Using 'for' to loop a loop

So, what you are trying to achieve is called nested loops. Although you could start b with 5 and end before 610, I think it's easier and simpler to understand if you do something else. For example:

// By doing so, it's easy to know how many times you are going to run the second for loop.
for (int i = 0; i < 20 ; ++i) {

     for (int j = 0; j < 100 ; ++j) {

     ax1[j] = 5 + j * 30;
     /* This is where it matters.
         You can use a bit of math to simplify your code, like so: */
     ay1[j] = 5+ax0[5 + i  * 30];
     /* In the first time you go around this second loop you have 5, then 35, then 65, then 95 and so on.
       And it's easier to draw more. */

     ax2[j] = (abs(curserx)/30 - j);
     ofSetColor(abs(curserx-300),abs(cursery - 300),180);
     ofSetLineWidth(2);
     ofCircle(ax1[j],ay1[j],ax2[j]);
     }
}

In your previous example, you also had an infinite loop. You can't do:

for ( b = 5; b < 610 ; b + 30)

You'll have to do:

for ( b = 5; b < 610 ; b += 30)