```
# NOT RUN {
## A random sparse "incidence" matrix :
m <- matrix(0, 400, 500)
set.seed(12)
m[runif(314, 0, length(m))] <- 1
mm <- as(m, "dgCMatrix")
object.size(m) / object.size(mm) # smaller by a factor of > 200
## tcrossprod() is very fast:
system.time(tCmm <- tcrossprod(mm))# 0 (PIII, 933 MHz)
system.time(cm <- crossprod(t(m))) # 0.16
system.time(cm. <- tcrossprod(m)) # 0.02
stopifnot(cm == as(tCmm, "matrix"))
## show sparse sub matrix
tCmm[1:16, 1:30]
# }
```

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