Simple Math Expression in ExprTk

Question

I am attempting to use a simple expression such as the following and the result should be that the value of z becomes 1. However nothing seems to be happening any suggestions on how I could resolve this issue ?

template<typename t>
void MyTestB()
{

    t x = 1.0;
    t z = 0;

    std::string e = "if((x + 2) == 3){z=1;}";
    exprtk::symbol_table<t> symbol_table;
    symbol_table.add_variable("x",x);
    symbol_table.add_variable("z",z);

    exprtk::expression<t> expression;
    expression.register_symbol_table(symbol_table);


    exprtk::parser<t> parser;

    parser.compile(e,expression);
    t y = expression.value();
    std::cout << z;
}

The program does finish however at y = NAN (which is understandable because expression is a conditional statement) However z still remains 0. I was expecting it to become 1

Citizenship

"nothing seems to be happening" is a somewhat vague problem description. How do you call this function? Do you get wrong outpu or no output at all? Does the program finish or hang? – Coronation

Does it work without templates? (e.g. fix t as int) – Cervelat

Just glancing at the examples online, it looks like your string should look like: "if((x+2) == 3, z := 1, 0)". Have you tried something like that? – Bevins

@Taylor yes that did the trick. Could you put that in the answe – Citizenship

Answer 1

Looking at the examples, it appears that if statements should have the form:

if (condition, expression if true, expression if false)

Also, assignment uses := instead of just =. So you should use the string:

if((x + 2) == 3, z := 1, 0)

Bevins

Answer 2

The expression when written out with proper indentation like so:

if( (x + 2) == 3 )
{
  z=1;
}

The issue is the value of the consequent statement: "z=1".

The = here is not assignment but rather equality (aka '==' ).

Hence the actual result of the if statement when the condition is true is false which converts to zero.

Note: Section 12.3 of the readme documentation details the how the conditional statements operate.

Hospice