Graphing Calculator Parsing

Question

This is for a school project where I am supposed to create a calculator that will graph functions. The graphing works, however my function that actually turns functions (the math kind) into data is not working for two major test case types, and I have no clue how to fix this. The test cases are input = -x^2 and input = x^2+2x+1, both of which when think through what I have written should work. I have been stumped by this for a while now, and I have not found a solution that does not mess up other equations.

double Solver(string input, double xValue)
{
int pos1;
int pos2;
int symbolpos;

string substring;

double value1;
double value2;

int count;

while (input.find_first_of(" ") != string::npos)
{
    input.erase(input.find_first_of(" "), 1);
}

while (input.find("(") != string::npos && input.find(")") != string::npos)
{
    pos1 = input.find_first_of("(");

    if (pos1 > 0 && input[pos1 - 1] != '-')
    {
        input.insert(pos1, "*");
        pos1++;
    }
    if (pos1 == 2 && input[0] == '-')
    {
        input.insert(pos1 - 1, "1");
        pos1++;
    }
    pos2 = pos1;
    count = 1;

    while (count != 0)
    {
        pos2++;
        if (input[pos2] == '(')
            count++;
        if (input[pos2] == ')')
            count--;
    }

    substring = to_string(Solver(input.substr(pos1 + 1, pos2 - pos1 - 1), xValue));
    input.replace(pos1, pos2 - pos1 + 1, substring);
}

while (input.find("x") != string::npos)
{
    pos1 = input.find_first_of("x");
    if (pos1 > 0)
    {
        input.insert(pos1, "*");
        pos1++;
    }
    if (pos1 == 2 && input[0] == '-')
    {
        input.insert(pos1 - 1, "1");
        pos1++;
    }
    input.replace(pos1, 1, to_string(xValue));
}

while (input.find("--") != string::npos)
{
    input.replace(input.find("--"), 2, "+");
}

input = parsing(input, '^');
input = parsing(input, '*');
input = parsing(input, '/');
input = parsing(input, '+');
input = parsing(input, '-');


return stod(input);
}


string parsing(string tInput, char searchChar)
{
string input = tInput;
int pos1, pos2, symbolpos;
double value1, value2;
string substring;

while (input.substr(1).find_first_of(searchChar) != string::npos)
{
    symbolpos = input.find_last_of(searchChar);
    pos1 = 0;
    for (int a = 0; a < symbolpos - 1; a++)
    {
        if (input[a] == '^')
            pos1 = a++;
        if (input[a] == '*')
            pos1 = a++;
        if (input[a] == '/')
            pos1 = a++;
        if (input[a] == '+')
            pos1 = a++;
        if (input[a] == '-')
            pos1 = a;
    }

    pos2 = input.size();
    for (int a = input.size(); a > symbolpos + 1; a--)
    {
        if (input[a] == '^')
            pos2 = a--;
        if (input[a] == '*')
            pos2 = a--;
        if (input[a] == '/')
            pos2 = a--;
        if (input[a] == '+')
            pos2 = a--;
        if (input[a] == '-')
            pos2 = a--;
    }
    value1 = stod(input.substr(pos1, symbolpos - pos1));
    value2 = stod(input.substr(symbolpos + 1, pos2 - symbolpos));
    if (searchChar == '^')
        substring = to_string(pow(value1, value2));
    if (searchChar == '*')
        substring = to_string(value1 * value2);
    if (searchChar == '/')
        substring = to_string(value1 / value2);
    if (searchChar == '+')
        substring = to_string(value1 + value2);
    if (searchChar == '-')
        substring = to_string(value1 - value2);
    input.replace(pos1, pos2 - pos1, substring);
}

return input;

}

Thanks for your help!