GIT 461 GIS Python Programming Fundamentals Python Programming

GIT 461 GIS Python Programming Fundamentals

Python Programming Environment • We will use the “Python. Win” version 2. 7 system • This programming environment includes a code editor and a code interpreter • A code interpreter executes statements lineby-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”

Python. Win Environment • Interpreter window is displayed first. • You can type and execute simple statements in this window.

Python. Win 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 “Python. Progs” just for source code scripts.

Python. Win Environment • To load an existing python script select “File > Open” to load script into an edit window.

Python. Win 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

Python Programming Fundamentals • Google “Python Language Reference” to load this help file

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. 6745 E 3

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 screen X=X+5 Print X # the number “ 6” will appear on the screen

Python Comments • • A python comment begins with a “#”. Anything after the “#” is ignored by Python The 1 st 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 x 1, y 1, x 2, y 2 from the command line x 1 param = float(Strip. Comma(sys. argv[1])) y 1 param = float(Strip. Comma(sys. argv[2])) x 2 param = float(Strip. Comma(sys. argv[3])) y 2 param = float(Strip. Comma(sys. argv[4])) # x 1 # y 1 # x 2 # y 2

Python Operators (in order of precedence) • • • Multiplication: * Division: / Modulus: % Addition: + Subtraction: -

Expressions • Expressions are combinations of data and operators:

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 window

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

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.

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.

Conditional Statements: More Complex IF/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”

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=0 While i <= 10 : print I i=i+1 # you could also use i += 1

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

Getting User Input: command line • Command line parameters are entered at run time in the “Run Script” window

Getting User Input: “Input” function • The “input” function prompts the user for input during the running of the script

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).

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

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