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I would like to integrate ExprTk into a C++ (open source) home automation application I am building.

I have a list of values that can change dynamically, for example a switch position (BOOL) or a temperature value (double) and I would like to be able to evaluate an expression using those values.

I already keep my own symbol table of variables? many of them are recomputed in real time.

for example I would like to evaluate

"OUTSIDE_TEMP > 60" or something like "TODAY_MINS > 3600 && RELAY_1 == TRUE"

do I need to specify all the variable names each time I run the parserpile() or can I have the values looked up and then evaluated at expression.value() time?

what is considered the canonical way to do this? should I create a LOOKUP function and pass the variable name in as a string? ie "LOOKUP("OUTSIDE_TEMP") > 60" ?

I also thought about running the parser without defining any symbols and making a list of needed symbols from the error? I am not sure if that buys me anything, because an unclear if I can serialize the ExprTk context for later?

many thanks

This works, but I'd rather not have to specify all the variables each time.. can I save the context?

void test1() {
    
    using T              = double;
    typedef exprtk::symbol_table<T> symbol_table_t;
    typedef exprtk::expression<T>   expression_t;
    typedef exprtk::parser<T>       parser_t;
    
    symbol_table_t immutable_symbol_table( symbol_table_t::symtab_mutability_type::e_immutable);
    
    double outside_temp = -4;
    
    immutable_symbol_table.add_variable("OUTSIDE_TEMP" , outside_temp );
    
    expression_t expression;
    expression.register_symbol_table(immutable_symbol_table);
    
    parser_t parser;
    
    if (!parserpile("OUTSIDE_TEMP > 60.0", expression))
    {
        cout << "Error: " << parser.error() << endl;
    }
    else
    {
        outside_temp = 99.0;
        
        cout << expression.value() <<endl;
    }
}

I would like to integrate ExprTk into a C++ (open source) home automation application I am building.

I have a list of values that can change dynamically, for example a switch position (BOOL) or a temperature value (double) and I would like to be able to evaluate an expression using those values.

I already keep my own symbol table of variables? many of them are recomputed in real time.

for example I would like to evaluate

"OUTSIDE_TEMP > 60" or something like "TODAY_MINS > 3600 && RELAY_1 == TRUE"

do I need to specify all the variable names each time I run the parserpile() or can I have the values looked up and then evaluated at expression.value() time?

what is considered the canonical way to do this? should I create a LOOKUP function and pass the variable name in as a string? ie "LOOKUP("OUTSIDE_TEMP") > 60" ?

I also thought about running the parser without defining any symbols and making a list of needed symbols from the error? I am not sure if that buys me anything, because an unclear if I can serialize the ExprTk context for later?

many thanks

This works, but I'd rather not have to specify all the variables each time.. can I save the context?

void test1() {
    
    using T              = double;
    typedef exprtk::symbol_table<T> symbol_table_t;
    typedef exprtk::expression<T>   expression_t;
    typedef exprtk::parser<T>       parser_t;
    
    symbol_table_t immutable_symbol_table( symbol_table_t::symtab_mutability_type::e_immutable);
    
    double outside_temp = -4;
    
    immutable_symbol_table.add_variable("OUTSIDE_TEMP" , outside_temp );
    
    expression_t expression;
    expression.register_symbol_table(immutable_symbol_table);
    
    parser_t parser;
    
    if (!parserpile("OUTSIDE_TEMP > 60.0", expression))
    {
        cout << "Error: " << parser.error() << endl;
    }
    else
    {
        outside_temp = 99.0;
        
        cout << expression.value() <<endl;
    }
}

• c++
• exprtk

Share Improve this question Follow asked Feb 24 at 5:01 vinthewrenchvinthewrench 1111 silver badge44 bronze badges 4

• Yes, reusing the symbol table between runs seems like a good approach. What happens if you try? – Botje Commented Feb 24 at 9:03
• My concern is that I don’t want to query every variable from my system and inserted it into the simple table each time I want to do an evaluation. What I’d rather do is have something call me and ask for the symbols value. I’m thinking that this is gonna have to be a function Wrapped around the symbol‘s name. Specifically, I’m thinking I’m gonna have to write a generic function where I pass in a string and return a value. – vinthewrench Commented Feb 24 at 16:53
• You can also configure an unknown_symbol_resolver on the parser, but it should resolve to a memory location. – Botje Commented Feb 24 at 20:18
• the unknown_symbol_resolver might be what I want, I don't know how I missed this. now I am trying to get my around the example code.. sigh this is great stuff but the examples could have been simpler. Many thanks – vinthewrench Commented Feb 25 at 0:47

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2 Answers 2

Sorted by: Reset to default 2

Initially, ExprTk is a Parse Once Evaluate Many times math engine.

Meaning once you have registered your variables with a symbol table instance and that symbol table instance is registered to an expression instance which is then used to compile a string form of an expression. Then all you need to do is update the variables used in the expression as you'd normally do in code, then call the value method of the compiled expression instance. It will ensure that only the current values stored in the variables are used.

I would recommend spending a few minutes browsing the examples, such as exprtk_simple_example01.cpp:

using T              = double;
using symbol_table_t = exprtk::symbol_table<T>;
using expression_t   = exprtk::expression<T>;
using parser_t       = exprtk::parser<T>;

const std::string expression_string =
   "clamp(-1.0, sin(2 * pi * x) + cos(x / 2 * pi), +1.0)";

T x;

symbol_table_t symbol_table;
symbol_table.add_variable("x",x);
symbol_table.add_constants();

expression_t expression;
expression.register_symbol_table(symbol_table);

parser_t parser;
parserpile(expression_string,expression);

for (x = T(-5); x <= T(+5); x += T(0.001))
{
   const T y = expression.value();
   printf("%19.15f\t%19.15f\n", x, y);
}

In the above a variable x of type double is registered with a symbol table (aka object binding). Then an expression in string form using the variable x is compiled. Later on in the subsequent for loop, the variable x is modified as one normally would using C++, after each modification the expression is evaluated and the value returned is printed to stdout.

All one has to decide is where they are going to store their symbol_table and expression instances, eg: as members of a class or somewhere else etc

It is all very simple, and there is nothing complicated to reason about.

I solved my problem above by using the unknown_symbol_resolver. this allows me to bind new variable names on the fly depending on what the equation asks for.

ExprTk is very powerful, it does take some time to walk through the tests and the documentation to find a working sample of how do do things that the author didn't intend. All and All I am very happy with the results.

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