[GPS] - Fri Jul 12, 2002: I've been curious about how interpreters work, and in particular how they evaluate sequences and magically return a result. The other day I looked at a little mathematical expression evaluator [http://www.snippets.org] and the concept finally made sense. I came up with the following Tcl prototype to teach me how it works and verify my thoughts. Using a method similar to this a person could probably write a Tcl-like interpreter. ---- #!/bin/tclsh8.3 #version 1 July 12, 2002 proc push {arPtr val} { upvar $arPtr ar set len $ar(length) incr len set ar($len) $val set ar(length) $len } proc pop {arPtr} { upvar $arPtr ar set len $ar(length) if {![info exists ar($len)]} { return -code error "no element to pop" } set res [set ar($len)] unset ar($len) incr len -1 set ar(length) $len return $res } proc evaluate {str} { array set operandStack {length 0} array set operatorStack {length 0} set digit "" set paren 0 set parenStr "" set count 1 foreach char [split $str ""] { if {$paren} { if {$char == "("} { incr count } elseif {$char == ")"} { incr count -1 if {$count == 0} { set res [evaluate $parenStr] set count 1 set paren 0 set parenStr "" puts "sub eval $res" push operandStack $res } } else { append parenStr $char } continue } if {$char == "("} { set paren 1 } elseif {$char == "."} { append digit $char } elseif {[string is digit $char]} { append digit $char } elseif {[string is space $char]} { if {$digit != ""} { push operandStack $digit set digit "" } } elseif {$char == "+"} { push operatorStack $char } elseif {$char == "-"} { push operatorStack $char } elseif {$char == "*"} { push operatorStack $char } elseif {$char == "/"} { push operatorStack $char } } if {$digit != ""} { push operandStack $digit } parray operandStack parray operatorStack puts "----" while {$operandStack(length) > 1} { set num2 [pop operandStack] set num1 [pop operandStack] set op [pop operatorStack] switch -- $op { "+" { push operandStack [expr {$num1 + $num2}] } "-" { push operandStack [expr {$num1 - $num2}] } "*" { push operandStack [expr {$num1 * $num2}] } "/" { push operandStack [expr {$num1 / $num2}] } } } if {$operatorStack(length) > 0} { return -code error "invalid expression: $str" } puts "returning $operandStack(1)" return $operandStack(1) } proc main {} { puts [evaluate "100 + 200 + (20 / 4) + (20.0 / 300) + 5"] } main ---- For an example of C code which parses Tcl commands, and C code with parses like [expr] does, see "Scratch" in the CriTcl library - README [http://www.equi4.com/critlib/scratch.README], download [http://www.equi4.com/critlib/critlib.tar.gz]. It has similarities with the above, yet also some differences and tricks. -[jcw]