What Is Infix Expression in Data Structure?


Heather Bennett

In data structure, an infix expression is a mathematical expression in which operators are placed between the operands. In simpler terms, it is the commonly used way of writing arithmetic expressions such as addition, subtraction, multiplication, and division.

Understanding Infix Expressions

An infix expression is characterized by the placement of operators between operands. For example, the infix expression “3 + 4” has the operator “+” placed between the operands 3 and 4.

Infix expressions are widely used in everyday mathematics and are familiar to most people. They follow the conventional order of operations (also known as BODMAS or PEMDAS), where multiplication and division take precedence over addition and subtraction.

Advantages of Infix Expressions

While infix expressions are commonly used by humans, they require additional processing for evaluation by computers. However, infix expressions have several advantages:

  • Readability: Infix expressions closely resemble how humans write mathematical equations, making them easier to read and understand.
  • Familiarity: Most people are already familiar with infix notation due to its widespread usage in mathematics education.

The Need for Conversion: Infix to Postfix

In computer science and programming languages, postfix (also known as Reverse Polish Notation or RPN) notation is often preferred over infix notation for evaluating mathematical expressions. This preference is due to the ease of parsing and evaluating postfix expressions using a stack-based algorithm.

To evaluate an infix expression efficiently, it needs to be converted into postfix notation first. The conversion process involves rearranging operators and operands based on their precedence rules.

Conversion Steps:

  1. Create an empty stack to store operators temporarily.
  2. Scan the infix expression from left to right.
  3. If an operand is encountered, output it directly.
  4. If an operator is encountered, push it onto the stack after considering its precedence.
  5. If a closing parenthesis is encountered, pop operators from the stack and output them until an opening parenthesis is found. Discard both the opening and closing parentheses.
  6. Repeat steps 3-6 until the entire infix expression has been scanned.
  7. Pop any remaining operators from the stack and output them.

Evaluating Postfix Expressions

Once an infix expression is successfully converted into postfix notation, it becomes easier to evaluate using a stack-based algorithm. The evaluation process involves scanning the postfix expression from left to right:

  1. Create an empty stack to store operands temporarily.
  2. Scan the postfix expression from left to right.
  3. If an operand is encountered, push it onto the stack.
  4. If an operator is encountered, pop two operands from the stack, perform the operation, and push the result back onto the stack.
  5. Repeat steps 3-4 until the entire postfix expression has been scanned.
  6. The final result will be stored on top of the stack after evaluating all operators in the postfix expression.


Infix expressions are widely used in mathematics but require additional processing for computer evaluation. Converting infix expressions into postfix notation allows for more efficient evaluation using a stack-based algorithm. By understanding infix expressions and their conversion process, you can enhance your knowledge of data structures and algorithms.

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