parse
Expression Parser
work in progress
# Expression Parser - Bauer-Samelson algorithm as described in BYTE magazine 1976-02
# Python implementation: Rick Companje, 30 July 2022
# usage: main("20+4*(5-(6-3))/8")
def precedence_check(operators,c):
if not operators or operators[-1] == '(':
return True
else:
order = "^*/+-"
return order.find(operators[-1]) > order.find(c)
def unstack(operators, operands):
op = operators.pop()
b = operands.pop()
a = operands.pop()
if op=="^":
operands.append(a**b)
elif op=="*":
operands.append(a*b)
elif op=="/":
operands.append(int(a/b))
elif op=="+":
operands.append(a+b)
elif op=="-":
operands.append(a-b)
def parse(s, vars={}):
operands = []
operators = []
pc = "" #prev char
for c in s:
if c==")":
while operators[-1]!="(":
unstack(operators,operands)
operators.pop() #remove (
if c.isnumeric():
if pc.isnumeric():
operands[-1] = int(str(operands[-1])+c)
else:
operands.append(int(c))
elif c=="(":
operators.append(c)
elif c in "^+*/-":
while not precedence_check(operators,c):
unstack(operators,operands)
operators.append(c)
# print(c,"\t",operands,"\t",operators)
pc=c # prev char for multidigit numbers
while operators:
# print(operators,operands)
unstack(operators,operands)
return operands[-1]
vars = {
"t": 10,
"i": 85,
"x": 5,
"y": 5
}
print(parse("t+4*(5-(6-3))/8"))