class RpnException(Exception):
pass
def evalRPN(expr):
stack=[]
for a in expr.split():
if (a=='+'):
if (len(stack) < 2):
raise RpnException("Лишний знак +")
a2=stack[0]
a1=stack[1]
r=a1+a2
stack=[r]+stack[2:]
elif (a=='-'):
if (len(stack) < 2):
raise RpnException("Лишний знак -")
a2=stack[0]
a1=stack[1]
r=a1-a2
stack=[r]+stack[2:]
elif (a=='*'):
if (len(stack) < 2):
raise RpnException("Лишний знак *")
a2=stack[0]
a1=stack[1]
r=a1*a2
stack=[r]+stack[2:]
elif (a=='/'):
if (len(stack) < 2):
raise RpnException("Лишний знак /")
a2=stack[0]
a1=stack[1]
if (a2==0):
raise RpnException("Деление на нуль!")
else:
r=a1/a2
stack=[r]+stack[2:]
else:
stack=[int(a)]+stack
if (len(stack) != 1):
raise RpnException("Лишнее число")
return stack[0]
try:
print(evalRPN("12 3 + 5 /"))
except RpnException as e:
print(e)
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