# What Data Structure Is Used in Converting an Infix Notation to Prefix Notation?

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Scott Campbell

What Data Structure Is Used in Converting an Infix Notation to Prefix Notation?

In computer science, infix notation is the most commonly used way of writing mathematical expressions, where operators are placed between operands. However, when it comes to evaluating and manipulating these expressions programmatically, converting them into prefix notation can be advantageous. Prefix notation, also known as Polish notation, involves placing operators before their corresponding operands.

## The Need for Conversion

Why would we want to convert an infix expression to prefix notation? There are a few reasons:

• Operator Precedence: Infix notation relies on operator precedence rules, which can sometimes introduce ambiguity or require the use of parentheses. Prefix notation eliminates this ambiguity by explicitly stating the order of operations.
• Easy Evaluation: With prefix notation, evaluating an expression becomes more straightforward and efficient.

It allows for easy implementation of algorithms like stack-based evaluation.

• Simpler Parsing: When working with complex mathematical expressions programmatically, parsing infix notation can be challenging and error-prone. Converting to prefix representation simplifies the parsing process.

## Data Structure: Stack

The key data structure used in converting infix notation to prefix notation is a stack. A stack follows the Last-In-First-Out (LIFO) principle, meaning that the last element pushed onto the stack is the first one to be popped off.

The algorithm for converting infix to prefix using a stack typically involves scanning the infix expression from right to left and performing the following steps:

1. Create an empty stack to store operators temporarily.
2. Iterate through each character in the infix expression.
3. If the character is an operand (number or variable), add it to the output expression.
4. If the character is an opening parenthesis ‘(‘, push it onto the stack.
5. If the character is a closing parenthesis ‘)’, pop operators from the stack and add them to the output expression until an opening parenthesis is encountered. Discard both the opening and closing parentheses.
6. If the character is an operator (+, -, *, /, etc.

), compare its precedence with the topmost operator on the stack:

• If the topmost operator has higher precedence or equal precedence with left associativity, pop it from the stack and add it to the output expression. Repeat this step until a lower precedence operator or an opening parenthesis is encountered.
• Push the current operator onto the stack.

After scanning all characters in the infix expression, pop any remaining operators from the stack and add them to the output expression. The resulting expression will be in prefix notation.

## Example

Let’s illustrate this process with an example:

We have an infix expression: 3 + 4 * (5 – 6)

1. Create an empty stack and output string: Stack = [], Output = “”
2. Start scanning from right to left: Current = ‘)’ -> Add ‘)’ to stack
3. Current = ‘-‘ -> Pop ‘(‘ from stack and add it to output
4. Current = ‘5’ -> Add ‘5’ to output
5. Current = ‘ ‘ -> Skip whitespace
6. Current = ‘)’ -> Pop ‘*’ from stack and add it to output
7. Current = ‘4’ -> Add ‘4’ to output
8. Current = ‘ ‘ -> Skip whitespace
9. Current = ‘+’ -> Push ‘+’ onto stack
10. Current = ‘3’ -> Add ‘3’ to output
11. Scanning complete, pop remaining operators from stack: Pop ‘+’ from stack and add it to output

The resulting prefix expression is: + 3 – 5 * 4 6

## Conclusion

In the process of converting infix notation to prefix notation, the stack data structure plays a crucial role. By utilizing the LIFO property of stacks, we can efficiently manipulate operators and generate the desired prefix expression. Understanding this conversion process can greatly benefit programmers when dealing with complex mathematical expressions in their applications.

So next time you encounter an infix expression that needs evaluation or manipulation, consider converting it to prefix notation using a stack!