CIS 130 - Week 12 - Monday, April 12, 2010

*   REPETITION!!!

*   the 4th "basic" structure of programming
    (along with sequence, branching, and procedure)

    ...because computers are better at doing things
    over and over and over and over ad infinitum
    than people!

*   the most general, basic C++ repetition statement:

    the while statement

syntax:

while (bool_expression)
    statement;

although since you can replace a single statement with
a block (a block is considered a single "statement"):
(a block is

{
   anything;
}
)

while (bool_expression)
{
    statement1;
    statement2;
    ...
}

semantics:

*   ideally, see the flow chart drawn on the board
    (hopefully in your notes)

    *   made a PDF after class; see 
        basic-while-flow-chart.pdf

*   this is called a leading decision loop,
    because the decision to do the body
    is made "first";

    ...note that you MIGHT not ever execute
    the while loop's statement!!

example:
this will be a loop where I know how many times
I want to repeat something, going in --
  a count-controlled loop

*   goal: to burp to the screen 15 times

*   if I want to do something a certain number of times,
    how can I set that up so a bool_expr will be
    true for that many times, and then false?

    ...for a count-controlled loop, 
    a local variable called a counter is often
    used for this;

int count = 0;
const int NUM_BURPS = 15;

while (count < NUM_BURPS)
{
    cout << "BURP!" << endl;
    count++;
}

*   this fragment is in burp.cpp, and you can see
    that it does perform as advertised...

*   do you see how count is CHANGED in the body
    of the while loop, so that eventually the loop
    condition becomes false?

    ...you need to make sure that the LOOP CONTROL
    VARIABLE(s), that are letting you control the
    loop repetition, get affected by the loop
    so that loop eventually will end
    (the loop condition will become false)

    ...if you don't, and INFINITE LOOP results!
    (hit ctrl-c as fast as you can!!!!!)

*   a second example:

    I want to ask a user for 5 grades,
    read them in,
    and compute their average

...that isn't pseudocode yet!

set up a local variable to hold the latest grade
set up a local variable to hold the sum of all the grades
    so far -- set it to 0 to start
set up a local variable to hold the count, set it to 0 to
    start
set up a local named constant for the number of grades

while count is less than the number of grades desired
    ask for a grade
    read in that grade
    add in that grade to the sum so far
    count++

print out the result of (sum so far / number of grades)

now let's try to convert our pseudocode to C++:

/* set up needed local variables */

double latest_grade;
double sum_grades = 0;
int count = 0;
const int NUM_GRADES = 5;

/* read in and adding up the grades */

while (count < NUM_GRADES)
{
    cout << "Please enter a grade: ";
    cin >> latest_grade;

    sum_grades = sum_grades + latest_grade;
    count++;
}

cout << "The average is: " << sum_grades / NUM_GRADES 
     << endl;

*   and some examples of these converted into
    non-main functions; see examples along with these
    notes