README.md

Infix-to-Prefix-Convertion-by-Python

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Postfix

Step 1: Add '')" to the end of the infix expression
Step 2: Push(o nto the stack
Step 3: Repeat until each character in the infix notation is scanned
IF a(is encountered, push it on the stack
IF an operand (whetheradigit oracharacter) is encountered, add it postfix expression.
IF a ")" is encountered, then
a. Repeatedly pop from stack and add it to the postfix expression until a "(" is encountered.
b. Discard the "(".That is, remove the(from stack and do not add it to the postfix expression
IF an operator O is encountered, then
a. Repeatedly pop from stack and add each operator ( popped from the stack) to the postfix expression which has the same precedence orahigher precedence than O
b. Push the operator to the stack
[END OF IF]
Step 4: Repeatedly pop from the stack and add it to the postfix expression until the stack is empty
Step 5: EXIT

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Prefix

Step 1: Reverse the infix string. Note that while reversing the string you must interchange left and right parentheses.
Step 2: Obtain the postfix expression of the infix expression Step 1.
Step 3: Reverse the postfix expression to get the prefix expression

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CODE

def isOperator(c):
    return (not (c >= 'a' and c <= 'z') and not(c >= '0' and c <= '9') and not(c >= 'A' and c <= 'Z'))
 
def getPriority(C):
    if (C == '-' or C == '+'):
        return 1
    elif (C == '*' or C == '/'):
        return 2
    elif (C == '^'):
        return 3
    return 0
 

def infixToPrefix(infix):
    operators = []
    operands = []
 
    for i in range(len(infix)):
        
        if (infix[i] == '(' ):
            operators.append(infix[i])
 
        elif (infix[i] == ')'):
            while (len(operators)!=0 and (operators[-1] != '(' )):
                op1 = operands[-1]
                operands.pop()
                op2 = operands[-1]
                operands.pop()
                op = operators[-1]
                operators.pop()
                tmp = op + op2 + op1
                operands.append(tmp)
            operators.pop()
        elif (not isOperator(infix[i])):
            operands.append(infix[i] + "")
 
        else:
            while (len(operators)!=0 and getPriority(infix[i]) <= getPriority(operators[-1])):
                op1 = operands[-1]
                operands.pop()
 
                op2 = operands[-1]
                operands.pop()
 
                op = operators[-1]
                operators.pop()
 
                tmp = op + op2 + op1
                operands.append(tmp)
            operators.append(infix[i])
 
    while (len(operators)!=0):
        op1 = operands[-1]
        operands.pop()
 
        op2 = operands[-1]
        operands.pop()
 
        op = operators[-1]
        operators.pop()
 
        tmp = op + op2 + op1
        operands.append(tmp)
    return operands[-1]

while(1):
    s = input("Infix Expression : ")
    print("Prefix Expression : ", infixToPrefix(s))
    
Step 1: Reverse the infix string. Note that while reversing the string you must interchange left and right parentheses.
Step 2: Obtain the postfix expression of the infix expression Step 1.
Step 3: Reverse the postfix expression to get the prefix expression