Ruby, 61 lines, includes console input
puts class RHEvaluator
def setup e
@x = e
getsym
rhEval
end
def getsym
@c = @x[0]
@x = @x.drop 1
end
def flatEval(op, a, b)
case op
when ?* then a*b
when ?/ then a/b
when ?+ then a+b
when ?- then a-b
when ?^ then a**b
end
end
def factor
t = @c
getsym
t = case t
when ?- then -factor
when ?0..?9 then t.to_f - ?0
when ?(
t = rhEval
getsym # eat )
t
end
t
end
def power
v = factor
while @c == ?^
op = @c
getsym
v = flatEval op, v, factor
end
v
end
def multiplier
v = power
while @c == ?* or @c == ?/
op = @c
getsym
v = flatEval op, v, power
end
v
end
def rhEval
v = multiplier
while @c == ?+ or @c == ?-
op = @c
getsym
v = flatEval op, v, multiplier
end
v
end
RHEvaluator # return an expression from the class def
end.new.setup gets.bytes.to_a
