如何将逆波兰表示法(RPN)转换为指定API的条件集合对象？

First, let's break down how to map the Reverse Polish Notation (RPN) to your API's CriteriaCollection structure using a stack-based approach—this is the standard way to evaluate RPN, and it translates perfectly to building nested logical collections.

Step-by-Step RPN Evaluation & Structure Building #

The RPN A B OR C AND corresponds to the logical expression (A OR B) AND C. Here's how to build the CriteriaCollection step by step:

1. Initialize an empty stack: We'll use this to hold operands and intermediate CriteriaCollection objects.
2. Process each token in order:
   • Token "A": Push a SimpleCriteria object representing your condition A (e.g., field, operator, value) onto the stack. Stack: [SimpleCriteria(A)]
   • Token "B": Push a SimpleCriteria object for condition B. Stack: [SimpleCriteria(A), SimpleCriteria(B)]
   • Token "OR": Pop the top two elements (B then A), create an OR-type CriteriaCollection containing both, then push this collection back to the stack. Stack: [CriteriaCollection(OR: [A, B])]
   • Token "C": Push a SimpleCriteria object for condition C. Stack: [CriteriaCollection(OR: [A, B]), SimpleCriteria(C)]
   • Token "AND": Pop the top two elements (C then the OR collection), create an AND-type CriteriaCollection containing both, then push this back to the stack. Stack: [CriteriaCollection(AND: [OR(A,B), C])]

The final stack element is exactly the CriteriaCollection you need—it represents (A OR B) AND C.

Concrete Code Example (C#) #

Assuming your API defines CriteriaBase, SimpleCriteria, and CriteriaCollection like this (adjust field names to match your actual API):

// Base class for all criteria types
public abstract class CriteriaBase { }

// Represents a single condition (e.g., A, B, C)
public class SimpleCriteria : CriteriaBase
{
    public string Field { get; set; }
    public string Operator { get; set; } // e.g., "EQ", "GT"
    public object Value { get; set; }
}

// Represents a collection of criteria joined by a logical operator
public class CriteriaCollection : CriteriaBase
{
    public string LogicalOperator { get; set; } // "AND", "OR"
    public List<CriteriaBase> Criteria { get; set; } = new List<CriteriaBase>();
}

Here's the conversion function that implements the stack logic:

public CriteriaBase ConvertRpnToCriteria(string rpnExpression)
{
    var stack = new Stack<CriteriaBase>();
    var tokens = rpnExpression.Split(' ', StringSplitOptions.RemoveEmptyEntries);

    foreach (var token in tokens)
    {
        // Check if token is a logical operator
        if (token.Equals("AND", StringComparison.OrdinalIgnoreCase) || 
            token.Equals("OR", StringComparison.OrdinalIgnoreCase))
        {
            // Pop right operand first (stack is LIFO)
            var rightOperand = stack.Pop();
            var leftOperand = stack.Pop();

            // Build the collection with the correct operator and operands
            var collection = new CriteriaCollection
            {
                LogicalOperator = token.ToUpper(),
                Criteria = new List<CriteriaBase> { leftOperand, rightOperand }
            };

            stack.Push(collection);
        }
        else
        {
            // Parse operand into a SimpleCriteria (adjust this based on your actual operand format)
            // Example: If token was "Age GT 30", you'd split into Field="Age", Operator="GT", Value=30
            var simpleCriteria = new SimpleCriteria
            {
                Field = token, // Replace with actual field name
                Operator = "EQ", // Replace with actual operator for your condition
                Value = null // Replace with actual value for your condition
            };

            stack.Push(simpleCriteria);
        }
    }

    // The stack should have exactly one root element
    return stack.Count == 1 ? stack.Pop() : throw new InvalidOperationException("Invalid RPN expression");
}

Key Notes for Adaptation #

• Operand Parsing: If your RPN operands are more complex than single letters (e.g., Age GT 30), modify the operand handling block to split the token into Field, Operator, and Value as needed.
• API Structure Adjustments: Match the field names (like LogicalOperator or Criteria) to exactly what your API expects—don't assume the names I used are correct for your implementation.
• Case Sensitivity: Ensure the logical operator strings ("AND"/"OR") match the case required by your API (e.g., lowercase "and" vs uppercase "AND").
• Error Handling: Add checks for invalid RPN (e.g., insufficient operands for an operator) to avoid runtime crashes.

When you run this function with ConvertRpnToCriteria("A B OR C AND"), you'll get a CriteriaCollection that exactly represents (A OR B) AND C as required by your API.

内容的提问来源于stack exchange，提问作者Sam