Pairwise interaction matrix in R

Question

I am trying to compute a pairwise matrix in R that counts the number of times individuals interact with other individuals (so the matrix will include N number of rows and columns corresponding to number of individuals). I have a dataframe that lists "actors" and "partners" in separate columns. ``` nn <- data.frame(actors=c('DOL','DOL','DOL','DOL','DOL','NOR','NOR','NOR','NIN','JOJ'),partners=c('JOJ','JOJ','NOR','NOR','NIN','NIN','DOL','JOJ','NOR','NOR')) ``` The data are such that direction of the interaction is irrelevant, so each cell should count the number of times individual X acts on Y plus the number of times Y acts on X. Ideally, the data frame above should give a matrix that looks like this: ``` DOL JOJ NOR NIN DOL 0 2 3 1 JOJ 2 0 2 0 NOR 3 2 0 2 NIN 1 0 2 0 ``` I started writing a loop to cycle through each individual in my dataset and to count his/her interactions both from actor->partner and partner->actor. I'm sure this would work, but is not ideal as the full dataset is…

Accepted Answer

Base R's `table()` will get you what you're after:

```
x <- with(nn, table(actors, partners))
x + t(x)
#       partners
# actors DOL JOJ NIN NOR
#    DOL   0   2   1   3
#    JOJ   2   0   0   2
#    NIN   1   0   0   2
#    NOR   3   2   2   0
```

Pairwise interaction matrix in R

Solution

Problem