fill entire widget opengl - c++

Question

So I'm working with a 2d array, and I'm trying to display it on a widget using opengl. It seems to work fine, but it does not fill the widget properly. Rather than filling it evenly it's moved to the top right as seen in the image below. How can I get this to be centered?
int x = -0.1;
int y = -0.1;
float lengthX = 0.9 / ROW;
float lengthY = 0.9 / COLM;
for (int i = 0; i < ROW; i++) {
for (int j = 0; j < COLM; j++) {
if (arr[i][j] == 1) {
glColor3f(1.0f, 1.0f, 1.0f);
} else {
glColor3f(0.0f, 0.0f, 0.0f);
}
glBegin(GL_QUADS);
// Q3 Q4 Q1 Q2
glVertex2f((-x) + 2 * i * lengthX, (-y) + 2 * j * lengthY);
glVertex2f(x + (2 * i + 1) * lengthX, (-y) + (2 * j + 1) * lengthX);
glVertex2f(x + (2 * i + 1) * lengthX, y + (2 * j + 1) * lengthY);
glVertex2f((-x) + 2 * i * lengthX, y + 2 * j * lengthY);
glEnd();
}
}

Answer 1

First off, your code is wrong with it's variables; Such as int x = -0.1.
Now, to fix the problem just add this to the beginning of your code:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 1, 1, 0, -1, 1);
glMatrixMode(GL_MODELVIEW);
The problem was that you're using the default matrix setup. Basically, it ranges from -1, -1 to 1, 1, instead of 0, 0, to 1, 1. I can't quite read your code, but if you can edit the glOrtho function's first 4 parameters (the last two shouldn't effect anything) to change the visible range of what you're drawing.
Just to explain a bit more, the first parameter sets the left side, the second right, the third bottom, and the fourth top. So glOrtho(0, 600, 800, 0) means setting a vertex to 0, 0 means it's showing on the top left, while a vertex set to 800, 600 will be showing on the bottom right.
Your setup was only showing a part of what it was supposed to show because the center of it, was the corner of your screen.