[R] Pairwise Distances -- How to vectorize the loop
Timothy W. Victor
tvictor at dolphin.upenn.edu
Tue Nov 16 04:26:30 CET 2004
R-List,
I'm trying to compute pairwise distances among pairs of observations,
which each pair containing data from 2 groups. There are more than
100000 unique pairs. I have programmed a distance function that has
three parameters, a vector of covariates from the ith observation in
Group 1, a vector of covarites from the jth observation in Group 2, and
a weighting matrix.
I have used expand.grid to create a matrix containing all possible pairs
of Keys (unique identifiers) from Group 1 and Group 2 and a placeholder
for each pairwise distance in an effort to pre-allocate memory.
The data containing the covariates are currently in a matrix that looks
like:
x1 x2 x3 ... xn Key Group
where x1,...,xn are the covariates of interest. The Key values
correspond to the row number.
I'm trying to figure out a way to calculate all of the distances by
vectorizing a loop. I'm trying to avoid the nested-loop paradigm. I'm
sure I'm missing something quite obvious.
Any insights would be appreciated.
Best,
Tim
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