.character(df$Taxon)), 6) 

Next I want to also include all the other columns of information, so I use this additional line of code that for each Taxon selected, it adds the other columns of data, and also works well:

mysamples <- apply(combos, 2, function(vec) df[ df$Taxon %in% vec, ] )

So then we reach my problem. From "mysamples" (which should now be a list of 3003 dataframes), I would like to select all the dataframes that include at 3 Taxon that are "grazer" and 3 Taxon that are "weevil", and to store these dataframes in a new list.

Therefore, I would like this new list to contain only dataframes that include a balance of 3:3 weevil:grazer Taxon,

Many thanks, M

Answer 1

I think you're looking for all elements of mysamples that have exactly 3 weevil and exactly 3 grazer. You can do this with:

# Get list of bool for whether to include
include.list <- lapply(mysamples, function(x) sum(x$func.group == "weevil") == 3 &
                                              sum(x$func.group == "grazer") == 3)

# Limit mysamples to the selected ones
mysamples <- mysamples[do.call(c, include.list)]