Parsing logic expressions in strings with known variables - c++

Question

What I want to do is create a function that would take an input string with 1-3 variables (and it would require at least one), then parse that input for an expression. This expression would then be considered logical and return true or false.
For example,
"X == Y && 0.1 <= X < 1.0 && Z > 0.0"
Then first the variables in that string would be iterated through and replaced with actual values. After all of the variables have been replaced the modified string would be like this:
"1.0 == 1.0 && 0.1 <= 1.0 < 1.0 && 0.6 > 0.0"
Then that string would be logically tested. If it is true the function would return true, else false.
I was thinking the function would be something like this:
IsLogical(string, float X, float Y, float Z)
I've been searching for the past few hours for ways to do this and all I have found were arithmetic math equation parsers. So far all I have got done is the part where the variables are replaced with the three input values.
So if anyone is willing to help, can you tell me how it would be done? Showing me examples would definitely be great too.
This function can't use any external resources like boost or embedded Lua.
EDIT: This should include all the basic comparison operators in C++, such as less than, more than, less than or equal to, more than or equal to, equal to, not equal to, and, or, parenthesis. It would all be in C++ format. If the format of the input isn't correct the function would automatically return false.

Answer 1

This is a wild stab at your problem. I'm not going to code it in detail, and I'm not focusing on language semantics, perfect compilation etc.
Wrap your variables with a new type (ExpresionVar). Register each variable with an ExpressionBuilder, that contains logical operators registered in the beginning of the program. Refer to the comments (psuedo code) in result...
ExpressionVar
{
float value_;
std::string name_;
};
LogicalOp
{
virtual std::string id() const = 0;
virtual bool operator()(
const ExpressionVar& lhs, const ExpressionVar& rhs) const = 0;
};
IsEqual : LogicalOp
{
std::string id() const override{ return "=="; }
bool operator()(
const ExpressionVar& lhs, const ExpressionVar& rhs) const override
{
returns lhs.value_ == rhs.value_;
}
};
//etc...
ExpressionBuilder
{
void registerVar(std::initializer_list<const ExpressionVar&>);
static void registerLogical(LogicalOp&&);
bool result(const std::string& expression) const
{
//run through expression.
// Search for known ExpressionVariable (EV), if variable found,
// store as lhs, else throw.
// Then search for LogicalOp until found, if another EV found
// throw, else store op, whereafter searching of rhs, etc...
}
};
bool IsLogical(
const std::string& input,
std::initializer_list<ExpressionVar> variables)
{
//Logical ops registered prior...
return ExpressionBuilder().registerVar(variables).result(input);
}