m <- matrix(0, 400, 500)
set.seed(12)
m[runif(314, 0, length(m))] <- 1
mm <- as(m, "dgeMatrix")
object.size(m) / object.size(mm) # smaller by a factor of > 200
## tcrossprod() is very fast:
system.time(tCmm <- tcrossprod(mm))
system.time(cm <- crossprod(t(m)))
system.time(cm. <- tcrossprod(m))
stopifnot(cm == as(tCmm, "matrix"))
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