Error: expected primary-expression before '/' token - c++

Question

I have a problem with this :
"expected primary-expression before ‘/’ token"
Help me please
Thanks
the error shows on : angle = i * 2.0f * PI / numSegments;
Here is my code:
void display() {
glClear(GL_COLOR_BUFFER_BIT); // Clear colour buffer
glMatrixMode(GL_MODELVIEW); // To operateon the model-view matrix
glLoadIdentity(); // Reset model-view matrix
glTranslatef(ballX, ballY, 0.0f); //translate to (xPos, yPos)
// use triangular segments to form a circle
glBegin(GL_TRIANGLE_FAN);
glColor3f(0.0f, 0.0f, 1.0f); // Biru
glVertex2f(0.0f, 0.0f); // center of circle
int numSegments = 100;
GLfloat angle;
for (int i = 0; i <= numSegments; i++) { // last vertex same as first vertex
angle = i * 2.0f * PI / numSegments;
glVertex2f(cos(angle) * ballRadius, sin(angle) * ballRadius);
}
glEnd();
glutSwapBuffers(); // Swap front and back buffers (of double buffered mode)
// animation control - compute the location for the next refresh
ballX += xSpeed;
ballY += ySpeed;
//Check if the ball exceeds the edges
if (ballX > ballXMax) {
ballX = ballXMax;
xSpeed = -xSpeed;
}
else if(ballX < ballXMin) {
ballX = ballXMin;
xSpeed = -xSpeed;
}
if (ballY > ballYMax) {
ballY = ballYMax;
ySpeed = -ySpeed;
}
else if (ballY < ballYMin) {
ballY = ballYMin;
ySpeed = -ySpeed;
}
}

Answer 1

Maddeningly, neither the C nor the C++ Standards define a value for pi, and the compiler error you're getting is entirely consistent with PI not being defined.
If you're not using Boost with C++, the easiest thing to do is to define PI yourself, to a very large number of decimal places in order to guard yourself against future precision changes in your types. In POSIX you could use M_PI, but then your code is not strictly portable.