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Python Syntax
Basic Python syntax
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Lists
Dictionaries
Looping
Conditional statements
Lists
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Lists are collection of objects
List can hold any type of object-numbers, string
List are indexed (zero based)
List can grow and shrink
Variables can hold a list
Example:
dList = [“Soils”, “Roads”, “Rails”, “Parcel”]
numList = [1, 2, 3, 4, 5, 6]
Basic List Operations
numList = [1, 6, 5, 3, 2, 4]
• Get the number of items in a list
len(numList) Result: 6
• Sorting the list: orders a list
numList.sort()
Result:[1,2,3,4,5,6]
• Append the list: adds an object to the end of a list
numList.append(7)
Result:[1,2,3,4,5,6,7]
Remember that sort, append, etc are the methods for list
but len is a built in function. What are the other methods of list?
Application of list
Example:
fList = [“Water”, “Streams”, “Roads”, “Schools”]
for lyr in fList:
print lyr
What would be the output?
Examp le:
for name in [“Carter”, “Regan”, “Bush”]
print name + “was a US president.”
What would be the output?
Dictionaries
• Dictionaries are similar to list storing objects in pairs
• It is composed of a set of key & value pairs separated
by commas and enclosed by curly braces
• Like lists, it can grow and shrink
• Variables can hold a dictionary
Example:
dictList = {“Soils” : ”Polygon” , “Roads” : “Polyline”, “Wells” : ”Point”}
fList = {‘food’ : ’bread’, ‘quantity’ : 4, ‘type’ : ’wheat’}
Basic Dictionaries Operatios
dictList = {“Soils”:”Polygon” , “Roads”:“Polyline”, “Wells”:”Point”}
• Get a list of Keys
dictList.keys()
Result:['Soils', ‘Roads', 'Wells']
• Get a list of Values
dictList.values()
Result:[‘Polygon', ‘Polyline', ‘Point']
dicList = dicList.Keys( )
for lyr in dicList:
print lyr
What would be the output?
Conditional statements (if/elif/else)
• Conditional statements are used to see if a condition is
true or false
• Very common in decision making
• if….elif….else: Python’s conditional logic
if y == 1:
print “y is 1”
elif y == 2:
print “y is 2”
else:
print “y is neither 1 nor 2”
Conditional statements
y = 1
If y > 2:
print “y is greater than 2”
else:
print “y is less than or equal to 2”
Question: What would be printed?
• Colon used at the end of each condition
• Indentation defines what executes for each condition
• Python automatically indents
y = 2:
#assignment
if y == 2:
#testing condition
• One equal sign (=) for assignment, two(==) for conditions
Looping!
Looping allows your program to repeat over and
over as necessary
Two basic types of loop:
1. For loops
2. While loops
• For loops execute a block of statements a predetermined
number of times
• A While loop executes until some condition is met
For loop
For loops execute a block of statements a
predetermined number of times
for i in range (1,10):
print i
What would be the result ?
for i in range (10):
print i
Will you get the same result ?
for i in range (1,10,2):
print i
What would be the result ?
For loop
Python’s for loop can also operate on a list of items
for name in [‘Carter’,’Regan’,’Bush’]:
print name + “was a US president”
What would be the result ?
Looping the loop
for n in range (1,11):
for m in range (1,11):
print n, “*”, m, “=“, n*m
print “……………………………”
What would be the result ?
While loop
x =0
while x < 11:
x = x + 1
print x
x =0
while x < 11:
x = x + 1
print x
Same or different result ?
• Python executes the entire block of code after the
colon until the condition is true
• Python detects the block with colon & indentation
While loop
password = “nothing”
while password != “GIS”:
password = raw_input(“Enter your Password: “)
While loop
password = “nothing”
while password != “GIS”:
password = raw_input(“Enter your Password: “)
if password == “GIS”:
print “Congratulations! You’re in”
else:
print “Please try again!”
Pay attention to the indentation, colon, etc
Using a counter with a loop
password = “nothing”
count = 0
while password != “GIS”:
password = raw_input(“Enter your Password: “)
count = count + 1
If password == “GIS”:
print “Congratulations! You’re in”
else:
print “Please try again!”
print “It takes you” + str(count) + “trial”
Python Modules
• Python extends its capability by
incorporating functions from external modules
• You use import to bring modules
Example:
import math (import math module)
import arcpy (import arcpy module)
Useful Modules
To generate random number
import random
random.random()
#will generate floating point number between 0 & 1
Result: 0.4466987867
random.randomint(1,10)
Result: 4
random.choice([‘Chair’ , ’Table’ , ’Book’]}
Result: ‘Table’
Some useful functions
import math
math.pi
Result: 3.1415926535897931
math.sqrt(144)
Result: 12.0
math.trunc(22.6758)
Result: 22
int(22.6758)
Result: 22
round(22.6758)
Result: 23.0
round(22.2344)
Result: 22.0
Common Python Operator
Operator
Symbol
Example
• Addition
+
7 + 3 = 10
• Subtraction
-
7–3=4
•Multiplication
*
7 * 4 = 28
• Division
/
7/3=2
• Remainder
%
7%3=1
• Equal to
==
a == b
• Not Equal to
!= or < >
a != b or a < > b
Read relevant sections from
online resources as needed!