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@Lata Patwal

Evaluate the Infix expression /-*25*12-119 ?

Prefix and Postfix expressions can be evaluated faster than an infix expression. This is because we don’t need to process any brackets or follow operator precedence rule.

In postfix and prefix expressions which ever operator comes before will be evaluated first, irrespective of its priority. Also, there are no brackets in these expressions. As long as we can guarantee that a valid prefix or postfix expression is used, it can be evaluated with correctness.

We can convert infix to postfix and can convert infix to prefix. In this article, we will discuss how to evaluate an expression written in prefix notation. The method is similar to evaluating a postfix expression.

Please read Evaluation of Postfix Expression to know how to evaluate postfix expressions Algorithm EVALUATE_PREFIX(STRING)

Step 1: Put a pointer P at the end of the end

Step 2: If character at P is an operand push it to Stack

Step 3: If the character at P is an operator pop two elements from the Stack.

Operate on these elements according to the operator, and push the result back to the Stack

Step 4: Decrement P by 1 and go to Step 2 as long as there are characters left to be scanned in the expression.

Step 5: The Result is stored at the top of the Stack, return it

Step 6: End Example to demonstrate working of the algorithm

Expression:

+9*26 Character | Stack | Explanation Scanned | (Front to | | Back) ….6 6 6 is an operand, push to Stack 2 6 2 2 is an operand, push to Stack * 12 (6*2) * is an operator, pop 6 and 2, multiply them and push result to Stack 9 12 9 9 is an operand, push to Stack + 21 (12+9) + is an operator, pop 12 and 9 add them and push result to Stack Result: 21 ….

Examples:

Input : -+8/632 Output : 8 Input : -+7*45+20 Output : 25 ….

Complexity

The algorithm has linear complexity since we scan the expression once and perform at most O(N) push and pop operations which take constant time....

https://gateoverflow.in/30887/postfix-evaluation

https://web.stonehill.edu/compsci/CS104/Stuff/Infix%20and%20%20postfix%20expressions.pdf