How does C++ Opengl circle function work - c++

Question

I am reading a book of opengl and there is a function to draw a circle but I don't know how to put this function in my code and run it and also I don't know what parameter I put in this.
I am new to opengl and I am trying to figure it out.
Code
#include <stdlib.h>
#include <GL/glut.h>
#include <cmath>
void keyboard(unsigned char key, int x, int y);
void display(void);
void drawCircle(float cx, float cy, float r, int num_segments);
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutKeyboardFunc(&keyboard);
glutDisplayFunc(&display);
glutMainLoop();
return EXIT_SUCCESS;
}
void keyboard(unsigned char key, int x, int y)
{
switch (key)
{
case '\x1B':
exit(EXIT_SUCCESS);
break;
}
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
//drawCircle(, , , );
glFlush();
}
void drawCircle(float cx, float cy, float r, int num_segments)
{
glBegin(GL_LINE_LOOP);
for (int i = 0; i < num_segments; i++)
{
float theta = i * (2.0f * PI / num_segments); // get the current angle
float x = r * cos(theta); // calculate the x component
float y = r * sin(theta); // calculate the y component
glVertex2f(x + cx, y + cy); // output vertex
}
glEnd();
}

Answer 1

You're missing window creation and a setting the color of the circle you are
drawing:
#include <stdlib.h>
#include <gl/glut.h>
#include <math.h>
#define M_PI 3.14159265359
void keyboard(unsigned char key, int x, int y);
void display(void);
void drawCircle(float cx, float cy, float r, int num_segments);
int main(int argc, char** argv)
{
int width = 1280;
int height = 720;
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);
glutInitWindowSize(width, height);
glutCreateWindow("circle");
glutKeyboardFunc(&keyboard);
glutDisplayFunc(&display);
glutMainLoop();
return EXIT_SUCCESS;
}
void keyboard(unsigned char key, int x, int y)
{
switch (key)
{
case '\x1B':
exit(EXIT_SUCCESS);
break;
}
}
void display()
{
glColor3f(1.0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT);
gluOrtho2D(0.0, 1280, 0.0, 720);
glColor3f(1.0, 1.0, 1.0);
drawCircle(640, 360, 100, 200);
glFlush();
}
void drawCircle(float cx, float cy, float r, int num_segments)
{
glBegin(GL_LINE_LOOP);
for (int i = 0; i < num_segments; i++)
{
float theta = i * (2.0f * M_PI / num_segments); // get the current angle
float x = r * cos(theta); // calculate the x component
float y = r * sin(theta); // calculate the y component
glVertex2f(x + cx, y + cy); // output vertex
}
glEnd();
}