def infix_2_postfix(infix_expr):
postfix = "" # empty string where operands operators and popped items from stack will go
operator_stack = Stack() # Empty stack to push items
for symbol in infix_expr:
if symbol == " " or symbol == '\t': # Ignore white spaces
continue
if symbol == '(':
operator_stack.push(symbol)
elif symbol == ')':
top_symbol_stack = operator_stack.pop()
while top_symbol_stack != '(':
postfix += top_symbol_stack
top_symbol_stack = operator_stack.pop()
elif symbol in '+-*/%^':
while not operator_stack.is_empty() and precedence(
operator_stack.peek()) >= precedence(symbol):
# If the precedence in the stack is >= scanned symbol pop from stack and add to postfix list
postfix += operator_stack.pop()
operator_stack.push(
symbol) # When finished push the scanned symbol
else: # Its an operand add it to postfix string
postfix += symbol
while not operator_stack.is_empty():
postfix += operator_stack.pop()
return postfix
def infix_to_postfix(infix_expr):
# Prec holds the precedence
prec = {}
prec['**'] = 4
prec['*'] = 3
prec['/'] = 3
prec['+'] = 2
prec['-'] = 2
prec['('] = 1
operator_stack = Stack() # Empty stack for the operators
postfixList = [] # Empty stack for the post fix list
tokenList = infix_expr.split() # Lets split the expression
# Lets read each token
for token in tokenList:
if token in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' or token in '0123456789':
# Append token to the postfixList
postfixList.append(token)
elif token == '(':
# Push the token to operator stack
operator_stack.push(token)
elif token == ')':
# pop last item from operator stack
topToken = operator_stack.pop()
while topToken != '(':
postfixList.append(topToken)
topToken = operator_stack.pop()
else:
while (not operator_stack.is_empty()) and (
prec[operator_stack.peek()] >= prec[token]):
postfixList.append(operator_stack.pop())
operator_stack.push(token)
while not operator_stack.is_empty():
postfixList.append(operator_stack.pop())
return " ".join(postfixList)