1.  Wednesday, 9/18/02, Slide #1 CS106 Introduction to CS1 Wednesday, 9/18/02  QUESTIONS?? HW #1 due today at 5!!  Today: Loops, and two new data types  Reading (for Mon): 2.3, 2.5, 6.1-6.2b  Exercises: p. 50 #15, 16  New files/handouts: WhileEx1.cpp, WhileEx2.cpp, NestedLoop.cpp

2.  Wednesday, 9/18/02, Slide #2 Repetition (Looping) Structures  Provides the ability to repeat a list of statements (an "action") more than once  Three looping structures in C++  while structure  for structure  do-while structure

3.  Wednesday, 9/18/02, Slide #3 Looping tasks With loops you can:  Repeat an action a specific number of times  Play a game 5 times  Repeat until some condition is met  Play game until user says quit  Play game until user loses 3 times  Accumulate a total  Add up a grocery bill and compute tax  Compute the average of a set of marks  Count how many occurrences of a letter there are in a given passage

4.  Wednesday, 9/18/02, Slide #4 The while structure  The syntax of the while structure is:  The statements of the loop (the “body” of the loop) repeat while the T-F condition remains true -- possibly forever!  How do we change the true/false value of the condition?? while (T-F condition) or while (T-F condition) single statement;{ list of statements }

5.  Wednesday, 9/18/02, Slide #5 Initialize, Update, Exit!  Initialize variables in loop condition before entering loop;  Update these variables inside the loop, so it can eventually terminate;  Check that the loop condition can be made false inside loop, so that loop will end. If you accidentally execute an infinite loop in VCPP, press Ctrl-Break to stop it. Statement to initialize loop condition variables while (condition) { Loop body contains statements to update loop condition variables }

6.  Wednesday, 9/18/02, Slide #6Examples  Write a program that inputs a number from the user, representing a restaurant bill, that adds 7% tax, then 15% tip to that, and prints the total. The process should repeat until the user enters a number <= 0.  Write a program that writes “TGIF” ten times, using only one output statement

7.  Wednesday, 9/18/02, Slide #7 First while-loop example  Write a program that inputs a number from the user, representing a restaurant bill, that adds 7% tax, then 15% tip to that, and prints the total. The process should repeat until the user enters a number <= 0.  See WhileEx1.cpp for full program float tax = 0.07; float tip = 0.15; float amount; cin >> amount; //while positive amount, compute tax and tip while (amount > 0) { float taxtotal = amount * (1 + tax); float total = taxtotal * (1 + tip); cout << total << endl ; cin >> amount; } Initialize loop condition objects before loop! Update loop condition objects inside loop!

8.  Wednesday, 9/18/02, Slide #8 Sentinels and Counters  The object amount in the last program is called a sentinel, because its value signals when to end the loop.  If we have a loop that should repeat a fixed number of times, we can use a loop counter to control the loop.  The loop counter starts at 0  It is increased by 1 each time the loop executes  The loop ends when the counter reaches the desired number of executions

9.  Wednesday, 9/18/02, Slide #9 Second While Loop Example  Write a program that writes “TGIF” ten times, using only one output statement  See WhileEx2.cpp for complete program.  Off-by-one errors: A common error is to write loops that execute one too many or one too few times – always check!! float numTimes = 10; float count = 0; //count = # loop executions while (count < numTimes) { cout << "TGIF" << endl; count = count + 1; }

10.  Wednesday, 9/18/02, Slide #10 Accumulating values  The statement count = count + 1; inside the loop, causes the object count to increase by 1 each time  We can accumulate other values too:  Write a program to input a list of positive numbers and compute their sum; 0 or negative number ends input  Modify to compute average of the numbers  Modify to count how many are over 50

11.  Wednesday, 9/18/02, Slide #11 “Nested Loops”: An example  Assume that the objects numRows and numCols have the values 3 and 5, resp. Trace the following code segment by hand, showing values in memory and on monitor.  See NestedLoop.cpp for complete program float row = 0; while (row < numRows) { float col = 0; while (col < numCols) { cout << '*'; col = col + 1; } cout << endl; row = row + 1; }

12.  Wednesday, 9/18/02, Slide #12 Nested Loops  If we want to repeat a process that uses a loop, then we put one loop inside another as part of its action.  For each iteration of the outer loop, the inner loop must be completely executed.  Example: Modify the previous program to ask the user if she/he would like to draw another rectangle.

13.  Wednesday, 9/18/02, Slide #13 Choosing the right data type: int, float, and double  In many of the previous programs, the float objects we used were always whole numbers.  C++ provides different data types for whole numbers and numbers with fractional parts:  int: An object of type int may only be a whole number  float: An object of type float may have a fractional part  double: Similar to float, but with larger number of decimal places, and larger magnitude permitted  Look at programs we did today. Which objects should be type int, and which should be float or double?

14.  Wednesday, 9/18/02, Slide #14 More on int, float, and double  C++ constants: If we use a numeric constant value in C++ that is a whole number, it is automatically given type int. If it has a fractional part, it is given type double.  Integer arithmetic: Any arithmetic operation involving only int type objects produces an int type object. For example, what is  5 / 3 + 3 / 5