如何将if命令的条件语句保存为字符串？

Question

I have a numerical algorithm. I create a set of x, and y coordinates for points in R^2. Then, I use a rule to see which points are suitable for my purpose, a simple if statement like below:

int N = 0;
for (int i1 = -xN; i1 < xN + 1; i1++)
{
    for (int i2 = -yN; i2 < yN + 1; i2++)
    {
        double x = dist(0) * i1;
        double y = dist(1) *i2;
        
        if (sqrt(pow(x, 2) + pow(y, 2)) > 0.8 && abs(x) < 3.1 && abs(y) < 3.91)
        {
            Points.row(N) = trans(vec{ x, y});
            N = N + 1;
        }
    
    }
}

Since I change the rule (that is, sqrt(pow(x, 2) + pow(y, 2)) > 0.8 && abs(x) < 3.1 && abs(y) < 3.91) in each example, I was wondering if I could store it as a string variable and then save it with other parameters of the example in a single text file. something like this:

ofstream myfile;
myfile.open("resultEXAMPLE2D01.txt");
myfile << "xN = [" << xN << "];" << endl;
myfile << "N = [" << output.N << "];" << endl;
myfile << "% Condition is " << cond << endl;
myfile.close();

where "cond" should be "sqrt(pow(x, 2) + pow(y, 2)) > 0.8 && abs(x) < 3.1 && abs(y) < 3.91" in this case. So, in "cond" x and y are not variables, I just look at the whole thing as a text/string. but then inside the if statement I want to evaluate some criteria for x and y, so this "cond" should be a function with two doubles as input and a bool as output. Is there some straight forward way to have this kind of type conversion?

Answer 1

Looks like you need Stringification . You can read more about it here:

• https://gcc.gnu.org/onlinedocs/gcc-4.8.5/cpp/Stringification.html
• Stringification of a macro value

So, you can define macros and pass to it your condition. Inside macros you can put your computation code, use passed condition and also stringify your condition (place #cond for cond paramater) for output to somewhere (file, console, ...).

Here is example:

int N = 0;

#define Calculate(cond) \
{ \
    N = 0; \
    for (int i1 = -xN; i1 < xN + 1; i1++) \
    { \
        for (int i2 = -yN; i2 < yN + 1; i2++) \
        { \
            double x = dist(0) * i1; \
            double y = dist(1) *i2; \
            \
            if (cond) \
            { \
                Points.row(N) = trans(vec{x, y}); \
                N = N + 1; \
            } \
     \
        } \
    } \
    \
    std::cout << "Condition: " << #cond << std::endl; \
}


int main()
{
    Calculate(sqrt(pow(x, 2) + pow(y, 2)) > 0.8 && abs(x) < 3.1 && abs(y) < 3.91)
    Calculate(x > 0)

    return 0;
}

Program output:

Condition: sqrt(pow(x, 2) + pow(y, 2)) > 0.8 && abs(x) < 3.1 && abs(y) < 3.91
Condition: x > 0