Question

I am making my own simple drawing engine. I am trying to determine if a variable has been set to a specific value using what I think is called bitwise comparison but I maybe wrong.

I’ve always been a bit confused about what the following is and how I use it:

int DRAW_REPEAT_X = 70001; // I have a feeling I should make this value binary instead of a unique number, ie, 0
int DRAW_REPEAT_Y = 70002; // I have a feeling I should make this value binary instead of a unique number, ie, 2
int drawMethod    = DRAW_REPEAT_X | DRAW_REPEAT_Y; // this means I want to repeat an image on both the x and y axis doesn't it?

// Now I want to check if drawMethod has DRAW_REPEAT_X set: this is where I struggle to know how to check this
// Is the following correct?
if (drawMethod && DRAW_REPEAT_X) {
  // the user wants me to repeat an image along the x axis
}

// Now I want to check if drawMethod has DRAW_REPEAT_Y set: this is where I struggle to know how to check this
if (drawMethod && DRAW_REPEAT_Y) {
  // the user wants me to repeat an image along the x axis
}

Is the following code correctly checking if DRAW_REPEAT_X is set? It always returns 1 in my anding check.

EDIT
And to check whether both bits are set do I do this?

if (drawMethod & DRAW_REPEAT_X & DRAW_REPEAT_Y) {
   // both set
}

// OR

if (drawMethod & DRAW_REPEAT_X && drawMethod & DRAW_REPEAT_Y) {
   // both set
}

Answer 1

No it isn’t, you should use the bitwise AND operator instead – & and set the flags as binary values – your intuition is correct on that side.

A common trick to setting specific bits is using the shift operator:

int DRAW_REPEAT_X = 0x1 << 0;  //first bit set to 1, others 0
int DRAW_REPEAT_Y = 0x1 << 1;  //second bit set to 1, others 0

and check the int as

if (drawMethod & DRAW_REPEAT_X)  //check it that particular flag is set, ignore others
{
}