How to initialize only few elements of an array with some values?

Solution 1:

Is it possible to skip this values and only assign the values 1, 2 and 3?

In C, Yes. Use designated initializer (added in C99 and not supported in C++).

int array[12] = {[0] = 1, [4] = 2, [8] = 3};  

Above initializer will initialize element 0, 4 and 8 of array array with values 1, 2 and 3 respectively. Rest elements will be initialized with 0. This will be equivalent to

 int array[12] = {1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0};   

The best part is that the order in which elements are listed doesn't matter. One can also write like

 int array[12] = {[8] = 3, [0] = 1, [4] = 2}; 

But note that the expression inside [ ] shall be an integer constant expression.

Solution 2:

Here is my trivial approach:

int array[12] = {0};
array[0] = 1; array[4] = 2; array[8] = 3;

However, technically speaking, this is not "initializing" the array :)

Solution 3:

An alternative way to do it would be to give default value by memset for all elements in the array, and then assign the specific elements:

int array[12];
memset(array, 0, sizeof(int) * 12); //if the default value is 0, this may not be needed
array[0] = 1; array[4] = 2; array[8] = 3;