error: expression must have pointer-to-object type

unsigned int v = w;
v[i] = 1; // error, v is not an array

This is not correct because v is not an array. The solution might be using a std::bitset or simply shifting bits and using some bit manipulation - this would be faster.

unsigned int setBit(unsigned int w,unsigned int i) {
    unsigned int v = ( w |= 1 << i);
    return v;
}

usage:

int main(int argc, char *argv[])
{
    unsigned int in = 0;
    unsigned int res = setBit(in,1); // set 1st bit in 0 to 1, results in 2
    return 0;
}

meaning of unsigned int v = ( w |= 1 << i);

| - the bitwise OR

<< - the bitwise shift

v = ( w |= 1 << i) is the same as v = ( w = w | 1 << i) so it means: v is equal to (take w and OR it with 1 left shifted by i, and assign this to w)

about C/C++ bit manipulation