process user data input next user data often the code to input the first data and the later data is the samebut it must appear in both places(dsentinel loops [interactive while loops (page )often the end of the repetition of data-reading loop is indicated by sentinel in the dataa data value known to both the user and the program to not be regular datathat is specifically used to signal the end of the data (enesting control flow statements [nesting control-flow statements (page ) if statements may be nested inside loopsso the loop does not have to execute all the same code each timeit just needs to start with the same test ii loops may be nested the inner loop completes its repetitions each time before going back to the outer loop heading (fbreaking repeating pattern into loop [graphical applications (page )since loop is basically circularthere may be several choices of where to split it to list it in the loop body the split point needs to be where the continuation test is ready to be runbut that may still allow flexibility when you choose to change the starting point of the loopand rotate statements between the beginning and the end of the loopyou change what statements need to be included before and after the loopsometimes repeating or undoing actions taken in the loop (gtuples in lists [loops and tuples (page ) list may contain tuples for-each loop may process tuples in listand the for loop heading can do multiple assignments to variables for each element of the next tuple (htuples as return values [loops and tuples (page ) function may return more than one value by wrapping them in tuple the function may then be used in multiple assignment statement to extract each of the returned variables graphics (azelle' graphics graphwin method checkmouse(allows mouse tests without stopping animationby testing the last mouse clicknot waiting for new one [graphical applications (page )(bthe most finished examples of using zelle' graphics are in [loops and tuples (page )and [graphical applications (page ) more on flow of control