How to resolve the algorithm Parsing/Shunting-yard algorithm step by step in the C# programming language

Published on 12 May 2024 09:40 PM

How to resolve the algorithm Parsing/Shunting-yard algorithm step by step in the C# programming language

Table of Contents

Problem Statement

Given the operator characteristics and input from the Shunting-yard algorithm page and tables, use the algorithm to show the changes in the operator stack and RPN output as each individual token is processed.

Add extra text explaining the actions and an optional comment for the action on receipt of each token.

The handling of functions and arguments is not required.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Parsing/Shunting-yard algorithm step by step in the C# programming language

Explanation of the provided C# code:

Problem Statement: The code is designed to convert an infix mathematical expression into its corresponding postfix notation (also known as Reverse Polish Notation). Postfix notation is a notation in which operators appear after their operands.

Detailed Explanation:

  1. Input and Output: The program takes an infix expression as input and outputs the corresponding postfix expression.

  2. Constants: Predefined constants used in the program include:

    • infix: The input infix expression.
    • operators: A dictionary that maps operator symbols to their precedence and associativity. The operators and their precedence levels are defined in this dictionary.
  3. ToPostfix Extension Method: This method extends the string type and converts an infix expression to postfix notation.

    • Tokenization: The input expression is split into tokens (individual elements) using spaces as separators.
    • Stacks and Lists: Two stacks and a list are used:
      • stack: Stores operators and parentheses.
      • output: Stores thepostfix expression.
    • Looping Through Tokens: The code iterates through each token in the input expression and performs the following actions based on the type of token:
      • Numbers: If the token is a number, it is added to the output list.
      • Operators: If the token is an operator, it is compared to the operators in the stack. The precedence and associativity of operators are used to determine the order in which they are added to the output list.
      • Opening Parenthesis: Opening parentheses are pushed onto the stack.
      • Closing Parenthesis: Closing parentheses pop operators from the stack until an opening parenthesis is encountered.
    • Stack Cleanup: After processing all tokens, any remaining operators in the stack are popped and added to the output list.
  4. Output Formatting: The code includes a method called Print that prints the current state of the stack and output list after each action. This helps visualize the conversion process.

  5. Result: The method returns the postfix expression as a string by joining the elements in the output list.

Usage: The code demonstrates the conversion by taking an infix expression as input and outputting the corresponding postfix expression.

Example:

Input infix expression: 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3

Output postfix expression: 3 4 2 * 1 5 - 2 3 ^ ^ / +

Note: The code includes two printing methods, one with a compact format and the other with more readability. The compact format is used in the code for conciseness, but you can uncomment the more readable format if desired.

Source code in the csharp programming language

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main() {
        string infix = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3";
        Console.WriteLine(infix.ToPostfix());
    }
}

public static class ShuntingYard
{
    private static readonly Dictionary<string, (string symbol, int precedence, bool rightAssociative)> operators
        = new (string symbol, int precedence, bool rightAssociative) [] {
            ("^", 4, true),
            ("*", 3, false),
            ("/", 3, false),
            ("+", 2, false),
            ("-", 2, false)
    }.ToDictionary(op => op.symbol);

    public static string ToPostfix(this string infix) {
        string[] tokens = infix.Split(' ');
        var stack = new Stack<string>();
        var output = new List<string>();
        foreach (string token in tokens) {
            if (int.TryParse(token, out _)) {
                output.Add(token);
                Print(token);
            } else if (operators.TryGetValue(token, out var op1)) {
                while (stack.Count > 0 && operators.TryGetValue(stack.Peek(), out var op2)) {
                    int c = op1.precedence.CompareTo(op2.precedence);
                    if (c < 0 || !op1.rightAssociative && c <= 0) {
                        output.Add(stack.Pop());
                    } else {
                        break;
                    }
                }
                stack.Push(token);
                Print(token);
            } else if (token == "(") {
                stack.Push(token);
                Print(token);
            } else if (token == ")") {
                string top = "";
                while (stack.Count > 0 && (top = stack.Pop()) != "(") {
                    output.Add(top);
                }
                if (top != "(") throw new ArgumentException("No matching left parenthesis.");
                Print(token);
            }
        }
        while (stack.Count > 0) {
            var top = stack.Pop();
            if (!operators.ContainsKey(top)) throw new ArgumentException("No matching right parenthesis.");
            output.Add(top);
        }
        Print("pop");
        return string.Join(" ", output);
        
        //Yikes!
        void Print(string action) => Console.WriteLine($"{action + ":",-4} {$"stack[ {string.Join(" ", stack.Reverse())} ]",-18} {$"out[ {string.Join(" ", output)} ]"}");
        //A little more readable?
        void Print(string action) => Console.WriteLine("{0,-4} {1,-18} {2}", action + ":", $"stack[ {string.Join(" ", stack.Reverse())} ]", $"out[ {string.Join(" ", output)} ]");
    }
}


  

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