The line inside the loop:
a(((y(i+50)-y(i))/(x(i+50)-x(i)))>10);
extracts a piece of the array a and then essentially throws away the extracted piece. It's like you were in the soup aisle at your local grocery store. You may have picked up a can of chicken noodle soup to look at it, but you put it back on the shelf. When you get to the register to buy your groceries, there won't be anything in your shopping cart.
Add code inside your for loop that actually does something with the extracted piece of the array -- put it in your shopping cart, hand it to another shopper, start juggling it (actually, the employees of the store probably wouldn't like it if you did that last action.) If you're not sure how to do something with that piece of the array, describe what you want to do and the readers may offer some guidance. We know lots of MATLAB noodle soup recipes (with L-shaped membrane noodles, naturally :)
