def post_to_pre(strs):
s = strs.split()
St = Stack()
final = ""
for value in s:
if value in ['+', '-', '*', '/', '%']:
a = St.Pop()
b = St.Pop()
final = str(value) + " " + str(b) + " " + str(a)
St.Push(final)
elif value.isalpha():
St.Push(value)
elif is_number(value):
St.Push(value)
else:
print("Error")
exit(0)
return St.Top()
def isMatched(expr):
"""Checks if the expression 'expr' has matching opening/closing symbols."""
S = Stack()
n = len(expr)
for i in range(0, n):
symb = expr[i] #next symbol
# print(symb)
if symb in ['{', '(', '[']:
S.Push(symb)
elif symb in ['}', ')', ']']:
if S.isEmpty():
return False
if S.Top() == '{' and symb == '}':
S.Pop()
elif S.Top() == '(' and symb == ')':
S.Pop()
elif S.Top() == '[' and symb == ']':
S.Pop()
else:
continue
if S.isEmpty():
return True
else:
return False
def eval_prefix(strs):
strs = reverse(strs)
s = strs.split()
St = Stack()
count = 0
for value in s:
value = reverse(value)
if value in ['+', '-', '*', '/', '%']:
b = St.Pop()
a = St.Pop()
if value == '+':
result = a + b
St.Push(float(result))
elif value == '-':
result = a - b
St.Push(float(result))
elif value == '/':
result = a / b
St.Push(float(result))
elif value == '*':
result = a * b
St.Push(float(result))
elif value == '%':
result = a % b
St.Push(float(result))
elif is_number(value):
St.Push(float(value))
else:
print("Error")
return St.Top()