# Which Data Structure Is Used for Postfix?

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Larry Thompson

Which Data Structure Is Used for Postfix?

When it comes to evaluating mathematical expressions, two popular methods are infix notation and postfix notation. In infix notation, operators are placed between operands, while postfix notation places operators after the operands.

Postfix notation, also known as reverse polish notation (RPN), has certain advantages over infix notation. One of the key factors that make postfix notation efficient is the data structure used to evaluate expressions.

## The Stack Data Structure

In postfix evaluation, the stack data structure plays a crucial role. A stack follows the Last-In-First-Out (LIFO) principle, meaning that the last element added is the first one to be removed. This property aligns perfectly with how postfix expressions are evaluated.

To understand how a stack is used in postfix evaluation, let’s consider an example expression: `5 3 + 8 *`. To evaluate this expression using a stack:

1. Create an empty stack.
2. Read each element from left to right:
• If the element is an operand (a number), push it onto the stack.
• If the element is an operator (+, -, *, /), pop two operands from the stack, perform the operation on them, and push the result back onto the stack.
3. The final result will be left on top of the stack.

In our example expression, we start with an empty stack and read each element one by one:

1. We encounter ‘5’, which is an operand. We push it onto the stack.
2. We encounter ‘3’, another operand.

We push it onto the stack.

3. We encounter ‘+’, an operator. We pop ‘3’ and ‘5’ from the stack, perform the addition, and push ‘8’ onto the stack.
4. Finally, we encounter ‘*’, another operator. We pop ‘8’ and ‘8’ from the stack, perform the multiplication, and push ’64’ onto the stack.

After processing all elements, we are left with only one element on top of the stack: ’64’. This is our final result for the postfix expression.

## Why Use a Stack?

The use of a stack in postfix evaluation allows us to efficiently process operators and operands in their correct order. By pushing operands onto the stack and popping them when encountering an operator, we ensure that operators always have their required operands available for evaluation.

The LIFO property of a stack also helps in maintaining proper precedence between operators. Since postfix notation eliminates the need for parentheses to indicate precedence, a stack simplifies the evaluation process by ensuring that higher-precedence operators are evaluated before lower-precedence ones.

### Conclusion

In conclusion, postfix evaluation relies on the efficient implementation of a stack data structure. The LIFO property of a stack allows for proper evaluation of postfix expressions by maintaining operand order and operator precedence. Understanding how stacks work in evaluating postfix expressions is essential for anyone working with mathematical computations or programming languages that support postfix notation.