#include <iostream>
#include <string>
#include <stack>
using namespace std;
float postix_evalute(string expr)
{
    stack<float> stk;

    for (int x = 0; x < expr.length(); x  )
    {
        if (isdigit(expr[x]))
        {
            stk.push((expr[x] - '0'));
        }
        else
        {
            float val;
            float op2 = stk.top();
            stk.pop();
            float op1 = stk.top();
            stk.top();
            switch (expr[x])
            {
            case ' ':
                val = op1   op2;
                break;
            case '-':
                val = op1 - op2;
                break;
            case '*':
                val = op1 * op2;
                break;
            case '/':
                val = op1 / op2;
                break;
            }
            stk.push(val);
        }
    }
    return stk.top();
}
int main()
{
    string line;
    cout << "The Value Of experssion" << endl;
    cin >> line;
    cout << postix_evalute(line) << endl;
    return 0;
}

When I Enter This Expression for example: input=213 output=4 but i expect to get 24 which equal (21 3) so what should i do to make operation with more than on digit which equal in infix -> 21 3 anyone have anyidea how to do that

CodePudding user response：

You need a way to differentiate the tokens of your postfix expressions. For example, if the input is 213 , a parser would typically read that as 213 and . For these simple expressions, you could separate the different tokens with whitespaces, as in 21 3 . Then, having that input in a string, you could read each token with an input string stream.

float postix_evalute(std::string input)
{
    std::stack<float> stk;

    std::istringstream iss{input};
    std::string token{};
    while (not iss.eof())
    {
        iss >> token;

        try
        {
            stk.push(std::stof(token));
        }
        catch (const std::invalid_argument& ex)
        {
            assert(stk.size() >= 2);
            float op2 = stk.top(); stk.pop();
            float op1 = stk.top(); stk.pop();

            float val{};
            if (token == " ") { val = op1   op2; }
            else if (token == "-") { val = op1 - op2; }
            else if (token == "*") { val = op1 * op2; }
            else if (token == "/") { val = op1 / op2; }
            else { throw std::runtime_error{std::string{"unknown token: "   token}}; }

            stk.push(val);
        }
    }

    return stk.top();
}

Demo 1: using 21 3 as an input.
Demo 2: using 21 3 # as an input.