Setting multiple array values after declaration

Question

Is there any way to set the values in an array after it has been declared?

unsigned char values[16];
// Some other code to decide what to put into the values array
values = {0xAA,0xBB,0xCC,0xDD};

I saw this solution but it is for a C++ implementation. I'm trying to be efficient on memory and avoid a memcpy as well as I don't always need to fill the entire array like in the example above. So far this is the only method that I know works but its very kludgy.

unsigned char values[16];
// Some other code to decide what to put into the values array
values[0] = 0xAA;
values[1] = 0xBB;
values[2] = 0xCC;
values[3] = 0xDD;

memcpy(values, (int[]){0xAA, 0xBB, 0xCC, 0xDD}, 4*sizeof *values); see ideone.com/9ruF7z — pmg
– pmg, Commented Mar 17, 2022 at 20:34
you are efficient when using memcpy()... not when avoiding it :) — pmg
– pmg, Commented Mar 17, 2022 at 20:39
@pmg OP's array is an unsigned char array, so a memcpy from an int array won't work. — user3386109
– user3386109, Commented Mar 17, 2022 at 21:14
My bad @user3386109... but the point is still valid for unsigned char as long as you change the type of the unnamed object. Thank you! — pmg
– pmg, Commented Mar 17, 2022 at 21:18
@pmg Yes sorry, that comment was more for the benefit of the OP, who would be disappointed if they tried it without fully understanding it. I agree that your suggestion is valid. — user3386109
– user3386109, Commented Mar 17, 2022 at 21:22

marcluque · Accepted Answer · 2022-03-18 07:13:16Z

0

Maybe something like this would do:

unsigned char* values;
// Some other code to decide what to put into the values array
values = (unsigned char[]) {0xAA,0xBB,0xCC,0xDD};

Appendix: Note that it really depends on what you need. Since values is not an array anymore in this case.

edited Mar 18, 2022 at 7:13

answered Mar 17, 2022 at 23:54

marcluque

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2 Comments

Lundin Over a year ago

This possibly enforces an extra level of indirection though, so this could be less efficient, in case the compiler can't optimize away the values variable.

marcluque Over a year ago

@Lundin thanks for the hint. I was assuming that any compiler would likely make this to a memcpy, making it equivalent to your answer. Like in the case of, e.g., assigning structs. Maybe I'm confusing or missing something. Could you elaborate what this indirection would be?

Lundin · Accepted Answer · 2022-03-18 07:47:04Z

0

Either use memcpy or assign each index manually, there is no other sensible solution. You don't have to memcpy the whole array at once, just restrict it to some indices:

memcpy(&values[0], (unsigned char[]){0xAA,0xBB,0xCC,0xDD}, 4);

where values[0] can easily be changed to any values[i]. This is also safe as far as alignment is concerned. Note that there should be no benefit in terms of efficiency between this code and assigning all 4 indices manually. Strive to make the code as readable as possible - if you think compound literals are hard to read for example, then stick to what you already have.

answered Mar 18, 2022 at 7:47

Lundin

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