Moving a ship and calculating direction to translate

I am trying to move a ship across the screen using the arrow keys. But everytime I move the ship it moves in the wrong direction. I need it to move in the direction it is pointing. I beleive the problem is in the calculation of xd and yd, the coordinates that I am translating too...it seems to move up and down correcting but anyother direction it moves, it seems, the opposite direction...

/************************************************** *********/
void rotate(){
if(rot > 360.0) rot -= 360.0;
if(rot < 0.0) rot += 360.0;
if(Left_isPressed)
rot += 5.0;
if(Right_isPressed)
rot -= 5.0;
if(Up_isPressed){
yd += (cos((rot/180*3.14)) * speed);
xd += (sin((rot/180*3.14)) * speed);
}

/*******function to draw ship********/
glPushMatrix();
glTranslatef(xd,yd,0.0);
glRotatef(rot,0.0,0.0,1.0);
glBegin(GL_LINES);
for(i=0;i<=size;i++)
glVertex2f(x[i],y[i]);
glEnd();
glPopMatrix();
glutSwapBuffers();

[cg-hci_novice]

Let me first say I am not an expert.

That being said, I think you need to have both a concept of "facing direction" and "facing movement" to implement the ship's movement properly. The ship movement is a vector (i.e. it has both magnitude and direction). These could be variables in your application's code (i.e. direction_face, direction_move).

Also, you might want to look at:
http://www.evl.uic.edu/aej/488/oldhw/fall2k.hw2.html

Hope this helps.

Good luck.

[cg-hci_novice]

The "facing movement" above should read "movement direction". Sorry.