.Open pages 181-196 Advanced Data
.Open Reading
. Introduction
The most useful part of the reading is the material on
type conversions in section 5.7 and on multidimensional array in section
5.9.  Using enumerated types makes you programs more readable, however,
so you should look at section 5.8 -- it is not that difficult.  I'm hoping
to skimp on the rest of the reading.
. 5.5 Complicated declarations and typedef
I don't expect you to use this section in labs, quizzes, classes, or the final
but typedef may be used later in the part of the book we are covering.  It
is definitely part of CSCI202.

Purpose:  to give your own name to a new data type.

Syntax: Just like a variable declaration.

Effect the name becomes the short hand name for a data type and can be used to declare
variables and constants later in your program

I've must used it for horrible things like arrays of function pointers that you can
forget about for now.
. 5.6 The type void and void* -- voidstar
This is the weirdest data type in C++/C.  It indicates the absence of data.
It is used in function definitions:
.As_is 	void name(parameters)....
.As_is 	Type name(void)....

The text also mentions something I don't want to talk about!  C++ and C allows you to have
pointers that indicate a single place in memory without saying what type of data
is to be found there.  They are declared using 'void*' and there is not much you
can do with them.

Again let's skip this topic.

.Open 5.7 Type conversions
. 5.7.1 Automatic type conversion
Worth putting a PostIt note on this page!
. 5.7.2 Explicit Type conversions and casts
.As_is 		(typename) expression
.As_is 		typename ( expression )
.Close
. 5.8 Enumeration types
These are a useful tool for making your programs more readable.
They also tend to make your programs less buggy.

The
.Key enum
statement declares a new data type.  It can have any values you like!

Syntax of an enumeration declaration:
.As_is 		enum name { list_of_values };
Notice the squiggly braces.  Parentheses and square brackets do not work.


. Output and input of enumerated values
However, the compiler codes the values as int's anyway.  This means that if
you output an enumerated value you get a number.  You have to convert it
to a string and a clever way to do this is on page 188 by using an array of char texts.
You can use an array of strings as well.

Page 189 shows a clever solution to the problem of inputting enumerated values.  Again
the compiler converts values to integers.  To be able to input the values you
must write code like `get_enum` that looks up a input string in the table of
values.
.Open 5.8 Tables
. Introduction
A very powerful idea needed in lots of projects
. 5.9.1 Multidimensional arrays
Notice the syntax
.As_is 		array[rows][columns]
Do not write `array[row, col]`.  It compiles, runs and gives you `array[col]`!.
. 5.9.2 Tables with the help of vectors and matrices
.Close 5.8 Tables
. 5.10 Pairs
I think we can skip this data type!  Used in CSCI202.

.Close Reading
.Open Glossary
cast::="to force a value in one data type to an equivalent value in a different data type".
char::datatype=8 bit.
enumeration::="To list a set of values and have them numbered...", a C/C++ data type.
enum::lexeme, "Used to define a new type of int with constant names for values".
.As_is 	enum Glass={empty, half_empty, half_full, full};
.Close
.Open Questions
. What are declarations
A declaration reserves some storage and gives it a name.  It may also
put data in the storage.
. Please explain the void parameter and its use in more detail
If you declare a function like this
.As_is .... fred (void )...
then the compiler will object if you ever write any call of `fred` with data
in it:
.As_is 	...fred (42);
is now illegal.

Similarly, if a function is declared to return "void" then you may not
use it in an expression to get a value.
. How are pointers and arrays useful in computer programming
First: arrays are a very common type of data.   See below for examples, and
the book.

Second pointers provide a simple and fast way to access items in the array.
In this class I refered back to
.See ./13.html#How do pointers and text strings work together
in the previous class.  Here is an example with an array of
ints being added up by using a pointer
.As_is int sum(int a[], const int N)
.As_is {
.As_is 	int sum =0;
.As_is 	for( int* p = a; p < a+N; p++)
.As_is 		sum = sum + *p;
.As_is 	return sum;
.As_is }
. What does the command char do
The keyword `char` is the name of a predefined type -- the computers
standard character type -- these days coded as 8 bit ASCII.  Because
"char" is the name of a data type you use it primarily to declare
variables and constants:
.As_is 	char input;
.As_is 	const char QUOTES = '\"';
These statements reserve a piece of primary memory and give it a temporary
name.  It is just big enough to contain one byte (8 bits) of information.

You can also use "char" to declare arrays of characters and to
specify parameters and returned types from functions:
.As_is 	char example ( char para ) { return para + 1; }
. What is typedef
The typedef key word (notice the spelling) is used to give a new name
to a complex data type.  Giving things well chosen 
names makes them easier to understand.

You use "typedef" by writing it in front of any normal looking
variable declaration:
.As_is 	typedef vector <int> intvect;
and it totally changes the meaning.  In stead of creating a new
variable (storage, address, ...) it defines a new data type.
You can then use it to declare variables
.As_is 	intvect example_int_vector;

Typedefs can make some declarations a lot easier to understand
However I don't intend to require them in any project, use them in
example, or test your knowledge of them in any quiz or final.
.What is the function of enumertaion types
They make programs more readable. See next Question.
. How does an enum reduce bugs
Enumerated data makes you code more readable.  It makes it
harder to do stupid things like forgetting how you coded a
particular value.  It makes it easier to see a mistake.  For example
if you saw:
.As_is 		light=0;
.As_is ...
.As_is 		light=1;
.As_is ...
.As_is 		light=2;
you can only guess at what is in my mind.  But if first create an
enumerated type with good names you know precisely what I'm writing about:
.As_is 	enum StopLight {red,yellow,green};
.As_is ...
.As_is 	   StopLight light;
.As_is ...
.As_is 		light=red;
.As_is ...
.As_is 		light=yellow;
.As_is ...
.As_is 		light=green;

And if you find me doing somethin like
.As_is 			light = light * green;
you can be pretty sure I've made a mistake!

Here more examples:
.As_is enum TicTacToeSymbol {empty, oh, cross};
.As_is enum LifeState {dead, alive};
.As_is enum Month{jan, feb,mar,apr,may,jun,jul,aug,sep,nov,dec};
.As_is enum DayInWeek { mon, tue, wed,thu, fri, sat, sun };

. When using enukeration types does the compiler assign a values to the different words
YES!

The compiler removes the names and replaces them by numbers
so you don't have to learn anything new about how enumerated data
works -- it is an int in disguise. 

The normal numbers are: 0,1,2,3,....

You can specify any int you like for an enumerated value.
. Can you explain the example on Page 188
This shows you a common technique that is used when you have an enumerated
type and want to input and output its values.  The technique is to declare
an array of strings that show the outside view of the data -- inside the
computer enumerated values are just numbers.   Then you use the array
to output them.

Example.... I declare TicTacToe to have three values: empty, oh, and cross.
(see above).  This does not help me output the values.  So I also declare
.As_is 	const string tttout[3]={" ", "O", "X"};
So no I can output a particular place on the board like this:
.As_is 		cout << tttout[empty];

More below...


. What is a multidimensional array
A one dimensional array is a row of items numbered [0], [1], [2], ...,
[SIZE-1].  Examples include an array of characters "abc" and this array
of ints:
.As_is 	const int DAYS_IN_MONTH[12]={31,28,31,30,31,30,31,31,30,31,30,31}; // not leap year
You access an element like this DAYS_IN_MONTH[jan].

Note:  I used an enumerated value in the above example...

Multidimensional arrays have two or more subscripts.  They
have two or more dimensions.  They
are arrays of arrays.

Examples of two-dimensional arrays.
.As_is 		TicTacToe board[3][3]; // 9 cells in a square array
.As_is 		LifeState plane[100][100]; // for conway's game of life
You access elements in these arrays like this: board[0],[1], plane[24][42],
etc.

Once you have got a TicTacToe board and a array tttout defining how to
output the symbols you display a board like this
.As_is for(int row=0; row < 3 ; row++)
.As_is {
.As_is 	for(int col=0; col < 3; col++)
.As_is 	{
.As_is 		cout << " " << tttout[ board[row][col] ] ;
.As_is 		if(col < 2)
.As_is 			cout << " | ";
.As_is 	}
.As_is 	cout << endl;
.As_is 	if( row < 2 )
.As_is 		cout << "-----" << endl;
.As_is }
(This code needs combining with previous snippets and testing...)

Another example is a single work sheet for a spreadsheet!
In fact, most of the tables that users want see are two dimensional
tables:
.As_is 		double sales[WEEKS][DAYS_IN_WEEK];
You access the cells like this sales[8][fri]

Three diemnsional arrays are also common.   A modern spreadsheet
has many worksheets and each of those is a two diemnsional array
of formulas.  Similarly, many scientific models of objects -- a
body, a bridge, a block of uranium, etc.  are modeled by a large
three-dimensional array.  The cells in the array can store the
ammount of bine, the stress, the radiation, etc. in the cell... and
the computer can calculat what happens.   This kind of volume
modelling and computation is expensive and common!

.As_is 	double bone[10][10][10];
declares an array of 1,000 cells organized like a cube.  You access
a particular cell like this bone[row][col][slice].

.Box
Be happy you have multidimensional arrays.  When I started out
I didn't have them in my programming languages.  I will not forget
in hurry a friend who need a 5><5 square and declared 5
different arrays:
.As_is 		double a[5],b[5],c[5],d[5],e[5];
And the pages and pages of special code for each row of the
array...   I went in a different direction.  I declared
.As_is 		double a[25];
and had to mentally transcribe `a[r,c]` into
.As_is 			a[ 5 * r + c ]
every where...
.Close.Box
. If we want to combine 2 different arrays do the rules of matrix addition and mulitplication apply
NO.   Sadly, no. .... You can not add and multiply arrays in C++
unless you have included definitions of the operations.   You'd
have to hunt for these on the Internet.   

Don't get me on the topic of prgramming matrix operations.  I have
had to do it in two different jobs and did four years of college
work on them....
.Close
. Next -- Quiz 7 + Lab 8 + Project 6
.See ./projects.html#P6
.Close