if anyone has the book learning processing, maybe they can help me understand this

i am just learning about pixels[loc] and kind of stuck at exercise 15-16, which isnt published in his answers online

trying to make a simple radial gradient

here's what i have so far

size(255,255);
loadPixels();

//Loop through every pix column

for(int x = 0; x < width; x++) {
 
//Loop through every pix row
 for(int y = 0; y < height; y++) {
   int loc = x + y*width;
   float distance =     ;
   pixels[loc] =      ;
 }
}

thanks

i am still getting my head around using the looped x,y values and pixels[loc] (1D format)  

so the bottom would be something like this?

distance = dist(x,y,width/2,height/2);
color c = color(distance);
pixels[loc] = c;