评估 PostFix 函数

Question

下面我正在尝试编写一个计算后缀表达式的程序。但是，我注意到我设置要操作的两个元素没有设置为正确的值。我仍在学习 c++ 库中的堆栈功能，所以如果有人能解释为什么会这样，我将不胜感激！

/*Sample output:
Enter an infix or prefix expression:3+4-1
Postfix is: 34+1-
postFix[i]: 3
postFix[i]: 4
element1: 52
element2: 51
val is: 103
postFix[i]: 1
element1: 49
element2: 103
val is: 54
Value is 54*/

int evaluatePostfix (string postFix)
{
     stack<int> operands;

        int length = postFix.length();
        int val = 0;
        for ( int i = 0; i < length; i++ )
        {
                //if the char is a digit push the digit onto the stack
                if( isdigit( postFix[i] ))
                {
                        operands.push( postFix[i] );
                        cout << "postFix[i]: " << postFix[i] << endl;
                }
                else if ( isOperator(postFix[i]) )
                {
                        //
                        //element 1 and element 2 will be the element on the top of the stack
                        //
                        int element1 = operands.top();
                        cout << "element1: " << element1 << endl;
                        operands.pop();
                        int element2 = operands.top();
                        cout << "element2: " << element2 << endl;
                        operands.pop();
                        //
                        //create a switch statement that evaluates the elements based on the operator
                        //
                        switch( postFix[i] )
                        {
                                case '+':
                                        val = element2 + element1;
                                        cout << "val is: " << val << endl;
                                        operands.push(val);
                                        break;
                                case '-':
                                        val = element2 - element1;
                                        cout << "val is: " << val << endl;
                                        operands.push(val);
                                        break;
                                case '*':
                                        val = element2 * element1;
                                        operands.push(val);
                                        break;
                                case '/':
                                        val = element2 / element1;
                                        operands.push(val);
                                        break;
                                default:
                                        return 0;
                        }

                                                                                 }

        }
        return val;
}

Answer 1

问题是您的后缀是一个字符串，当您压入这些字符（有效整数）时，您会得到 ascii 值（例如 51、52）。这些对应于正确的数字（例如 3、4）

您最终得到的值偏移了 48，对应于 0 符号。