Most efficient way to initialize a large block of doubles in C

Question

What's this easiest / most efficient way to initialize these blocks of doubles, preferably at compile time:

#define N 1000
double mul1[N][N] __attribute__ ((aligned (64)));
double mul2[N][N] __attribute__ ((aligned (64)));

They're used for "const" read only test data.

Answer 1

Static array initialisation (which I take it is what you want) is ALWAYS performed at compile time, so you initialise them as you would any other array.

Answer 2

If you need this for only this two arrays I prefer to generate a file filled with the default value from the make file.

If you need something more complicated, you can use Boost.Preprocessor library. You may use BOOST_PP_WHILE and BOOST_PP_ARRAY_PUSH_FRONT.

I think it will work :)

Answer 3

There is a GCC (not standard C!) feature called Designated Initializers

For 1D array it would be just:

double array[N] = {[0 ... (N-1)] = MY_DOUBLE_VALUE};

For 2D a bit trickier:

double array[N][N] = { [0 ... (N-1)] = {[0 ... (N-1)] = MY_DOUBLE_VALUE}};

Answer 4

Easiest and most efficient are sometimes incompatible. How efficient does it need to be?

Easiest is a loop. Are a million assignments in a simple loop really too slow for your purposes?

int i, j;
for (i = 0; i < N; ++i) {
  for (j = 0; j < N; ++j) {
    mul1[i][j] = INITIAL_VALUE_1;
    mul2[i][j] = INITIAL_VALUE_2;
  }
}

Have you timed the simple solution to see if it's fast enough? If not, how much faster does it need to be?