GIT461 GIS - PowerPoint Presentation

Slide1

GIT461 GIS

Python Programming FundamentalsSlide2

Python Programming Environment

We will use the “

PythonWin

” version 2.7 systemThis programming environment includes a code editor and a code interpreterA code interpreter executes statements line-by-line as you type them – very useful for testingA Code editor is used to type a complete program that is later tested and “de-bugged”Slide3

PythonWin Environment

Interpreter window is displayed first.

You can type and execute simple statements in this window.Slide4

PythonWin Environment

To type a Python script program select “File > New > Script”.

Type in code and save with a .

py extention.A good idea to have a folder such as “\PythonProgs

\” just for source code scripts.Slide5

PythonWin Environment

To load an existing python script select “File > Open” to load script into an edit window.Slide6

PythonWin Environment

To run a script you should load it, and then select “File > Run”

Note that you may enter “command line arguments” at this point if needed

Select “OK” button to run the scriptSlide7

Python Programming Fundamentals

Google “Python Language Reference” to load this help fileSlide8

Python Data Types

String: a sequence of alphanumeric characters

Integer: a whole number that has no fractional component

Float: a number that contains a fractional componentString example: “105 Glendale Avenue” (note that strings are enclosed in quotes)Integer examples: 100, -19, 0, 9999999Float examples: 1.0, -123.678, 1.6745E3Slide9

Python Assignment Statement

The “=“ sign is the assignment operator as it is in most programming languages

X = 1

Print X # the number “1” will appear on the screenX = X + 5Print X # the number “6” will appear on the screenSlide10

Python Comments

A python comment begins with a “#”.

Anything after the “#” is ignored by Python

The 1st line in the below script is a comment line- it is ignored by PythonThe characters to the right of the “#” on lines 2-5 are ignored

# get x1, y1, x2, y2 from the command line

x1param = float(

StripComma

(

sys.argv

[1])) # x1

y1param = float(

StripComma

(

sys.argv

[2])) # y1

x2param = float(

StripComma

(

sys.argv

[3])) # x2

y2param = float(

StripComma

(

sys.argv

[4])) # y2Slide11

Python Operators (in order of precedence)

Multiplication: *

Division: /

Modulus: %Addition: +Subtraction: -Slide12

Expressions

Expressions are combinations of data and operators:Slide13

Built-in Python Functions

A function takes an “argument” or “arguments” and returns a value that can be used in an assignment statement

In the below

satements abs(x) and pow(x,y) are built-in functions in every implementation of the Python language

x = abs(-8)

print x # the number “8” would appear in the interactive window

y = pow(2,3)

print y # the number “8” would appear in the interactive windowSlide14

Python Built-In Functions

abs(x) # returns the absolute value of x

float(x) # returns the string x converted to a floating point number

int(x) # returns the string x converted to a integer numberpow(x,y) # returns the number x rasied to the y power

round(

x,n

) # rounds the number x to n decimal places

str

(x) # returns the string equivalent of the object xSlide15

More Complex Expressions using Functions and Exponentiation

Note that trig functions use radian angular values.

You must convert degrees to radians before using the trig functions ( radians = degrees * 3.1416/180.0).

Note that before using the trig functions the math “module” had to be imported.Slide16

Controlling Program Flow: Conditional Statements

A statement uses “reserved” python language words to initiate an action

A conditional statement makes a decision at run-time

X = 10

If x == 10 :

print “X=10

”

Note: everything indented

Below the “If” statement is

Part of the statement.Slide17

Conditional Statements: More Complex IF/ELIF/ELSE construct

The “

elif

” and “else” keywords can be used to construct more complex decision structuresx =

random.randint

(1,10)

If x == 1 :

print “you are in first place”

Elif

x == 2 :

print “you are in second place”

Elif

x == 3 :

print “you are in third place”

Else :

print “ sorry, you didn’t place this time” Slide18

Controlling program flow with a loop: While statement

While statements can be used to repeat a section of code until some condition is reached.

i

= 0While

i

<= 10 :

print I

i

=

i

+ 1

# you could also use

i

+= 1Slide19

For Loops

A “For” loop uses a “list”. First a list must be built before it can be used in the For loop

Mylist

= [“A”, “B”, “C”, “D”]For letter in Mylist

:

print letter # the letters “A” through “D” would print to the screenSlide20

Getting User Input: command line

Command line parameters are entered at run time in the “Run Script” windowSlide21

Getting User Input: “Input” function

The “input” function prompts the user for input during the running of the scriptSlide22

User Input: Input function

Note that if you use the “input” function and you enter a string value it must be enclosed in

quotes (single or double).Slide23

Functions

A function is a stand-alone section of code that is designed to accomplish a specific task based on one or more parameters passed to the function

The function returns a calculated result so a function normally appears in the main code to the right of an assignment (=) statement so the returned value is stored in the variable on the left side of the assignment statementSlide24

Function Placement

Functions are normally placed at the top of the main program file because they need to be defined before they are referenced in the main program

Below is an example function that converts a longitude string value (“-0883015”) to its decimal degree number equivalent