When you want a random line from a file, and don't know how many lines are in the file to start with, there's a neat trick to save the memory consumption of:

  proc random_line {chan} {
    while {![eof $chan]} {
      lappend l [gets $chan]
    }
    lindex $l [expr {rand()*[llength $l]
  }

I'm not sure who this algorithm comes from, and I hesitate to publish it without attribution -- perhaps it was http://okmij.org/ftp/%|%Oleg Kiselyov%|% or maybe even [Knuth].  I first learned of it as "choosing a random element from a linked list in a single traversal", so tend to think of it coming from a Schemer ... if anyone can find an authoritative reference, please insert it here!


  proc random_line {chan} {
    set i 0
    while {![eof $chan]} {
      set l [gets $chan]
      incr i
      if {rand()<1./$i} {
        set result $l
      }
    }
    set result
  }

.. the curious reader is encouraged to demonstrate to him/herself that all the elements of the input will be chosen with equal probability.  Enjoy!

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