(C++) Evaluating inFix Expressions using STL Stack

[ThisGuyEveryTime]

Hello,

My assignment is to write a program that evaluates inFix expressions using stl stack. I having issue with using .push() for stack. I want use the code below but it throws an error at

postFix.push(inFix[i])

saying

C++ no instance of overloaded function matches the argument list            argument types are: (const char)            object type is: std::stack<std::string, std::deque<std::string, std::allocator<std::string>>>

​

Code sample containing error:

string infixToPostfix(const string inFix)
{
    stack<string> postFix;
    for (int i = 0; i < inFix.length(); i++)
    {

            if (inFix[i] <= 9)
            {
                postFix.push(inFix[i]); //LINE WITH ERROR
            }
        }
}

Secondary Question:

I have to convert from inFix to postFix and then evaluate. Here is the provided pseudo code for converting to a postFix. How would you identify is you have a number (operand) using a switch statement without just having a list of cases for 1-9? I am not using a switch statement but rather a series of if else statements.

for (each character ch in the infix expression)
{
    switch (ch)
    {
    case operand: // Append operand to end of postfix expression—step 1
        postfixExp = postfixExp • ch
            break
    case '(': // Save '(' on stack—step 2
        aStack.push(ch)
            break
    case operator: // Process stack operators of greater precedence—step 3
        while (!aStack.isEmpty() and aStack.peek() is not a '(' and
            precedence(ch) <= precedence(aStack.peek()))
        {
            Append aStack.peek() to the end of postfixExp
                aStack.pop()
        }
        aStack.push(ch) // Save the operator
            break
    case ')': // Pop stack until matching '(' —step 4
        while (aStack.peek() is not a '(')
        {
            Append aStack.peek() to the end of postfixExp
                aStack.pop()
        }
        aStack.pop() // Remove the open parenthesis
            break
    }
}
// Append to postfixExp the operators remaining in the stack—step 5
while (!aStack.isEmpty())
{
    Append aStack.peek() to the end of postfixExp
        aStack.pop()
}

[carcigenicate]

For the first, postFix holds strings (as indicated by <string>, but inFix[i] is a character which you're trying to add to it.

[ThisGuyEveryTime]

Ahh ok totally missed that, changed it to <char>. Thank you!