Optimizing a loop in C for a microcontroller

I am trying to draw a Checkerboard pattern on a LCD using a GUI library called emWin. I have actually managed to draw it using the following code. But having this many loops in the program body for a single task, that too in the internal flash of the microcontroller is not a good idea.

For those who have not worked with emWin, I will try and explain a few things before we go for the actual logic.

• GUI_RECT is a structure which id define source files of emWin and I am blind to it.

• Rect, Rect2, Rect3.. and so on until Rect10 are objects.

• Elements of the Rect array are {x0,y0,x1,y1}, where x0, y0 are starting locations of the rectangle in the X-Y plane and x1, y1 are end locations of the Rectangle in the X-Y plane.

Rect={0,0,79,79} is a rectangle that starts at the top left of the LCD and is up to (79,79), so it's basically a square.

• The function GUI_setBkColor(int color); sets the color of the background.

• The function GUI_setColor(int color); sets the color of the foreground.

• GUI_WHITE and DM_CHECKERBOARD_COLOR are two color values, #defineed

• GUI_FillRectEx(&Rect); will draw the rectangle.

The code below works fine but I want to make it smarter.

```
GUI_RECT Rect = {0, 0, 79, 79};
GUI_RECT Rect2 = {80, 0, 159, 79};
GUI_RECT Rect3 = {160, 0, 239, 79};
GUI_RECT Rect4 = {240, 0, 319, 79};
GUI_RECT Rect5 = {320, 0, 399, 79};
GUI_RECT Rect6 = {400, 0, 479, 79};
GUI_RECT Rect7 = {480, 0, 559, 79};
GUI_RECT Rect8 = {560, 0, 639, 79};
GUI_RECT Rect9 = {640, 0, 719, 79};
GUI_RECT Rect10 = {720, 0, 799, 79};

WM_SelectWindow(Win_DM_Main);
GUI_SetBkColor(GUI_BLACK);
GUI_Clear();

for(i = 0; i < 6; i++)
{
if(i%2 == 0)
GUI_SetColor(GUI_WHITE);
else
GUI_SetColor(DM_CHECKERBOARD_COLOR);

GUI_FillRectEx(&Rect);

Rect.y0 += 80;
Rect.y1 += 80;
}

for(i = 0; i < 6; i++)
{
if(i%2 == 0)
GUI_Set

It's unlikely that your app's performance concerns are due to the excessive looping. They are all small loops and should rip through pretty quickly, even on an embedded controller. I suspect your expensive call is the GUI_FillRectEx() calls.

That said, there are techniques to clean up your code and make it more maintainable.

• Look for the duplicated logic.

Notice how you have the same loop structure + code for each of those blocks?

for(i = 0; i < 6; i++)
{
    if(i%2 == 0)
        GUI_SetColor(DM_CHECKERBOARD_COLOR);    
    else
        GUI_SetColor(GUI_WHITE);

While the cost isn't necessarily great in this case, you are repeating it for each of your rectangles.

-
Look for the actions that don't need to occur.

-
In this case, do you really need to paint both the white and black squares? Could you "cheat" by using a black canvas and only paint the white squares? That would cut your FillRectEx() calls in half.

-
Do you really need to declare 10 Rect's and increment coordinates on each loop? How about declaring a fixed array ahead of time and pre-populate / hard code all the coordinates?

StackExchange Code Review Q#27889, answer score: 4