Question

I saw the code below in this answer to What is your favorite C programming trick?

What does this piece of code do? How is it useful?

double normals[][] = {
    #include "normals.txt"
};

Answer 1

This uses the preprocessor to initialize an array.

It’s basically inlining the code that populates the array with values, by reading it from an external text file. Note that this requires that “normals.txt” be filled with values that match the C syntax, ie:

{0, 0, 1},
{0, 1, 0},
{1, 0, 0}