Two level loop over a matrix's row in R -


i need convert iteration below matrix operation.

a = certain_matrix(3,5) mat = matrix(0, 3,3)  ( in 1:3 )   ( j in 1:3 )     mat[i,j] = certain_distance(a[i,], a[j,])

roughly need use apply(). however, cannot hold of row index in 

apply(a, 1, function(x) xxx)

so how perform operation fast?


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