freeGLUT grid drawing not drawing fully at specific values

Question

I am trying to draw a 2 dimensional grid depending on a given gridSize given by the user (the grid is always a square). However, for some values of the grid, the top line and the right line of the grid do not draw. For the life of me, I cannot figure out what is wrong with my code here.

This is my code in my render method to draw the grid:

glBegin(GL_LINES);

for (float i = -1.0f; i <= 1.0f; i += 2.0f/(gridSize)) {
    glVertex2f(i, 1.0f);
    glVertex2f(i, -1.0f);

    glVertex2f(-1.0f, i);
    glVertex2f(1.0f, i);
}
glEnd();

gridSize is just the user input for the size of the grid. I divide by 2.0, since I want my grid to have a constant width and height of 2.0x2.0.

Values like 10, 25, 100 all work fine, but values such as 50 don't draw the top and left line, as shown here

It might be some dumb detail that i'm missing, but i just can't see anything wrong with my code.

Answer 1

Use an integer loop variable & calculate the floating-point position inside the loop:

glBegin(GL_LINES);
for( unsigned int j = 0; j <= gridSize; ++j )
{
    const float i = -1.0f + ( j * 2.0f/(gridSize) );
    glVertex2f(i, 1.0f);
    glVertex2f(i, -1.0f);
    glVertex2f(-1.0f, i);
    glVertex2f(1.0f, i);
}
glEnd();

All together:

#include <GL/glut.h>

void display()
{
    glClearColor( 0, 0, 0, 1 );
    glClear( GL_COLOR_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho( -1.1, 1.1, -1.1, 1.1, -1, 1 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    const unsigned int gridSize = 50;
    glBegin(GL_LINES);
    for( unsigned int j = 0; j <= gridSize; ++j )
    {
        const float i = -1.0f + ( j * 2.0f/(gridSize) );
        glVertex2f(i, 1.0f);
        glVertex2f(i, -1.0f);
        glVertex2f(-1.0f, i);
        glVertex2f(1.0f, i);
    }
    glEnd();

    glutSwapBuffers();
}

int main( int argc, char** argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutCreateWindow( "GLUT" );
    glutDisplayFunc( display );
    glutMainLoop();
    return 0;
}