[GPS] 4 Dec 2003:

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======
 # Purpose: convert a decimal number to a binary representation
 
 proc to.binary n { 
  incr n 0
  set r ""
  while {$n > 0} { 
   set r [expr {$n & 1}]$r
   set n [expr {$n >> 1}]
  } 
  return $r
 }
======

Example:
 % to.binary 3
 11
 % to.binary 4
 100
 % to.binary 200
 11001000

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[JPT]
Here's a recursive one-liner that could certainly be optimized:

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  proc to.binary n {expr {!$n ? 0 : "[to.binary [expr {$n>>1}]][expr {$n&1}]"} }
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[AMG] Here's another method:

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 proc to.binary {n} {
    binary scan [binary format I $n] B* out
    return [string trimleft [string range $out 0 end-1] 0][string index $out end]
 }
======

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See also: [Binary representation of numbers]

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