. 13 Logic and Conditions
. Previous -- Control Structures
.See ./12.html
.Open Prepare
. 5.8 Logical Operators
VITAL!
. 5.9 Confusing the Equality (==) and Assignment (=) Operators
All too common.
. 5.10 Structured Programming Summary
Review.
. 5.11  Software Engineering Case Study: Identifying Objects’ States and Activities in the ATM System
. 5.12 Wrap-Up
Review chapter
.Close Prepare
. Deliver a question
.Open Questions and Answers
. Is there an if-if structure
Example
.As_is 		if(if(.....) )....
No.

However this is OK
.As_is 	if(....)
.As_is 	{
.As_is 		if(....)
.As_is 		...
.As_is 	}
. If you repeat a loop does it restart
Yes.
. If you use return in a loop does it terminate the loop
Yes.
. When combining small programs into a large one do you modify the variables
Yes.  You also can put the parts between {braces} and then refactor out the
shared variables:
.As_is 	{ declare shared
.As_is 		{ declare local only
.As_is 		}
.As_is 		{ declare local only
.As_is 		}
.As_is 		{ declare local only
.As_is 		}
.As_is 	}

The best way to combine small programs is to make them into functions
and call them as needed.... coming up next.
. Explain boolalpha
This is only used in quizzes, exams, and examples.

It is part of <iomanip> and uses namespace std.  So you may need
.As_is 	using std::boolalpha;

Normally, when you output a bool it displays as either 1 or 0. 1 is true
and 0 is false.

After boolalpha you get a truncated form of "false" and/or "true".

Use it when you want to display bool values/conditions as "t" or "f".
This is rare -- mainly: debugging, classroom exercises, lab work, examples
in books, ...  Mostly it is better to show spell out what the
condition means with something like this
.As_is 		cout << .... << (single? " Single " : " Married ") << ...;

. Are break and continue only used on loops
The "break" is used with switch.
The continue only with loops.
. When to use negation rather than the negated relation
When you want the opposite of a complex condition
.As_is 		if ( ! (x>3 or y <2) )
Note -- the "!"/"not" operator doesn't cost anything much in time or space3
and saves brain power.... which is more expensive.

When the condition is not based on relations you can not just change == to !=
etc..  For example
.As_is 		if(!(n%p)){ /*n is not divisible by p*/}

As an
example in the lab you met: 
.As_is 	while(cin >> value){....}
which reads each `value` until the end of file (or when some one types a vlaue that
doesn't fit).  The `cin>>value` is a combined command and condition.  You can test
for the presence of data and also get the first one:
.As_is 	if(!(cin>>value))
.As_is 	{
.As_is 		cout << "No data! \n";
.As_is 		return 1;
.As_is 	}
.As_is 	double max=value; double min=value; double sum=value;
.As_is 	double sumsquares=value*value;
.As_is 	while(cin>>value)  // proces the other values
.As_is 	{
.As_is 		...
.As_is 	}

. Why can we only combine the structures using nesting and stacking
Because it is not clear what it means.... and because C++ syntax
won't let you do it.
. Why are nonzero values treated as true
The designers of C thought that zero ( in binary: all the bits a zero)
was clearly the way to encode "false".   They threfore decided that all other
values (which would have a non-zero bit somewhere in them) should be
"true" in a condition.  It turns out that this was a smart choice making
code simpler and faster in a lot of programs.

The alternative was to choose a value (say one(1)) to mean "true" and
add a run time error if ever a condition was neither 1 or 0.  This would
make a lot of programs more complex and a bit slower.

. bool forms of negative numbers
These are treat as "true" because they are nonzero.

. When can you use logical operators && || !
In any condition -- in if, if-else, while, for, ..., and do-while.  

You can also use then to give values assigned to bool variables (flags):
.As_is 	bool flag = false;
.As_is 	...
.As_is 		flag = (n>2)&& !(n%p);

. Can you write the exclusive or function in C++
Yes.  The `exclusive or` is true when two Boolean variables have different
values.  And C++ is happy to apply '==' and '!=' to bools:
.As_is 		bool a, b, c;
.As_is 		....
.As_is		c = (a != b);

"Believe it or not."
. Is using && || ! vs and or not just a personal preference
These days yes.  In some older compilers we don't have and/or/not.
. Is bool like truth tables
Yes.

Indeed one way to design a logical (bool) expression is to write out
a truth table first and then code it using the rows that are true...
. Why logical negation
It saves thinking!



. Why can't the compiler reject assignments in conditions
Because  many programmers, in the last 30 years, have
put assignments in conditions with care and great success. A classic
maneuver is
.As_is 		while ( (data = cin.get()) != EOF )....



. Explain UML State Machine Diagrams
State Machine diagrams are elaborated from the "finite state machines"
invented by the very first computer scientists to model brains, circuits,
and computers.  They are used in software engineering, in networking,
and in computer science theory.

They show how the `state` of something changes under the
influence of `events`.  For example, a door is either open or closed.
These are the two states for a door (we ignore "half open") for this
example.  Suppose we are talking about an elevator door, then the
events happen when buttons are pushed.  There are buttons labelled
"open" and "closed".  As a result we get the first diagram below
that describes how the door behaves.

Here is another example.  I have a ball point pen.  If the refill is "out"
I can write with it.  If the refill nib is "in" it is safe to put in my pocket.
In this case there is a single type event "click" when I click the button on
top.  Sometimes the nib comes out and sometimes it goes in.... 
See the second state machine below.

State machines can have more than 2 states.  They can have any finite number.
In my inlaws house there was a lamp in the bathroom.  It had a pull chain.
It had for states:
.List
 Light off, heat off
 Light on, heat off
 Light off, heat on
 Light on, Heat on
.Close.List
By pulling the chain you could get to any one of these for states -- see
the third state machine below.

.Image StateMachines.png [Three state machines]

For a couple of more practical examples try
.See ../samples/uml0.html#State

State machines are a marvelous way to analyse dynamic problesm, complex
protocols, and design intricate solutions that work.

. Explain lvalues and rvalues
Ever since the ancient language of BCPL (1970s) we have distinguished the
kind of value that can appear on the left of an assignment from the
kind of value that is normally needed on the right hand side of an assignment.
.As_is 		lvalue = rvalue;

We say that a variable has an lvalue and an rvalue.  The lvalue is the
address in the computer that is given to the variable by the compilation
process.   It indicates where data is found, and where it is put.  This is
the value that is needed on the left of an assignment.  This is also
what is needed in cin statements -- we need a place to put things.

A variable has an address of a piece of storage or memory.  This piece of
memory holds the `rvalue` of the variable.   When we put a `lvalue` on the right
of an assignment we get the data that is in the memory -- not the address.  This
also happens in cout statements where we need a value to be printed.

So far the only `rvalues` you have met have been ints, doubles, floats, strings,
and so on.  None of these is the address of some data.  So it is not a
valid `lvalue`. You can not compile
.As_is 		3.14159 = 17;
for example.  It is trying to change a constant!  So this is forbidden.

In C++ we have an operator that explicitly gives the lvalue of a variable
where we would normally get its rvalue.
.As_is 		cout << & variable;
In CS202 you meet a kind of varaible that stores addresses -- and these
can get confusing.... they are called pointers.  




.Close
. Exercises
. Next
.See ./14.html
.See ./lab07/