infixToPrefix.py

"""
Output:

Enter an Infix Equation (space separated) = a + b ^ c
 Symbol  |  Stack  | Postfix
----------------------------
   c     |         | c
   ^     | ^       | c
   b     | ^       | cb
   +     | +       | cb^
   a     | +       | cb^a
         |         | cb^a+

	 a+b^c (Infix) ->  +a^bc (Prefix)
"""

def infix_2_postfix(infix):
    Stack = []
    Postfix = []
    priority = {'^':3, '*':2, '/':2, '%':2, '+':1, '-':1}       # Priority of each operator
    print_width = len(Infix) if(len(Infix)>7) else 7

    # Print table header for output
    print('Symbol'.center(8), 'Stack'.center(print_width), 'Postfix'.center(print_width), sep = " | ")
    print('-'*(print_width*3+7))

    for x in infix:
        if x.isalpha() or x.isdigit() : Postfix.append(x)       # if x is Alphabet / Digit, add it to Postfix
        elif x == '(' : Stack.append(x)                         # if x is "(" push to Stack
        elif x == ')' :                                         # if x is ")" pop stack until "(" is encountered
            while Stack[-1] != '(' :
                Postfix.append( Stack.pop() )                   #Pop stack & add the content to Postfix
            Stack.pop()
        else:
            if len(Stack) == 0 : Stack.append(x)                  #If stack is empty, push x to stack
            else:
                while len(Stack) > 0 and priority[x] <= priority[Stack[-1]] :      # while priority of x is not greater than priority of element in the stack
                    Postfix.append( Stack.pop() )                                   # pop stack & add to Postfix
                Stack.append(x)                                 # push x to stack

        print(x.center(8), (''.join(Stack)).ljust(print_width), (''.join(Postfix)).ljust(print_width), sep = " | ")         # Output in tabular format

    while len(Stack) > 0 :                                  # while stack is not empty
        Postfix.append( Stack.pop() )                       # pop stack & add to Postfix
        print(' '.center(8), (''.join(Stack)).ljust(print_width), (''.join(Postfix)).ljust(print_width), sep = " | ")         # Output in tabular format

    return "".join(Postfix)             # return Postfix as str

def infix_2_prefix(Infix):
    Infix = list(Infix[::-1])       # reverse the infix equation
    for i in range(len(Infix)):
        if Infix[i] == '(' : Infix[i] = ')'         # change "(" to ")"
        elif Infix[i] == ')' : Infix[i] = '('       # change ")" to "("
    return (infix_2_postfix("".join(Infix)))[::-1]  # call infix_2_postfix on Infix, return reverse of Postfix

Infix = input("\nEnter an Infix Equation (space separated) = ")       #Input an Infix equation
Infix = "".join(Infix.split())                      #Remove spaces from the input
print("\n\t", Infix, "(Infix) -> ", infix_2_prefix(Infix), "(Prefix)")