GTECH 731 Programming for Geographic Applications

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• GTECH 731 Programming for Geographic
  Applications
• Tuesdays 535 p.m. - 9.15 p.m. Room 1090B-HN

Professor Sean Ahearnsahearn_at_hunter.cuny.edu212-
772-53271023 Hunter North CARSI LabTeaching
AssistantGordon Greenggreen_at_ecteon.com212-543-4
478
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• GTECH 731 Programming for Geographic
  Applications
• Texts

Required Learning C 2005 Get Started with C
2.0 and .NET Programming by Jesse Liberty.
Optional Java Programming for Spatial Sciences
by Jo Wood. Java is very similar to C, but not
similar enough for us to use this as the primary
text. However, it may be useful to read in
parallel with the Liberty text. Other readings
may be given out in the form of handouts.
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• GTECH 731 Programming for Geographic
  Applications
• Attendance and Exercises

Assignments There will be short assignments
almost weekly. It is very important to stay
up-to-date with these, because each assignment
will build directly on the last one. These will
account for most of the grade. Absences
Especially in the first half of the semester, any
missed material will be problematic since each
topic depends on the preceding topics. Plagiarism
It is important to do your own work and work
through the problems yourself. Lab policies
Always delete your working files from you local
machine and keep all your files on the network
drives. Dont install any software, and
otherwise abide by the lab policies http//www.g
eography.hunter.cuny.edu/tbw/spars/rules.html
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• 1. Programming Background

• Computer basics
• Programs
• Languages
• Role of the operating system
• C program elements

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• Computer Basics
• The basic concept of a computer was first
  envisioned by Turing in 1936, when he described
  an abstract model of the modern computer

From Wood, 2002.
Details
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• Computer Basics
• Turing then devised the Universal Turing
  Machine, a related thought-experiment where the
  machine can run any other defined Turing machine.
  
• This corresponds most closely to an actual
  computer, where any algorithm can be run on a
  single machine.
• For an interesting and more in-depth discussion,
  of this topic see

Martin Davis. Engines of Logic Mathematicians
and the Origin of the Computer. Chapter 7
"Turing Conceives of the all-purpose computer"
Norton, 2001. ISBM 0393322297.
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Computer Basics
Rather than using arbitrary symbols, computers
represent everything as a zero or a one (a bit),
usually grouped into multiples of 8 (a byte).
Most PCs are now have a 32- or 64-bit
architecture, which means that data is most often
treated in units of four or eight bytes, for
example 0001 1000 1010 0111 0111
1111 0000 0000 What exactly these numbers
mean depends on the context. They can
represent An instruction in a program ADD
two numbers MOVE this information from this
location in memory to another An integer
123,456 -12 A floating-point number
1.1234 123,456.789 A location in the
computers memory The place where the text of
the constitution is stored Letters ABCD Et
c., etc.
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Computer Basics System diagram
Program memory refers to the memory storing the
actual program, which is normally loaded from
disk into memory, and data memory, which is where
information manipulated by the program is stored.
The processor treats these two kinds of
information differently. Programs are a sequence
of instructions executed by the processor data
is information altered and stored by the
program. Diagram from Hordeski, 1990.
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Computer Basics Processor diagram
The processor contains registers which contain
the information it is currently operating on, the
current program location, and other critical
information. Registers are located in the heart
of the CPU (central processing unit) and
represent the fastest-working part of the system.
Information is then transferred from a cache,
or short-term memory on the main chip, which in
turn transfers information to and from main
memory (RAM). Information in RAM may then be
passed on to disk (e.g., in file/save), or a
network, printer, etc. More information on
registers...
Diagram from Hordeski, 1990.
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Computer Basics Memory hierarchy
This results in a hierarchy of storage
areas Registers and Cache are not
usually managed by the programmer directly.
fastest Registers Cache Memory (nanoseconds)
Disk (milliseconds) Network Drives (eg,
fileservers) (seconds) Slowest

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• Programs
• A program is a series of instructions that
  operate on data. The central processing unit
  reads instructions in sequence from memory, and
  executes them one by one, in a kind of loop

Diagram from http//en.wikipedia.org/wiki/ImageCP
U_block_diagram.svg
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Programs Each instruction, or group of bits
understood as a command by the processor, is
loaded from memory by the processor, and results
in a particular action being taken. Rather
than using the long binary number or machine
code, a programmer can represent the instruction
with a mnemonic. Each kind of
processor has its own Instruction Set, which
means that these instructions are different for
different chip makers. This is a large part of
why, for example, code written for Motorola and
Intel chips had such a hard time cooperating.
Example from http//www.compilers.net/paedia/assem
bly_language/index.htm.
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Programs These mnemonics are the basis of
assembly language. In assembly language, you
have to explicitly deal with low-level details
like registers and locations in memory, which
allows you to write very efficient code.
However, it is extremely time-consuming and
impractical for most applications.
Usually programming is done in a higher-level
language, which is automatically translated into
machine code. In a high-level language, very few
words represent many lines of assembly code.
Example from http//www.pcmag.com/encyclopedia_ter
m/0,2542,tcompileri40105,00.asp
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Languages These high-level languages are all
basically variations on replacement algorithms,
or grammars, wherein rules are implemented which
govern what system of replacements generates the
final program. There are many ways a language
syntax can be described. The following is a
sample of the kind of grammar diagram that can
succinctly describe a statement. Each valid
language statement can be replaced either by
other language statements, or by entities that
can be ultimately distilled to machine code.

Language grammars are a very large topic, but not
really necessary when actually programming. See
the relevant Wikipedia entry for more details.
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Languages Lineages

• Languages follow lineages, where each language
  shares characteristics of its predecessors. In
  this course we will be using C, which is closely
  related to Java and descends from C and C.
• C is a very low-level, systems-oriented
  procedural language that made it easy for
  programmers to write code as economical as
  assembly language. It also made it easy to make
  mistakes and write buggy software.
• C added some more advanced features, making it
  possible to write more high-level code, but it
  but left all the original problems of C in place.
  In some ways this made things worse by making
  them more complicated.
• Java rectified most of these problems, but the
  Java system is geared toward cross-platform
  development, and is awkward when using
  system-specific features (like Windows user
  interfaces).
• C has the benefits of Java, but is closely
  linked to Microsofts .NET framework, which
  allows you to write fully functional Windows
  programs that use all of the features of the OS.

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Languages Lineages
For a more up-to-date diagram, go to
http//www.levenez.com/lang/history.html05
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Languages High-level versus low-level
There are a variety of ways of classifying
languages. High-level and low-level is sometimes
a useful distinction, although it can be
misleading because you can, for example, write
high-level functions using a low-level language,
and some high-level languages fully support
low-level functions.
From Wood, 2002.
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Languages Compiled versus interpreted
In interpreted languages, the code is not
compiled to machine code, but, when the program
is run, the instructions are translated into
system commands by a separate program (the
interpreter) on the fly.
Most scripting languages, like JavaScript, are
interpreted. They are usually slower than
compiled languages, but not always. The speed of
a program depends on many factors, and whether it
is interpreted may or may not be determinative.
Diagram from http//web.cs.wpi.edu/gpollice/cs544
-f05/CourseNotes/maps/Class1/Compilervs.Interprete
r.html
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Languages Compiled versus interpreted
Languages can be compiled or interpreted. In
compiled languages, the code is compiled to
machine code, and the operating system manages
running and terminating the program
Diagram from http//web.cs.wpi.edu/gpollice/cs544
-f05/CourseNotes/maps/Class1/Compilervs.Interprete
r.html
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Languages Hybrid languages
Java, C, and Visual Basic are examples of
hybrid languages. In these cases the compiler
generates an intermediary code which depends on a
separate software infrastructure to run. This
allows for more flexibility because the
intermediate code is not resolved to machine
code, so is more independent of the particular
platform it runs on. At the same time, it can be
highly optimized because it is compiled. In C
and .NET, the Intermediate Code is MSIL, or
Microsoft Intermediate Language. The
Interpreter is the Just-In-Time Compiler, or JIT,
which creates executable code from the MSIL on
the fly.
Diagram from http//www.codeproject.com/KB/dotnet/
clr.aspx?df100forumid3272exp0select412238.
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Role of the Operating System Running a program
Once a program is compiled, it is run by the
operating system. The operating system is
responsible for allocating memory for the
program, loading its first instructions into the
processor, managing the process as it runs, and
cleaning up after it terminates. It also
provides the interfaces by which it communicates
with devices which would otherwise require more
specialized code.
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Role of the Operating System
In modern operating systems, the OS is
responsible for many functions that would
otherwise require programmers to rewrite basic
operations like drawing a letter on the screen.
One key role of the operating system is to
launch and manage programs, which, when running,
become processes that the operating system
juggles. Other functions include interacting
with the user (managing the mouse, keyboard, and
display), managing communication with peripheral
devices (disk drives, networks, etc), displaying
graphics, managing windows, and many other
functions that once required specialized
programs. That means most of what most programs
do is interact with the operating system. The
Microsoft .NET architecture provides a convenient
way of accessing OS and network resources. Since
we will be working in that environment, a large
part of the code we write will involve
interacting with .NET.
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Sample C program
Here is a very simple C program. It consists
of components that we will get into in more
detail in later classes. All of these elements
are required in any C program. using
System   namespace HelloNameSpace public
class HelloWorld static void Main(string
args) Console.WriteLine("Hello
World!")
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Sample C program
The USING statement defines what part of the
.Net framework (or other external components)
will be incorporated into this program using
System   NAMESPACE says that any names (more
on this later) created here are part of the given
unit, not another (e.g., System). namespace
HelloNameSpace Curly braced define the beginning
and end of any block of code. A block of code
means different things in different contexts.
Here, the HelloNameSpace consists of anything
within the block. For clarity, blocks typically
share the same level of indentation.
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Sample C program
PUBLIC CLASS HELLOWORLD says this code unit, or
class (much more on classes later), that we are
calling HelloWorld, is available to any external
code to use. public class HelloWorld STATIC
VOID MAIN. More on static and void later, but
MAIN is a special function name that declares
this as the starting point of the program.
STRING ARGS is required for the starting point
of the program, and contains any command-line
parameters (for example, if we typed in
HelloWorld Banana at the command line, args
would contain Banana). static void
Main(string args) SYSTEM.CONSOLE.WRITELINE(
HELLO WORLD!) is telling the Console object
within the system namespace to write the line
Hello World!. System.Console.WriteLine("Hello
World!") Main() and WriteLine() are
functions, or ways of invoking code that take
some action. HelloWorld and Sytem.Console are
objects, or units of code that contain functions.