Simple C array declaration / assignment question

Question

In higher level languages I would be able something similar to this example in C and it would be fine. However, when I compile this C example it complains bitterly. How can I assign new arrays to the array I declared?

int values[3];

if(1)
   values = {1,2,3};

printf("%i", values[0]);

Thanks.

Answer 1

You can only do multiple assignment of the array, when you declare the array:

int values[3] = {1,2,3};

After declaration, you'll have to assign each value individually, i.e.

if (1) 
{
  values[0] = 1;
  values[1] = 2;
  values[2] = 3;
}

Or you could use a loop, depending on what values you want to use.

if (1)
{
  for (i = 0 ; i < 3 ; i++)
  { 
    values[i] = i+1;
  }
}

Answer 2

you can declare static array with data to initialize from:

static int initvalues[3] = {1,2,3};
…
if(1)
    memmove(values,initvalues,sizeof(values));

Answer 3

 //compile time initliaztion
 int values[3] = {1,2,3}

//run time assignment
 value[0] = 1;
 value[1] = 2;
 value[2] = 3;

Answer 4

In C99, using compound literals, you could do:

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

or

int* values = (int[3]){1, 2, 3};

Answer 5

It is also possible to hide the memcpy by using the compiler's block copy of structs. It makes the code ugly because of all the .i and i: but maybe it solves your specific problem.

typedef struct {
    int i[3];
} inta;

int main()
{
    inta d = {i:{1, 2, 3}};

    if (1)
        d = (inta){i:{4, 5, 6}};

    printf("%d %d %d\n", d.i[0], d.i[1], d.i[2]);

    return 0;
}

Answer 6

This works and optimizes better under gcc with -O3 (the compiler completely removes the code), whereas the memcpy forces the memory to be copied in all cases.

template <typename Array>
struct inner
{
    Array x;
};


template <typename Array>
void assign(Array& lhs, const Array& rhs)
{
    inner<Array>& l( (inner<Array>&)(lhs));
    const inner<Array>& r( (inner<Array>&)(rhs));
    l = r;
}

int main()
{
    int x[100];
    int y[100];

    assign(x, y);
}