The second field, in this case containing ‘index < 6’, is the test which is
done at the beginning of each loop through the program. It can be any
expression which will evaluate to a true or false. (More will be said
about the actual value of true and false in the next section.) The
expression contained in the third field is executed each time the loop is
executed but it is not executed until after those statements in the main
body of the loop are executed. This field, like the first, can also be
composed of several operations separated by commas.