Write a Program to Convert Expression from Postfix to Infix  using JavaScript in Compiler Design

In this article, we present a JavaScript program that converts postfix expressions to infix notation. We'll explore the theory behind postfix and infix notations, dive into the code implementation, and provide input/output examples.

Infix & Postfix Notation:

Postfix notation, also known as Reverse Polish Notation (RPN), is an expression format where operators follow their respective operands. For example, the postfix expression "2 3 +" represents the addition of 2 and 3.

In contrast, infix notation is the traditional way of representing expressions, with operators placed between operands. The equivalent infix expression for "2 3 +" is "2 + 3".

Converting a postfix expression to infix notation involves rearranging the operators and operands based on their positions. This process often requires using a stack to hold intermediate results.

Convert Expression from Postfix to Infix in JavaScript:

Let's explore the JavaScript code that converts postfix expressions to infix notation:

function detectOperand(ch) {
    return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');
}

function getInfix(postfix) {
    let stack = [];

    for (let value of postfix) {
        if (detectOperand(value)) {
            stack.push(value);
        } else {
            let firstOperand = stack.pop();
            let secondOperand = stack.pop();
            stack.push("(" + secondOperand + value + firstOperand + ")");
        }
    }

    return stack[stack.length - 1];
}

let postfix = "abc*+d+";
console.log(`The given postfix expression is: ${postfix}`);
console.log(`The corresponding infix expression is: ${getInfix(postfix)}`);

Input/Output:

The given postfix expression is: abc*+d+

The corresponding infix expression is: ((a*b)+(c+d))

Explanation:

Let's go through the code step by step to understand how it Converts Expression from Infix to Postfix :

The code begins with a helper function, detectOperand, which checks if a character is an operand (a letter). This function returns true if the character is an operand, and false otherwise.

The main function, getInfix, takes a postfix expression as input and iterates through each character. If an operand is encountered, it is pushed onto the stack. 

When an operator is encountered, two operands are popped from the stack. The operator is placed between the operands, enclosed in parentheses, and pushed back onto the stack. This process continues until the entire postfix expression is processed.

Finally, the top element of the stack contains the fully converted infix expression, which is returned as the result.

Conclusion:

In this article, we have explored the conversion of postfix expressions to infix notation using JavaScript. We learned about the theory behind postfix and infix notations, and how to implement the conversion algorithm using a stack and stack operations.

Happy coding!

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