git461 gis python programming fundamentals. python programming environment we will use the...
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GIT461 GIS
Python Programming Fundamentals
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Python Programming Environment• We will use the “PythonWin” version 2.7
system• This programming environment includes a
code editor and a code interpreter• A code interpreter executes statements line-
by-line as you type them – very useful for testing
• A Code editor is used to type a complete program that is later tested and “de-bugged”
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PythonWin Environment• Interpreter window is
displayed first.• You can type and
execute simple statements in this window.
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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.
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PythonWin Environment
• To load an existing python script select “File > Open” to load script into an edit window.
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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 script
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Python Programming Fundamentals
• Google “Python Language Reference” to load this help file
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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
component• String example: “105 Glendale Avenue” (note
that strings are enclosed in quotes)• Integer examples: 100, -19, 0, 9999999• Float examples: 1.0, -123.678, 1.6745E3
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Python Assignment Statement
• The “=“ sign is the assignment operator as it is in most programming languagesX = 1Print X # the number “1” will appear on the screenX = X + 5Print X # the number “6” will appear on the screen
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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 Python• The characters to the right of the “#” on lines 2-5 are ignored
# get x1, y1, x2, y2 from the command linex1param = float(StripComma(sys.argv[1])) # x1y1param = float(StripComma(sys.argv[2])) # y1x2param = float(StripComma(sys.argv[3])) # x2y2param = float(StripComma(sys.argv[4])) # y2
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Python Operators (in order of precedence)
• Multiplication: *• Division: /• Modulus: %• Addition: +• Subtraction: -
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Expressions• Expressions are combinations of data and operators:
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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 windowy = pow(2,3)print y # the number “8” would appear in the interactive window
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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
number• pow(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 x
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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.
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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 = 10If x == 10 :
print “X=10”
Note: everything indented Below the “If” statement isPart of the statement.
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Conditional Statements: More Complex IF/ELIF/ELSE construct
• The “elif” and “else” keywords can be used to construct more complex decision structures
x = 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”
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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 += 1
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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 screen
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Getting User Input: command line
• Command line parameters are entered at run time in the “Run Script” window
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Getting User Input: “Input” function• The “input” function prompts the user for
input during the running of the script
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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).
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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 statement
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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