Question re setting up an array of const

1

Hi, I have a project which uses a shaft encoder to monitor angle positions from 0 to 360 degrees. As part of the project I’ve set up an array of int and some pointers to facilitate a circular linked list. It all works fine, but the array should really be const.

int A[360];                  
int *currentAzPtr = &A[261]; // pointer to the value 261 which is the dome park position
int *targetAzPtr   = &A[261];

As you can see, there are 360 values, so my question is how to initialise the array as const type?

Currently i just have this for loop:

void initialiseList()
{

  for (int i = 0; i < 360; i++)
  {
    A[i] = i;
  }
}

So it’s not really a problem (as I have a non const version working) and not a PIO problem either, but just for the sake of keeping the data types in order, I thought I’d ask.

I did some web search but didn’t find anything I could use in AVR.
thanks for help.

2

C++ has template magic and constexpr ways of computing the array contents at compile-time so that they are const, see metaprogramming - Programmatically create static arrays at compile time in C++ - Stack Overflow.

In your case I can suggest to either

  • optimize away the array completely… replace A[i] = i for all i and the “dome park position” and whatever targetAzPtr represents, can’t that be saved as just a number instead of a pointer?
  • if only numbers 0-359 are present, use a smaller datatype like uint16_t (or int16_t if negative numbers are needed). Depending on the architecture where int is 32-bits, that would halve the memory usage
  • use a few commands in a Python shell to auto-generate the array…
>>> s = "const int A[360] = { "
>>> s += ", ".join([str(i) for i in range(360)])
>>> s += "};"
>>> s

const int A[360] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359};
  • careful if working with AVR, you have to use PROGMEM in the data declaration and special functions for loading the data – other architectures like ARM do not need this.
  • try on of the advanced C++ template or constexpr tricks above if the project and compiler is capable of such a high C++ version.
3

great, thanks for all this, much appreciated.