for(int i = 0; i < 20; i++) {
  println(i);
} 

/* in a for loop first you 
define the int, 
then you type the condition 
i is less than 20 loop is active 
3rd expression: what do you want to happen 
at the end of each loop, add 1 for instance
i = i + 1 aka I++ 
i = i + 2 
I += 2 
I += 3
n = n - 1 
n -- 
n = n - 2; 
n -= 2;
*/

In Processing: This is a basic example of a "for" loop. Relevant to almost any language.

Basic "for loop" Processing

Processing