How to move relatively to the angle? - c++

Question

I keep looking up on the internet how to move from point A to point B on an angle with a specified distance. When I tried to code it, however, the camera just gets messed up and I'm moving in random directions. I am using SDL/OpenGL with c++ and this is my player function. Right now, I'm trying to get the player to move forwards along the angle.
void player_action()
{
if (SDL_PollEvent(&event))
{
switch (event.type)
{
case SDL_MOUSEMOTION:
{
player.rotxl = (screen->w)/2;
player.rotxd = event.motion.x - player.rotxl;
player.rotx = player.rotx + (player.rotxd/4);
if (player.rotx < 0)
{
player.rotx = player.rotx + 360;
};
if (player.rotx > 360)
{
player.rotx = player.rotx - 360;
}
};
break;
case SDL_KEYDOWN:
switch (event.key.keysym.sym)
{
case SDLK_w:
{player.zvel = 8.0f;}; break;
case SDLK_a:
{player.xvel = 8.0f;}; break;
case SDLK_s:
{player.zvel = -8.0f;}; break;
case SDLK_d:
{player.xvel = -8.0f;}; break;
case SDLK_ESCAPE:
{running = false;}; break;
default: break;
}; break;
case SDL_KEYUP:
switch(event.key.keysym.sym)
{
case SDLK_w:
{player.zvel = 0.0f;}; break;
case SDLK_a:
{player.xvel = 0.0f;}; break;
case SDLK_s:
{player.zvel = 0.0f;}; break;
case SDLK_d:
{player.xvel = 0.0f;}; break;
default: break;
};
break;
};
};
player.x = player.x + float(player.zvel*cos((double)player.rotx));
player.z = player.z + float(player.zvel*sin((double)player.rotx));
glRotatef(player.rotx, 0.0f, 1.0f, 0.0f);
glTranslatef(player.x, player.y, player.z);
SDL_WarpMouse((screen->w/2), (screen->h/2));
};

Answer 1

If I'm correct the math functions sin and cos both take angles in radians, not degrees as player.rotx seems to be. Try the following:
player.x = player.x + float(player.zvel*cos((double)player.rotx*0.0174532925));
player.z = player.z + float(player.zvel*sin((double)player.rotx*0.0174532925));
We're multiplying player.rotx by pi/180, which is how we convert degrees to radians.
Not sure if this is your only problem, but it certainly appears to be one of them.

Answer 2

Two possible sources of the issue I see:
The standard C++ cos/sin function take a function in radians instead of degrees (one radian = 180/pi degrees).
While I'm not familiar with SDL a quick look at the docs makes me think you should be using event.motion.xrel instead of .x. You'll have to change your rotx calculation which should not need the screen width.