.Open 08 Control Structures
. Previous -- classes and objects
.See ./07.html
.Open Prepare
. 4.1 Introduction
Here we look at more advanced techniques for coding our `main()` functions
and `member functions` in our classes.

Tracing a program will now involve tracing instructions that are repeated
many times.
. 4.2 Algorithms
An algorithm is a precise description or recipe for achieving some goal,
given some data.  The description need to be precise enough that a machine
can carry out the instructions.

Algorithms often have complex structure.  
. 4.3 Pseudocode
Pseudo code was used after flow charts went out of fashion in the 1980's.
. 4.4 Control Structures
There are three classic types of control structure
.List
	Sequence	{....}
	Selection	if(...).... else ....
	Iteration	while(....) ...
.Close.List
. 4.5 if Selection Statement
. 4.6 if...else Double-Selection Statement
Two way choices: this or that.
.As_is 	if ( condition )
.As_is 		statement executed if condition was true
.As_is 	else
.As_is 		statement executed if condition was false

Example:  We want to count the number of odd and even numbers supplied
by our user:
.As_is 	if( i % 2 == 0 )
.As_is 		odd=odd+1;
.As_is 	else
.As_is 		even=even+1;

Exercise:  There is a logic error above.... what is it?
. 4.7 while Repetition Statement
.As_is 	while( condition )
.As_is 		statement repeated if condition is true and until it becomes false.

Example -- we want to read a series of numbers until one is less
than or equal to zero
and count the number of odd and even numbers.
.As_is 	...
.As_is 	cin>>i;
.As_is 	while (i > 0 )
.As_is 	{
.As_is 		if(i%2==0)
.As_is 			even=even+1;
.As_is 		else
.As_is 			odd=odd+1;
.As_is 		cin>>i;
.As_is 	}
.As_is 	cout << "#odd " << odd << "#even "<< even << endl;
.As_is 	...


.Close Prepare
. Deliver a question + P4
Project iteration 4 should have a print out of a program... complete with errors,
probably.
.Open Questions and Answers
. What are the differences between float and double
Both have addition, subtraction, multiplication, division, etc.... 
But this is because float numbers are converted to doubles
before the arithmetic is done!
Double take twice the space when the number is stored.  Float is
less accurate and doesn't have as large a range of numbers.  So you
are more likely to have errors with `float` than with `double`. We used
`float` when RAM was expensive and measured in Kb not Gb.

These days..... use `double` for all measurements and `int` for counting.
. We have if and else clauses, is there a then clause
No.
. Have there been calls for other control structures
Yes.  But they haven't survived the consensus.  When an idea has
lots of support it gets put into a language like C++.

Indeed the tendency has been to reduce the number of control structures.

One thing -- we have three or four statements (break, raise, goto, return,...) which
modify the standard behaviors of the three structures.   More later in this class and/or 
CS202.

The number of structures is pretty much standard these days.
. Can you prove that only Sequence + Selection + Iteration are needed
The proof takes several pages and the use of Boolean variables (later).  You
can find it by following this link
.See [BohmJaccopini66]
and (on campus) to the Assoc Computer Machinery's digital library at
.See http://doi.acm.org/10.1145/355592.365646
(off campus -- sign into library with your Coyote ID first).
. Other than if and else is there another way to check a condition
We use switch to choose between a set values.  The rest of the control structures
give you loops rather than choices.

There is a function in the <cassert> library that can checks is a condition is
true, and if not, aborts the program.  For example if I'm about the calculate
the square root of an unknown number `x` then I should put this command first:
.As_is 		assert(x>=0);
. How do you compile Pseudocode
You can't because it is a `fake` code.  It is read and written by humans, for humans,
only.

As a result Pseudocode omits a lot of things that compilers need -- declarations,
correct syntax, horrible details, predefined commands, etc. 

. How do you type Pseudocode
You can use any word processor or editor.

I often put my Pseudocode in a comment at the start of a function with the
heading: Algorithm.
.As_is /*Algorithm
.As_is 	1. Open data file
.As_is 	...
.As_is */
. When is it worth writing Pseudocode
Any time you are not sure how to code something.... write it in
strict English first and then translate it by hand into C++ or a real code.

. Can an if select several lines without using braces
NO!

. Can you use while for anything except repeating statement
I don't think so.
. Can you put while inside a while
Yes.

You can any control structure inside another one...  and so on.  Example in lab05.

. Can you make C++ start executing the last statement in the program
No.

The best you can do is to start with a command that jumps, somehow, to the last one.  But C++ programs
always start executing at "main".
. Do if-else and switch work the same way
No.  If-else is always 2 way and switch is n-way.  The machine code is quite different.  If-else
has a two way condition branch command (BNZ for example).  A switch is translated by computing the address of the next
statement from the value in the switch.

However, you can fake an if_else with a case:
.As_is 	switch(condition)
.As_is 	{
.As_is 		case true: true_part; break;
.As_is 		default: false_part;
.As_is 	}

.Close
.Open Exercises
 Without compiling and running this program
.See ./hailstone.cpp
and using pencil and paper, predict what it outputs.
.Close
. Next -- For statements
.See ./09.html
. Quiz 3 -- Classes and Objects

.Close 08 Control Structures