This is a small 4-function-calculator.
It uses [expr] directly to calculate the results, 
so you may need to append ".0" to numbers to do calculations in floating point.

E.g. "3/2" will be done on integers: "3/2=1", 
so you might want to do "3/2.0" instead.

Also, you cannot use the result of a calculation for further calculations,
i.e. if you have "17/4.0=4.25" in the display and you continue to type "*3="
you get an error.
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  # Small calculator

  package require Tk

  button .quit   -text "Off" -fg red -command exit
  label .readout -textvariable result -bg white
  set result " "
  button .key0  -text "0"    -width 3 -command {append result 0}
  button .key1  -text "1"    -width 3 -command {append result 1}
  button .key2  -text "2"    -width 3 -command {append result 2}
  button .key3  -text "3"    -width 3 -command {append result 3}
  button .key4  -text "4"    -width 3 -command {append result 4}
  button .key5  -text "5"    -width 3 -command {append result 5}
  button .key6  -text "6"    -width 3 -command {append result 6}
  button .key7  -text "7"    -width 3 -command {append result 7}
  button .key8  -text "8"    -width 3 -command {append result 8}
  button .key9  -text "9"    -width 3 -command {append result 9}

  button .point -text "."    -width 3 -command {append result .}
  button .plus  -text "+"    -width 3 -command {append result +}
  button .minus -text "-"    -width 3 -command {append result -}
  button .times -text "*"    -width 3 -command {append result *}
  button .div   -text "/"    -width 3 -command {append result /}
  button .equal -text "="    -width 3 -command {append result = [expr $result]}
  button .sign  -text "+/-"  -width 3 -command {set result [expr $result*-1]}
  button .clear -text "C/CE" -width 3 -command {set result ""}

  grid .quit .readout -sticky nsew
  grid .key7 .key8 .key9 .times .clear -sticky nsew
  grid .key4 .key5 .key6 .minus .div   -sticky nsew
  grid .key1 .key2 .key3 .plus  .equal -sticky nsew
  grid .key0 .sign .point              -sticky nsew

 #More than 4 columns:
 #grid configure .sbar    -columnspan 4 -padx 4
  grid configure .readout -columnspan 4 -padx 4
  grid configure .plus    -rowspan 2
  grid configure .equal   -rowspan 2

 #bind . <1> {append result 1}
 #bind . <2> {append result 2}
 #bind . <3> {append result 3}
  bind . <4> {append result 4}
  bind . <r> {append result 5}
  focus -force .
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See also: [A little calculator] - [A fancier little calculator]

[HJG] Changed "aufgabe" --> "result".

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