[Richard Suchenwirth] - As yet another midnight project, here's a small
calculator in Tcl/Tk. Known problem: arithmetics is done with ''expr'',
so e.g. ''1/2='' comes out as 0. When dividing, you should mark at least one
operand as double by appending a period. Then again, in addition to the
buttons you can use any of expr's other functionalities  via keyboard
input.

 wm title . Calculator
 grid [entry .e -textvar e -just right] -columnspan 5
 bind .e <Return> =
 set n 0
 foreach row {
    {7 8 9 + -}
    {4 5 6 * /}
    {1 2 3 ( )}
    {C 0 . =  }
 } {
    foreach key $row {
        switch -- $key {
            =       {set cmd =}
            C       {set cmd {set clear 1; set e ""}}
            default {set cmd "hit $key"}
        }
        lappend keys [button .[incr n] -text $key -command $cmd]
    }
    eval grid $keys -sticky we ;#-padx 1 -pady 1
    set keys [list]
 }
 grid .$n -columnspan 2 ;# make last key (=) double wide
 proc = {} {
    regsub { =.+} $::e "" ::e ;# maybe clear previous result
    if [catch {lappend ::e = [set ::res [expr $::e]]}] {
        .e config -fg red
    }
    .e xview end
    set ::clear 1
 }
 proc hit {key} {
    if $::clear {
        set ::e ""
        if ![regexp {[0-9().]} $key] {set ::e $::res}
        .e config -fg black
        .e icursor end
        set ::clear 0
    }
    .e insert end $key
 }
 set clear 0
 focus .e           ;# allow keyboard input
 wm resizable . 0 0
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[Arts and crafts of Tcl-Tk programming]