. 18 Arrays
. Previous -- Vectors
.See ./17.html
.Open Prepare
. 7.1 Introduction
. 7.2 Arrays
. 7.3 Declaring Arrays
Notice the syntax.
.Open 7.4 Examples Using Arrays
. 7.4.1 Declaring an Array and Using a Loop to Initialize the Array’s Elements
Notice the syntax for declarations.

Notice how arrays and for loops fit together
. 7.4.2 Initializing an Array in a Declaration with an Initializer List
Powerful.
. 7.4.3 Specifying an Array’s Size with a Constant Variable and Setting Array Elements with Calculations
THis is the best practice with array -- seeing they have a fixed size: give it a name
andt tell the compiler that it is a constant (const).  Do it.
. 7.4.4 Summing the Elements of an Array
A classic technique -- use it.
. 7.4.5 Using Bar Charts to Display Array Data Graphically
A classic technique -- use it.
. 7.4.6 Using the Elements of an Array as Counters
A classic technique -- use it.
. 7.4.7 Using Arrays to Summarize Survey Results
A classic technique -- use it.
. 7.4.8 Using Character Arrays to Store and Manipulate Strings
The original technique:  be very careful with this.  Nothing is quite what it seems.
. Skip 7.4.9 Static Local Arrays and Automatic Local Arrays
.Close 7.4 Examples Using Arrays
.Close Prepare
. Deliver a Question and P9
. Exercises
Given some vector declarations -- translate them into array declarations.
.Open Questions and Answers
. What is a data structure
Any organized collection of data -- all objects, arrays, vectors, lists, maps, sets, ....
and so on a classic data structures.

. Can you create an if statement with a vector
You can use vectors inside the conditions in vectors, because you can use the accessors
and subscripts to get normal values and test those.  Similary you can use vectors inside
the body of an if-statement.... just as if it was outside.  And you can declare a local
vector.... just like you declare any other local variable...

. Are than any more Accessor for vectors other than empty, size, front, and back
Just subscript and at.

. What happens if vector elements are not around square brackets
You have got into a separate universe, or you will get compile errors.

. Can an element of a vector, other than the last element by removed
Not efficiently.  The does exist an "erase" function which will in fact overwrite the
existing elements and pop the last one.   

If you really need to insert and erase elements in a vector.... you should be using a <list>.

. How to vectors and arrays relate to each other
Arrays are a lot more primitive and vectors.  They do share a common notation
for subscripts.  Arrays are OK when you know how many elements you need and
you don't want to do anything clever with them.

. Is there anything I can do with an array that I can't do with a vector
I know of onw thing that I can do with an array, but not with a vector -- use
an initializer list.  This means that
.As_is	int fib[] = {1,1,2,3,5,8,13};
is OK but
.As_is	vector <int> fib = {1,1,2,3,5,8,13};
does not compile it complains about not being given a sigle value for the
vector.  There are three obvious ways round this. (1) use an array
(when it doesn't grow). (2) Push the data directly into an empty vector.
.See ./initializeVector.cpp
Or (3) Put the data in an array with an initializer and then push it into
the vector:
.See ./initializeArrayFirst.cpp

With objects something similar works
.See ./vectorOfObjects.cpp
for example.

. Can I change the size of an array by ....
No.  No. Nothing will change the size of a local array once it is declared.

. Why can I go outside the bounds of an arrray
Because a compiled program doesn't see the boundaries.  It knows the
start of arrays, but not where they end.  It has no way of
calculating how far you can go.

A running program sees data as a gigantic array of words with the
beginning of the data for each variable is known...

. What happens if I go off the end of an array
You overwrite the adjacent memory.  It may crash the program or the computer.  But usually
you start to get very weird answers for no apparent reason.

. What about walking off the end of an array with a for loop
The results are unpredictable and never good.

. What about vectors -- can you go off the end without errors
Yes.  But the results are wrong, weird, and crash software.

. What is a Smart Array
It knows how big the array is.
It looks very like an array but any attempt to go ouside it is trapped
as an error.  It is a safe array.... but may be slower than a raw array.  One of the
neater things about C++ is that you can create a class that hides its data
and define the meaning of the subscript operator ([]) for objects in the 
class, so you can use a smart array just like a dumb one!

. What happens if someone inputs too much data into an array
In the worst case scenario they can insert there data into your program where it
will be executed.  This is called "A buffer overrun attack".  It is most
often caused by using "gets" or "cin>>" into an array.

. Does getline(cin, SIZE, ENDCHAR) help avoid buffer over runs
Yes.... except that you have problems when SIZE characters are read.  
You have to input the rest of the line after the SIZE characters.
I think it even
fails to terminate the array with a '\0' or something like that.  

Please use strings for unknown ammounts of data.  Or read it in one character at a
time...


. Can memory be reserved for several arrays with many decalarations
Yes.

. How many different types of arrays
Infinity.  One for each type of data at least.

. What happens if people supply too much data in an intializer
I think you get a compile error .... I hope so.
. How to avoid supplying too much data in an initializer
Let the compiler count the elements for you
.As_is 		double s[] = {1.23, 3.45, 5.67,7.89};
. Can you initialize arrays in any part of a program
You can initialize an array only in a daclation of a local
variables.  So any where where a local declaration is legal can have an array
and an initializer.

You can not initialize a data memebr/dtat field in a class... even if it is
an array.

. Is using an initializer fast than a for loop
Yes.  The compiler does the initializaitiopn before the program starts, I believe.

. Why is it better to store characters in char arrays rather than string
There are only two reasons:  speed and because we are forced to.  Arrays are
a bit faster because they have no frills and no safety features -- just like a sports
car.

We are forced to use a character array each time we type in a string literal with double
quotes:
.As_is 		"Hello"

This is converted into an unamed array of 6 characters (notice the 6(six) chars)
initialized to 
.As_is 		char noname[]={'H', 'e', 'l', 'l', 'o', '\0'};
Notice the extra NULL character at the end.

Personally -- I use STL <string>s as much as I possible can. They may be slower but they
are less confusing, more powerful, and safer.
. Which feature of character arrays is mostly used
Their is a CSCI202 technique known as a "char*" (pronoounced "char star") that gives
you tight, elegant, and fast code for many operations.  They also tend to be surprisingly
buggy.

. Can we us const to declare a function
First, you can declare parameters in a function to be constant
.As_is 		type function (...., const Type parameter, ...)...

Secod you can attach "const" to a member function of a class to tell
the compiler that it is an accessor -- a function that does not change its object
(examples for vectors: size() and empty()).  You do it like this inside a class
.As_is 	class Sample
.As_is 	{
.As_is 		...
.As_is 		double mean () const;
.As_is 		...
.As_is 	};
or outside the class
.As_is 	type Sample::mean()const { return sum()/size(); }

But I don't think you can define a "const function" -- it doesn't mean anything.

. Why do we need static_cast, why not write the expression so that it is in the right type
It is often possible to write expressions so that the give data of the right type.
For example
.As_is 		1/2
uses int arithmetic to calculate 0.  But
.As_is 		1.0/2
will use double arithmetic to calculate 0.5.

However, if you have to integers, `i` and `j`, and need their ratio
.As_is 		i/j
gives a rounded int answer...  A trick is to include a double:
.As_is 		(1.0*i)/j
for example.  And form the 1970's to the 1990's we could cast a number like this
.As_is 		((double)i)/j
or, later,
.As_is 		double(i)/j
however after 2000 the standard promotes the explicit static cast
.As_is 		static_cast<double>(i)/j
which may be longer but is quite clear as to what you need to do.

. Whay sort and serach since this is inefficient
Because people need the operations.
They need to search for data -- think Google!  And search can go faster if the data
is in the right order.

People also like to see data in different orders.... and each order means you have to sort
the data into that order.

Besides the algorithms are interesting and fun.

.Close
. Quiz 8 -- Vectors
. Next -- Wind up arrays
.See ./19.html