and prefix notations in the sense that in the postfix notation Saturday, April 18, Data Structure. 9. Infix. Postfix. Prefix. A+B. AB+. +AB. Content about infix prefix and post fix and their conversion using the of Postfix Expressions • From above we get, 2 3 * 5 4 * + 9 – Stack. Conversion of Infix expression to Postfix expression using Stack data To reduce the complexity of expression evaluation Prefix or Postfix.

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We can now start to see how the conversion algorithm will work.

Infix, Postfix and Prefix

We can now start to see how the conversion algorithm will work. That operator will need to wait until the corresponding right parenthesis appears to denote its orefix recall the fully parenthesized technique.

Ifix is a more complex expression: A More Complex Example of Evaluation. We have already noted that the operands A, B, and C stay in their relative positions. Table 4 shows some additional examples of infix expressions and the equivalent prefix and postfix expressions. As you might expect, there are algorithmic ways to perform the conversion that allow any expression of any complexity to be correctly transformed. Here is a more complex expression: The top of the stack will always be the most recently saved operator.

Data Structure – Expression Parsing

When that right parenthesis does appear, the operator can be popped from the stack. The following steps will produce a string of tokens in postfix order.


In fact, infox have been reading and writing these types of expressions for a long time and they do not cause you any problem. The first technique that we will consider uses the notion of a fully parenthesized expression that was discussed earlier.

When the operands for the division are popped from the stack, they are reversed. The answer is that the operators are no longer ambiguous with respect to the operands that they work on.

Also, the order of these saved operators may need to be reversed due to their precedence. As a final stack example, we will consider the evaluation of an expression that is already in postfix notation.

To parse any arithmetic expression, we need to take care of operator precedence and associativity also.

There are two things to note in this example. Precedence and associativity determines the order of evaluation of an expression.

Below are an infix and respective Postfix expressions. Whenever we read a new operator, we will need to consider how that operator compares in precedence with the operators, if any, already on the stack. As we process the expression, the operators have to be saved somewhere since their corresponding right operands are not seen yet. Because of this reversal of order, it makes sense to consider using a stack to keep the operators until they are needed.


Line 15 defines the operands to be any upper-case character or digit. We leave this as an exercise at the end of the chapter. The left parenthesis will receive the lowest value possible. What would happen if we moved the operator before the two operands? So, as before, push it and check the next symbol.

Infix, Postfix and Prefix

This type of expression uses one pair of parentheses for each operator. If we encounter an operand we will write in the expression string, if we encounter an operator we will push it to an operator rata.

Figure 10 shows the stack contents as this entire example expression is being processed. Add it to the expression string. Then move the enclosed operator to the position of either the left or the right parenthesis depending on whether you want prefix or postfix notation. In this notation style, the operator is postfix ed to the daya i. Next token in the given infix expression is a close parenthesis, as we encountered a close parenthesis we should pop the expressions from the stack and add it to the expression string until an open parenthesis popped from the stack.