How can I execute CString as a code line.

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2.33/5 (2 votes)

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I have a mathematical expression. I want to save it in CString. And after that I want to execute that CString as a code line.

Mathematical expression is like = 3 * (var1 + 10) + 100
if var1 > 100 then var1 = 1 else if var1 < 100 var1 = 2.

value of var1 is taken from some CSV / text file.

Suppose if var1 = 90
then CString strExp = 3 * (2 + 10) + 100;
and then I want to execute this CString and result would be saved in another string.

double Result = XXXX strExp XXXXX //whatever operation / parsing
// result = 136

Posted 13-Aug-15 20:15pm

Padmanabh Kulkarni

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3 solutions

Solution 3

XML

template <typename T>
void trig_function()
{
   typedef exprtk::symbol_table<T> symbol_table_t;
   typedef exprtk::expression<T>     expression_t;
   typedef exprtk::parser<T>             parser_t;

   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;
   parser.compile(expression_string,expression);

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

}

Posted 13-Aug-15 21:08pm

Padmanabh Kulkarni

Comments

Padmanabh Kulkarni 14-Aug-15 5:36am

What I understand from above answer is "In MFC it is not possible (with using our regular code / logic / libraries )for this there are 2 options
1) Develop special types representing nodes of expression tree and the tree itself - Sergey Alexandrovich Kryukov "
2) Use third party libraries / scripts - CPallini

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Sergey Alexandrovich Kryukov · Accepted Answer · 2015-08-13T20:33:00

You cannot do it in any simple way. In some interpretive languages, and even in some compiling languages, this is a fundamental feature, but C++ is not one of them.

You will need to create some compiler or interpreter to parse and execute your string. You can start here: http://www.codeproject.com/search.aspx?q=C%2b%2b+expression+parser&doctypeid=1%3b2%3b3%3b13%3b14%3b5[^].

Alternative way would be not using strings at all. You can develop special types representing nodes of expression tree and the tree itself, which your could use to create expressions right in C++ code. In other words, it's possible to develop something similar to .NET lambda expressions. But I guess it would take me writing a whole book to explain it. Your question strongly suggests that you are not ready for such works yet.

—SA

CPallini · Accepted Answer · 2015-08-13T21:01:00

Solution 2

As an alternative, you may embed a script language, like Lua in your application and then let the script engine evaluate expressions (and more complex constructs) for you. We have an article on this very topic: "Integrating Lua into C++" here at Code Project[^].
See also "Evaluating Mathematical Expressions using Lua" at Stack Overflow[^].

Posted 13-Aug-15 21:01pm

CPallini

Comments

CPallini 14-Aug-15 3:12am

That entirely depends on the framework you are using.

Sergey Alexandrovich Kryukov 14-Aug-15 10:31am

Hm... another idea. A 5.
—SA

Albert Holguin 14-Aug-15 11:37am

Embedding a scripting language is the way I would go about this too... not sure I'd use Lua though as there are a variety of heavy lifting mathematically intensive scripting engine options (Octave/Matlab) that would likely support math intensive scripting very well. Of course, I don't have much experience with Lua so that probably makes me a little biased against it. .....+5 for the scripting suggestion though...