ncol(matrix(1:6, 2, 3))

  nrow(matrix(1:6, 2, 3))

You can under to debug mode in any piece of code by calling ``browser``.

``traceback`` prints out the function call stack after an error occurs; does

nothing if there's no error.

``trace`` allows you to insert debugging code into a function a specific places.

``recover`` allows you to modify the error behavior so that you can browse the

function call stack.

Profiling

=========

How long execution of expression takes (in low sec/milisec resolution)::

  system.time(expr, gcFirst = TRUE)

  unix.time(expr, gcFirst = TRUE)

``Rprof`` function enable global profiling. ``summaryRprof`` function decrypt

profiling data::

  Rprof()       ## start profiling

  Rprof(NULL)   ## suspend profiling

  Rprof(append = TRUE)  ## resume profiling

  Rprof(NULL)   ## end profiling

  summaryRprof() ## investigate profiling report

Sampling from array::

  sample(x, size, replace = FALSE, prob = NULL)

  sample.int(n, size = n, replace = FALSE, prob = NULL)

  sample(1:10, 10)  ## permutation!!

  sample(1:10, 100, replace=TRUE)