Software often repeats a set of instructions over and over again. This repetition continues until some condition is met.

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2 Software often repeats a set of instructions over and over again. This repetition continues until some condition is met. A programming statement that does this is called a

3 : repeat "Enter user id and password" until valid values are entered. : after a game finishes, it asks the user if they want to play again. : data is repeatedly entered and processed, calculating a Sum, Average, Maximum, et. of the values entered.

4 conditionalexpression statements - when the conditionalexpression is FALSE: o skips to and continues. - when the conditional Expression is TRUE: o executes the statements o goes back to the while

5 a Counting Loop x = 1; % Displays: while x <= 5 % 1 disp(x); % 2 x = x + 1; % 3 end % 4 disp('done'); % 5 % Done

6 a Play again Loop % BEFORE LOOP: set answer to "yes" again = 'Y'; while strcmp(again,'y') % add code here to play a game end % at the bottom, ask to play again again = input('play again? (Y/N): ','s');

7 There are several "standard" algorithms used to solve problems with loops. We'll take a look at some of the most common.

8 Repeat some operation (ex: a game), and at the end, ask the user if they would like to "do it again" to decide whether or not to repeat.

9 again = 'Yes'; % execute at least once while strcmp(again,'yes') % do the "operation" to be repeated here end % at the bottom, inside the loop, ask again = input('do again? (Yes/No): ', 's' ); Note the loop will end when the user enters ANYTHING besides Yes

10 to repeat something a number of times When we want to count, there are three important numbers: - where to counting - where to counting - what to (called the i )

11 c = ; while c <= % do whatever is to be repeated here end % at the bottom, inside the loop c = c + ;

12 c = ; while c <= % do whatever is to be repeated here disp(c) % at the bottom, inside the loop c = c + ; end % prints:

13 To count the number of times a loop repeats when the counter variable does NOT control the loop (is not in the while condition).

14 c = 0; % set to 0 BEFORE the loop while something_not_involving_c % do whatever is to be repeated % add 1 to the counter c = c + 1; end % after the loop, c is now the % number of times the loop executed. disp(['repeated ',num2str(c),' times']);

15 with a Do Again Loop c = 0; % set to 0 BEFORE the loop again = 'Y'; while strcmp(again,'y') % do whatever is to be repeated c = c + 1; % add 1 to the counter again = input('again? (Y/N): ','s'); end % after the loop, c is now the % number of times the loop executed. disp(['we repeated ',num2str(c),' times']);

16 To add up numbers entered one at a time. sum = 0; % empty the bucket first!! while whatever_anykindofloop num = input('enter number: '); sum = sum + num; % add to sum end

17 Find the largest of numbers which are entered one at a time. Idea: when each number is entered, think: "Is this number greater than the largest number seen so far? If so, this number is now the largest number seen so far."

18 max = -9999; % a ridiculously low num while whatever_anykindofloop num = input('enter number: '); if num > max max = num; end; end

19 Find the lowest of numbers which are entered one at a time. Idea: when each number is entered, think: "Is this number less than the lowest number seen so far? If so, this number is now the lowest number seen so far."

20 min = 9999; % a ridiculously while whatever_anykindofloop num = input('enter number: '); if num min min = num; end; end num

21 Allow the user to enter an unknown number of values. The user indicates "no more values to enter" by entering a special value called a. The Sentinel Value should not "make sense" as a true value to be entered. The Sentinel Value should not be processed as a true value by the program.

22 num = anything_except_the_sentinel; while num ~= Sentinel num = input('enter number: '); if num ~= Sentinel % process num as a true value end; end;

23 -1 is not a true test score score = 0; % 0 is not -1 while score ~= -1 score = input('enter score (-1 to end): '); if score ~= -1 % process score as a true test score end; end;

24 To repeat a question until the user enters a valid response, usually with a "wrong answer" message. - ask the question - while wrong answer repeat: - print "wrong answer" message - ask the question again

25 % ask the question once num = input('enter number 1-10: '); % while WRONG ANSWER, repeat while num < 1 num > 10 disp('wrong answer try again'); num = input('enter number 1-10'); end

26 Term Loop Sentinel Definition a statement that repeats statements (ex: while, for) a value entered as data, but not processed as data; instead it indicates "end of data".