const char输入调用isdigit/isalnum报错及后缀计算器实现求助

Hey there! Let's work through your two problems step by step—sounds like you're building a postfix calculator per your professor's specs, and hit a snag with character validation first.

1. Fixing the isdigit()/isalnum() Error for const char #

The error happens because isdigit() and isalnum() expect an argument that's either an unsigned char or the special value EOF. If your char is signed (which it often is by default), passing a character with a value >127 will get converted to a negative integer, triggering undefined behavior—and even for valid digits, some compilers will throw a warning/error for the implicit conversion from const char to int.

You've got two solid options here, tailored to your use case (checking single-digit non-negative integers):

• Cast to unsigned char (cleanest for standard functions):
  Just wrap your const char in a cast to match what the functions expect:

  const char c = '5';
  if (isdigit(static_cast<unsigned char>(c))) {
      // It's a digit!
  }
  

• Manual character range check (simpler for your calculator):
  Since you only care about single digits ('0' to '9'), you can skip the standard library functions entirely and check directly:

  const char c = '7';
  if (c >= '0' && c <= '9') {
      // Confirmed digit—no conversion issues!
  }
  

  This is perfect for your use case because it avoids any type-related headaches and does exactly what you need.

2. Implementing the Postfix Calculator with Templates & Push/Pop #

Let's outline how to build this, keeping your professor's requirements in mind (template stack, push/pop methods, single-digit non-negative integers, specific allowed characters):

Step 1: Build a Template Stack Class #

First, create a generic stack template—this lets you reuse it for different types if needed, but we'll use int for your calculator:

#include <vector>
#include <stdexcept>

template <typename T>
class Stack {
private:
    std::vector<T> elements;

public:
    // Push an element onto the stack
    void push(const T& value) {
        elements.push_back(value);
    }

    // Remove the top element (throws if stack is empty)
    void pop() {
        if (empty()) {
            throw std::runtime_error("Cannot pop from empty stack");
        }
        elements.pop_back();
    }

    // Get the top element (throws if stack is empty)
    T top() const {
        if (empty()) {
            throw std::runtime_error("Cannot access top of empty stack");
        }
        return elements.back();
    }

    // Check if stack is empty
    bool empty() const {
        return elements.empty();
    }
};

Step 2: Process the Postfix Expression String #

Next, write a function that takes your input string, iterates over each character, and uses the stack to compute the result:

#include <string>

int evaluatePostfix(const std::string& expression) {
    Stack<int> stack;

    for (char c : expression) {
        // Skip whitespace (if your expression uses spaces to separate tokens)
        if (c == ' ') {
            continue;
        }

        // If it's a digit, convert to int and push to stack
        if (c >= '0' && c <= '9') {
            stack.push(c - '0'); // Convert char to int (e.g., '5' - '0' = 5)
        }
        // If it's an operator, pop two values and compute
        else {
            if (stack.empty()) {
                throw std::runtime_error("Invalid postfix expression: not enough operands");
            }
            int rightOperand = stack.top();
            stack.pop();

            if (stack.empty()) {
                throw std::runtime_error("Invalid postfix expression: not enough operands");
            }
            int leftOperand = stack.top();
            stack.pop();

            int result;
            switch (c) {
                case '+':
                    result = leftOperand + rightOperand;
                    break;
                case '-':
                    result = leftOperand - rightOperand;
                    break;
                case '*':
                    result = leftOperand * rightOperand;
                    break;
                case '/':
                    if (rightOperand == 0) {
                        throw std::runtime_error("Division by zero");
                    }
                    result = leftOperand / rightOperand;
                    break;
                default:
                    throw std::runtime_error("Invalid character in expression");
            }

            stack.push(result);
        }
    }

    // After processing all characters, stack should have exactly one result
    int finalResult = stack.top();
    stack.pop();
    if (!stack.empty()) {
        throw std::runtime_error("Invalid postfix expression: too many operands");
    }
    return finalResult;
}

Notes for Your Implementation #

• Adjust the allowed operators in the switch statement to match whatever your professor specified.
• Add error handling as needed (your professor might expect specific error messages or behavior).
• Test with simple postfix expressions first, like "34+" (should return 7) or "52*3+" (should return 13).

内容的提问来源于stack exchange，提问作者RyanJohnTyler