I have to write a code to evaluate a postfix notation(reverse Polish evaluation) in Smalltalk. I have gone through the documentation and have also implemented a stack. This is the code I have written so far:
Object subclass: #Rpcalc
instanceVariableNames: 'anArray top'
classVariableNames: ''
poolDictionaries: ''
category: nil !
pop:
| item |
item := anArray at: top.
top := top - 1.
^item!
push: item
top := top + 1.
anArray at: top put: item!
setsize: n
anArray := Array new: n.
top := 0!
evaluate:
| expression aToken op1 op2 operator answer|
Transcript show: 'Enter Expression' .
expression :- stdin nextLine.
| aStack |
aStack := Array new: 10 .
aToken := self getNextToken.
((aToken key) = 'operand')
ifTrue: [push : aToken].
aToken := self getNextToken.
((aToken key) = 'operator')
ifTrue: [op1 := pop.
op2 := pop.
operator := aToken.
"if(operator := +)"
"answer := op1 + op2"
I want to know how to tokenize each element in the expression. For e.g., for the expression, 10 3 + 3 7 * - I need to equate it to a token. If its an operand, it should push it into the stack. If an operator, pop the stack twice to get the operands and evaluate the expression. I am totally new to smalltalk, so I am clueless about the syntax.
You did not specify which Smalltalk dialect you are using. In Squeak, you could use the findTokens: method:
'336 8 4 2 1 + - * /' findTokens: ' '
==> an OrderedCollection('336' '8' '4' '2' '1' '+' '-' '*' '/')
Use isDigit to test if a token is a number:
'336' first isDigit
==> true
'+' first isDigit
==> false
To convert from a string to a number you would use asNumber:
'336' asNumber
==> 336
The whole RPN parser/evaluator can easily be implemented with less than 10 lines of code, but obviously that is your homework to do (hint: no need to implement a stack, there is already one).