r - Count every possible pair of values in a column grouped by multiple columns

Question

I have a dataframe that looks like this (this is just a subset, actually dataset has 2724098 rows)

> head(dat)

chr   start  end    enhancer motif 
chr10 238000 238600 9_EnhA1  GATA6 
chr10 238000 238600 9_EnhA1  GATA4 
chr10 238000 238600 9_EnhA1    SRF 
chr10 238000 238600 9_EnhA1  MEF2A 
chr10 375200 375400 9_EnhA1  GATA6 
chr10 375200 375400 9_EnhA1  GATA4 
chr10 440400 441000 9_EnhA1  GATA6 
chr10 440400 441000 9_EnhA1  GATA4 
chr10 440400 441000 9_EnhA1    SRF 
chr10 440400 441000 9_EnhA1  MEF2A 
chr10 441600 442000 9_EnhA1    SRF 
chr10 441600 442000 9_EnhA1  MEF2A 

I was able to transform my dataset to this format where groups of chr, start, end and enhancer represent a single ID:

> dat

 id motif 
 1  GATA6 
 1  GATA4 
 1    SRF 
 1  MEF2A 
 2  GATA6 
 2  GATA4
 3  GATA6 
 3  GATA4 
 3    SRF 
 3  MEF2A 
 4    SRF 
 4  MEF2A 

I want to find the count of every possible pair of motifs, grouped by id. So I want an output table like,

motif1 motif2 count
 GATA6  GATA4     3
 GATA6    SRF     2
 GATA6  MEF2A     2
 ... and so on for each pair of motif

In the actual dataset, there are 1716 unique motifs. There are 83509 unique id.

Any suggestions on how to proceed?

Answer 1

Updated: Here is a fast and memory efficient version using data.table:

• Step 1: Construct sample data of your dimensions approximately:

  require(data.table) ## 1.9.4+
  set.seed(1L)        ## For reproducibility
  N = 2724098L
  motif = sample(paste("motif", 1:1716, sep="_"), N, TRUE)
  id = sample(83509, N, TRUE)
  DT = data.table(id, motif)
  

• Step 2: Pre-processing:

  DT = unique(DT) ## IMPORTANT: not to have duplicate motifs within same id
  setorder(DT)    ## IMPORTANT: motifs are ordered within id as well
  setkey(DT, id)  ## reset key to 'id'. Motifs ordered within id from previous step
  DT[, runlen := .I]
  

• Step 3: Solution:

  ans = DT[DT, {
                tmp = runlen < i.runlen; 
                list(motif[tmp], i.motif[any(tmp)])
               }, 
        by=.EACHI][, .N, by="V1,V2"]
  

  This takes ~27 seconds and ~1GB of memory during the final step 3.

The idea is to perform a self-join, but make use of data.table's by=.EACHI feature, which evaluates the j-expression for each i, and therefore memory efficient. And the j-expression makes sure that we only obtain the entry "motif_a, motif_b" and not the redundant "motif_b,motif_a". This saves computation time and memory as well. And the binary search is quite fast, even though there are 87K+ ids. Finally we aggregate by the motif combinations to get the number of rows in each of them - which is what you require.

HTH

PS: See revision for the older (+ slower) version.