Declaring and initializing arrays in C

Question

Is there a way to declare first and then initialize an array in C?

So far I have been initializing an array like this:

int myArray[SIZE] = {1,2,3,4....}; 

But I need to do something like this

int myArray[SIZE];  myArray = {1,2,3,4....}; 

Answer 1

In C99 you can do it using a compound literal in combination with memcpy

memcpy(myarray, (int[]) { 1, 2, 3, 4 }, sizeof myarray); 

(assuming that the size of the source and the size of the target is the same).

In C89/90 you can emulate that by declaring an additional "source" array

const int SOURCE[SIZE] = { 1, 2, 3, 4 }; /* maybe `static`? */ int myArray[SIZE]; ... memcpy(myarray, SOURCE, sizeof myarray); 

Answer 2

No, you can't set them to arbitrary values in one statement (unless done as part of the declaration).

You can either do it with code, something like:

myArray[0] = 1; myArray[1] = 2; myArray[2] = 27; : myArray[99] = -7; 

or (if there's a formula):

for (int i = 0; i < 100; i++) myArray[i] = i + 1; 

The other possibility is to keep around some templates that are set at declaration time and use them to initialise your array, something like:

static const int onceArr[]  = {  0,  1,  2,  3,  4,..., 99}; static const int twiceArr[] = {  0,  2,  4,  6,  8,...,198}; : int myArray[7]; : memcpy (myArray, twiceArr, sizeof (myArray)); 

This has the advantage of (most likely) being faster and allows you to create smaller arrays than the templates. I've used this method in situations where I have to re-initialise an array fast but to a specific state (if the state were all zeros, I would just use memset).

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You can even localise it to an initialisation function:

void initMyArray (int *arr, size_t sz) {     static const int template[] = {2, 3, 5, 7, 11, 13, 17, 19, 21, ..., 9973};     memcpy (arr, template, sz); } : int myArray[100]; initMyArray (myArray, sizeof(myArray)); 

The static array will (almost certainly) be created at compile time so there will be no run-time cost for that, and the memcpy should be blindingly fast, likely faster than 1,229 assignment statements but very definitely less typing on your part :-).