The following emulates a simple reverse Polish notation calculator which
operates on numbers at the bottom of a stack, replacing them by the result.
How would you do it? (Beware of line wrap.)
clear =: 3 : ',.STACK =: '''''
enter =: 4 : ',.STACK =: STACK , x' NB. x a list of numbers
chs =: 3 : ',.STACK =: (_1 }. STACK), - _1 {. STACK'
dup =: 4 : ',.STACK =: STACK , (- x) {. STACK' NB. x a non-negative
integer
plus =: 3 : ',.STACK =: ( _2 }. STACK), +/ _2 {. STACK'
minus =: 3 : ',.STACK =: ( _2 }. STACK), -/ _2 {. STACK'
times =: 3 : ',.STACK =: ( _2 }. STACK), */ _2 {. STACK'
divide =: 3 : ',.STACK =: ( _2 }. STACK), %/ _2 {. STACK'
clear''
1 2 enter ''
1
2
chs '' NB. Change sign of bottom number
1
_2
2 dup'' NB. Can use dup to keep a copy of numbers about to be combined
1
_2
1
_2
plus'' NB. 1 plus _2 is _1
1
_2
_1
--
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