Launch erlang shell from the directory containing exp.erl :
1>c(exp).
{ok, exp}
2> Exp = exp:parse_expression("(2-3)+(4+2)").
{plus,{minus,{number,2},{number,3}},
{plus,{number,4},{number,2}}}
3> exp:evaluate_expression(Exp).
5
Created
April 12, 2016 08:36
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Erlang evaluation exercise
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| -module(exp). | |
| -export([parse_expression/1, evaluate_expression/1]). | |
| parse_expression(Exp) -> | |
| parse_acc(Exp, {unknown, unknown, unknown}). | |
| convert_operator(Char) -> | |
| case Char of | |
| $- -> minus; | |
| $+ -> plus; | |
| $* -> times; | |
| Other -> Other | |
| end. | |
| parse_acc([Head|Tail], Current) -> | |
| % io:format("~p / ~p / ~p~n", [parse_acc, [Head|Tail], Current]), | |
| case Head of | |
| $- -> parse_separator([Head|Tail], Current); | |
| $+ -> parse_separator([Head|Tail], Current); | |
| _ -> case string:to_integer([Head|Tail]) of | |
| {error, no_integer} -> parse_separator([Head|Tail], Current); | |
| {Number, Tail} -> parse_integer(Number, Current, Tail) | |
| end | |
| end; | |
| parse_acc([], Current) -> Current. | |
| parse_integer(Number, Current, Tail) -> | |
| % io:format("~p / ~p / ~p~n", [parse_integer, Tail, Current]), | |
| case Current of | |
| {unknown, unknown, unknown} -> parse_acc(Tail, {unknown, {number, Number}, unknown}); | |
| {Opearator, Left, unknown} -> parse_acc(Tail, {Opearator, Left, {number, Number}}); | |
| {_,_,_} -> {error, expression_already_complete} | |
| end. | |
| parse_separator([Head|Tail], Current) -> | |
| % io:format("~p / ~p / ~p~n", [parse_separator, [Head|Tail], Current]), | |
| case convert_operator(Head) of | |
| $( -> | |
| {NewExpression, NewTail} = parse_acc(Tail, {unknown, unknown, unknown}), | |
| case Current of | |
| {unknown, unknown, unknown} -> parse_acc(NewTail, {unknown, NewExpression, unknown}); | |
| {Operator, Left, unknown} -> parse_acc(NewTail, {Operator, Left, NewExpression}); | |
| {_,_,_} -> {error, expression_already_complete} | |
| end; | |
| $) -> {Current, Tail}; | |
| Other -> {_,Left,Right} = Current, | |
| parse_acc(Tail, {Other, Left, Right}) | |
| end; | |
| parse_separator([], Current) -> | |
| Current. | |
| evaluate_expression({Operator,Left, Right}) -> | |
| case Operator of | |
| minus -> evaluate_expression(Left) - evaluate_expression(Right); | |
| plus -> evaluate_expression(Left) + evaluate_expression(Right); | |
| times -> evaluate_expression(Left) * evaluate_expression(Right) | |
| end; | |
| evaluate_expression({number, Number}) -> | |
| Number. | |
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