.Open pages 48-58 Loops
. Podcst on reading
.See ./06.mp3
.Open Reading -- pages 48-59
Pages 48 to 58 introduce how C++ expresses
the third structure used to write algorithms:
.List
 Sequence
.As_is 		{ sequence }
 Selection
.As_is 		if ( condition ) { sequence }
 Iteration
.As_is 		while ( condition ) { sequence }
.As_is 		for (initial; condition; increment ) { sequence }
.Close.List
. The while-loop
Syntax
.As_is 		while ( A ) { B }
.Image while.gif [While A is true repeat B]

Example:
.As_is 		double x=1, sum = 0;
.As_is 		while( x > 0.0001)
.As_is 		{
.As_is 		   sum = sum + x;
.As_is		   x = x/2;
.As_is 		}
Exercise:  find out what the above does.

The loop has an expression that it treats as a condition.  It evaluates
this condition and if it is true then it enters the statement
that follows, and then goes back to the condition again.

Use while loops any time you need a program to repeat itself while
some condition is true.
. The for-loop
A for-loop is a short way to write a common type of while-loop.

.As_is 		for ( A; B; C ) { D }
.Image for.gif [Starting with A until B is false do D and then C]

Note the odd use of two semicolons (";") in the for loop syntax.
There must be exactly two of them and they must be inside the parentheses
that follow the reserved word "for".  This syntax started with C and has
been inhetitted by C++, Java, PHP, and other modern languages.

Use for-loops for counting, incrementing, and scanning across data:
.As_is 		int i=1, sum = 0;
.As_is 		for( i = 1; i <=N; i++)
.As_is 		{
.As_is 		   sum = sum + i;
.As_is 		}
(should add up all the numbers from 1 to N)

. Nested loops etc
You can have a loop inside a branch of an if.
.As_is    cout << "N= "; cin >> N;
.As_is    if( N > 0 )
.As_is    {
.As_is 		int i=1, sum = 0;
.As_is 		for( i = 1; i <=N; i++)
.As_is 		{
.As_is 		   sum = sum + i;
.As_is 		}
.As_is    }

You can have complete if-statements inside loops.
.As_is 		for( int i=1; i<=10; i++ )
.As_is 		{
.As_is 			if( i % 3 == 0)
.As_is 			{
.As_is 				cout << i << " divisible by 3 " << endl;
.As_is 			}
.As_is 			else
.As_is 			{
.As_is 				cout << i << " not divisible by 3 " << endl;
.As_is 			}
.As_is 		}//end for

You can even have loops inside loops.
.See ./mtable.cpp

In a recent presentation a student described the work he did with
biologists in Santa Barbara last summer (2007).  He noted that the
algorithms had for-loops nested 3 or 4 levels deep.

There is no limit to this nesting.

Key points:
.Set
 Use the structure that reflects your algorithm.
 Use indenting to display the nesting.
.Close.Set

. Hint -- do not use break
The C++ `break;` command leads to confusing programs that sometimes have 
lead to disasters -- like, once, shutting down all the telephones in a part 
of New York one day!
. Trap -- loops that never halt
Take time to prove to yourself that every loop in every program
can only repeat a finite number of times.  Otherwise you have
a program that runs forever -- not commonly useful.

We have Junior and Senior level courses that show how to estimate
how long an algorithm will take -- the analysis of algorithms.

. Relation between for and while
In theory you can replace
.As_is 		for ( A; B; C) { D }
by
.As_is 		{ A;
.As_is 		  while ( B )
.As_is 		  {
.As_is 		     D
.As_is 		     C;
.As_is 		  }
.As_is 		}
But in practice -- learn to use the for loop.
. Abbreviated for loops
You can omit each or all of the three parts of a for-loop and
it will compile and run.  A missing condition is taken to be `true`.
. Hint -- Write an algorithm first
.Close
.Open Syntax of for and while
.As_is 		while ( condition ) { body }
.As_is 		for ( initial; condition; increment ) { body }
.Close
. Example and exercise in class
.See ./06ex.cpp
.Open Questions
. Why are break statements so bad
They have been implicated in causing disasters.  The `break` has
two distinct meanings and some people for get this and produce code that
does not work properly.

Secondly, `break` skips oover statements to get to the end of the loop
and this is easily forgotten.

The fact is you can program anything with if and while.  So rather than code
like this
.As_is 	while(true)
.As_is 	{
.As_is 		A
.As_is 		if( B )
.As_is 		{
.As_is 			C
.As_is		}
.As_is 		else
.As_is 			break;
.As_is 		D
.As_is 	}
You can, instead use a bool flag.
.As_is 	bool ok = true;
.As_is 	while(ok)
.As_is 	{
.As_is 		A
.As_is 		if( B )
.As_is 		{
.As_is 			C
.As_is		}
.As_is 		else
.As_is 			ok=false;
.As_is 		if( ok )
.As_is 		{
.As_is 			D
.As_is		}
.As_is 	}
I'll be happy if you don't use break in projects.  
I'll only penalize you if it goes wrong!
It won't appear in
quizzes, finals, or labs.
. Can you explain for loops
For loops are compact way of writing counting and scanning loops.
. Should you use while or for
Use for for counting and scanning.  Use while for all other
iterations.
. How many Layers deep have you gone in nesting while statements
I'm not sure.... I've written for-loops at least 3 and
possible 4 deep.
. How does an if differ from a while
An if selects one of two alternatives. A while repeats a sequence
until a condition become false.
. What does fabs mean
It means "Floating-point Absolute Value".  It does something like this
.As_is 		if( x < 0 )
.As_is 			fabs=-x;
.As_is 		else
.As_is 			fabs=x;
. WHat is your opinion on using the Microsoft Visual C++ expression editor
I've found the "Visual" environments (except the original
VB) to need a lot of setting up -- I've usual given up.
. Are loops the same as while statements
No.  There are two kinds of loop (for and do-while) that are not
'while' statements.
. Is Linux/Unix an OS just for programmers or can average users use it
I think that Linux KDE is better than Vista!

But it is striking that Apple Macintoshes come with a version of
Unix and the users a smugly happy with it: "It just works".

On a lighter note.... here is a fun video on the topic
.See http://video.google.com/videoplay?docid=-5847227571896228342&q=linux+windows+macintosh&total=802&start=0&num=10&so=0&type=search&plindex=2

. Is there a limit to repetitions in a for loop
No.
. Is there a limit to repetitions in a while loop
No.
. Which is safest for or while
They are equally safe to use.  And both need care.
. Can you just use an if and something instead of a while
Yes.  You can use the "g*t*" statement -- now considered to be a rude
word!  Or you use recursion (later in the course).

Luckily -- we have a simple while!
. Is there a typo at the top of page 58
Yes the condition in the for loop should be "i<=n" not "i<=".
. How do logical operators function in C++
See
.See ./05.html
for my notes on these.

By the way -- I don't know precisely what out compilers and computers
do with them.  I just know that they work they way C++ says they should.
No.
.Close
.Open Next
. Quiz 3 -- Loops Logic Ifs
. Lab 04 -- Loops Logic Ifs
. Project 2  -- Ifs -- Due
.See ./projects.html#P2
.Close