Question

float Calculate(const string &query)
{
        std::cout << "Query: " << query << "\n";
        unsigned int size = query.length();
        char stack[70];
        float res;
        int m = 0;

        for (int i = 0; i < size; i++)
        {
                if (query[i] >= '0' && query[i] <= '9')
                {
                        stack[m] = query[i] - '0';
                        m++;
                        continue;
                }

                switch (query[i])
                {
                        case '+':
                        {
                                res = stack[m - 2] + stack[m - 1];
                                break;
                        }
                        case '-':
                        {
                                res = stack[m - 2] - stack[m - 1];
                                break;
                        }
                        case '*':
                        {
                                res = stack[m - 2] * stack[m - 1];
                                break;
                        }
                        case '/':
                        {
                                res = stack[m - 2] / stack[m - 1];
                                break;
                        }
                }

                    stack[m - 2] = res;
                m--;
                cout << "RES: " << res << "\n";
        }

        return res;
}

It calculates reverse polish notation.

When I call something like: Calculate("11+") it returns right result: 2.

But, when I pass a variable after getting of RPN string:

string inputStr;
string outputStr;

cout << "Put exercise\n";
getline(std::cin, inputStr);

outputStr = GetRPN(inputStr);
cout << "Output str :" << outputStr << ":\n";

float res = Calculate(outputStr);
std::cout << res << "\n";

So, when I input string: 1+1, function GetRPN returns 11+ and I see that in second cout. But result is 0!

What could it be?

---

string GetRPN(string input)
{
    vector <char> operation;
    string outputStr;      //output string, keep RPN
    int stack_count = 0;

    for(int i = 0; i < input.length(); i++)
    {
        if(input[i] >= '0' && input[i] <= '9')
        {
            outputStr += input[i];
        }
        else
        {
            if(operation.empty())
            {
                operation.push_back(input[i]);
                stack_count++;
            }
            else if(operation[stack_count - 1] == '+' || operation[stack_count - 1] == '-')
            {
                operation.push_back(input[i]);
                stack_count++;
            }
            else if ((operation[stack_count - 1] == '*' || operation[stack_count - 1] == '/') && (input[i] == '*' || input[i] == '/'))
            {
                outputStr += operation[stack_count - 1]; // move mark of operation to output str
                operation.pop_back(); // delet last element from vector
                operation.push_back(input[i]);// plus new operation mark to vector
                stack_count++;
            }
            else if (operation[stack_count - 1] == '*' || operation[stack_count - 1] == '/')
            {
                outputStr += input[i];
            }
        }
    }

    for(int i = operation.size(); i >= 0; i--)
    {
        outputStr += operation[i]; // move all operation marks to otput str
    }

    return outputStr;
}

Answer 1

Your cycle here

for(int i = operation.size(); i >= 0; i--)
{
    outputStr += operation[i]; // move all operation marks to otput str
}

does not make any sense. You are obviously attempting to access the vector at invalid index. It is illegal to access the element at operation[i] when i is equal operation.size(). The index is out of range.

Any self-respecting implementation would immediately report this problem with an assertion. In any case, as I said in the comment, the problems like that are resolved by debugging the code. Why are you asking other people to debug your code instead of doing it yourself?