Some quirky C++ statements.

AlexBB · 01-09-2016, 05:12 PM

Hi there. My environment is Linux Ubuntu 14.04. I am trying to adjust a C++ Gauss-Legendre Quadrature (one of hundreds of implementations) to my project. The code can be had at this link:

http://www.holoborodko.com/pavel/num...l-integration/

There are a couple of variants there, to get the code I use, one has to click on this link (download):

Gauss-Legendre Quadrature.zip in the section Gauss-Legendre Quadrature for C/C++

I tried to include one of the files: gauss_legendre.c into my project file and it did not compile with a bunch of compilation errors. I look at some statements there and hardly understand how they work.

(1) Statement on line 455:

Code:

x0 = cos(PI*(double)((i<<2)-1)*t1)*t0;

The error:

invalid type argument of unary * (have double)

Is this statement a bitwise shift to the left?

(2) I also need to understand this code (lines 63-65):

Code:

#ifndef FABS
  #define FABS(a) ((a)>=0?(a):-(a))
#endif

I will appreciate any comment.

Thanks, - Alex

norobro · 01-10-2016, 03:35 PM

Hi Alex

Quote:

Is this statement a bitwise shift to the left?

Yes. In effect multiplying i by 2^2.

Code:

#ifndef FABS   // if FABS is not defined, define it.
  #define FABS(a) ((a)>=0?(a):-(a))     
#endif

Everywhere the preprocessor finds FABS(a) it will substitute ((a)>=0?(a):-(a))
?: is the conditional operator. FABS(a) determines the absolute value of a.

To illustrate, run the preprocessor (gcc -E fabs.c) on the following:

Code:

#ifndef FABS
	#define FABS(a) ((a)>=0?(a):-(a))
#endif

int main()
{
    FABS(-2);
}

I assume the author uses that code, to avoid function calls to pow() and abs(), in the interest of processing speed.

Regarding the error, you haven't supplied enough information. Can you show us how you are implementing the code?