CEGE0096: Geospatial Programming
Lecture 1: Introduction
September 30, 2019
Aldo Lipani
Welcome!
CEGE0096
Geospatial Programming
Tutor: Dr. Aldo Lipani ([email protected])
Use Moodle forum for questions!
https://moodle.ucl.ac.uk
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This course provides an introduction to programming in
the context of geospatial science and technology.
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Aims and Learning Outcomes
By the end of the module, you will:
• Understand the basic principles of imperative programming
• Understand the basic principles of object-oriented programming
• Automate geospatial analysis workflows and map production
• Become familiar with:
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Python
Anaconda Python Notebooks
PyCharm
Git
Fast Paced Course
Position yourself to succeed!
• Read the assignments when they come out and come back to them later
• The practicals (after the assignments are released) are dedicated to improve
skills useful to solve them
New to programming? PRACTICE. PRACTICE? PRACTICE!
• You can’t passively absorb programming as a skill
• Run the python notebooks yourself, play with the code
• Do the exercises
• Experiment with new Python commands!
Experienced programmer? HELP. HELP? HELP!
• By helping your colleagues you will consolidate your knowledge and get better
in solving the assignments
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Course Structure – Part I
Week Date AM/PM Content Room Type
1
30 Sep PM Introduction (Programming and Python) Lecture
3 Oct AM Anaconda, Python Notebooks and Python Basics Practical
2
7 Oct PM Python (Continue), IDE and Debugger Lecture
10 Oct AM PyCharm, Debugger and Python Practical
3
14 Oct PM Python Data Structures and Version Control Lecture
17 Oct AM Git, GitHub and Problem Solving Practical
4
21 Oct PM Object Oriented Programming and GIS Data Structures with Python Lecture
24 Oct AM Drawing with Python Practical
5
28 Oct AM The Individual Project is Released
28 Oct PM Geometry with Python Lecture
31 Oct AM Review Practical
6
Reading Week
10 Nov PM Individual Project Deadline (11:59 PM)
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Course Structure – Part II
Week Date AM/PM Content Room Type
1
11 Nov PM Trees and Graphs Lecture
14 Nov AM To Define Practical
2
18 Nov PM To Define Lecture
21 Nov AM To Define Practical
3
25 Nov PM Working with Real Data in Python Lecture
28 Nov AM To Define Practical
4
2 Dec AM The Group Project is Released
2 Dec PM To Define Lecture
5 Dec AM To Define Practical
5
9 Dec PM Copyright and GDPR Lecture
12 Dec AM Review Practical
13 Jan PM Group Project Deadline (11:59 PM)
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Course Venues
• Lectures are at 2 PM on Mondays,
• Practicals are at 11 AM on Thursdays,
• Use campus route finder if you get lost (https://www.ucl.ac.uk/maps):
WARNING: Practicals rooms change often! Always check where it is!
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This Thursday:
Christopher Ingold
Building G20
Assessments
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• 1 individual programming assignment (50%)
• Geometric problem
• 1 group programming assignment (50%)
• GIS problem
• Using real data
• In both assignments you will be evaluated based on:
• The solution you found
• The use of git
• A report
• Etc. (details will be provided in the assignments sheets)
Recommended Readings
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• From Moodle (https://ucl.rl.talis.com/modules/cege0096.html)
• For the first assignment this book and the course material will suffice
PROGRAMMING
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What is Programming?
• It’s a way to tell a computer what to do.
• It is done via a programming language:
– It’s a set of rules for communicating an algorithm
– It provides a linguistic framework for describing computations
• Enables complex data handling and processing
• Not solely the pursuit of specialists: programming lies at the heart of
almost everything we do.
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What is Algorithm?
• Algorithms resemble recipes. They
take “inputs” (the ingredients),
perform a set of simple steps, and
then terminate after producing an
“output” (the meal)
• But algorithms are not like recipes
because their steps are well-defined
and expressed in a precise way (1/2
cup!?)
• An algorithm is a precise description
of sequence of steps that given an
input terminates returning an output
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Why Programming?
• Teaches you how to break down a complex problem and construct
methods for achieving a solution – computational thinking
• Allows you to access a wide range of already developed tools
• Makes you more employable
You will work with multiple large geospatial (or spatio-temporal) datasets:
• Processing
• Analysis
• Visualization and Mapping
… but what about GUI-based software, e.g. Excel, ArcMap?
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Programming vs. GUI-based Software
GUI-based software can only take you so far:
• You can create a map of restaurants in a town,
but what if you need the same map for every town in the UK?
• You can assign a missing coordinate reference system to a single dataset
when you load it in GIS application,
but what if you have thousands of datasets with missing CRS?
• You can carry out Kriging using a toolbox of GIS applications,
but what if the parameters in the tool don’t give you the flexibility you need?
What if you need to analyse/visualise the results in a different way?
GUI-based software will only allow you to do what the developer has
programmed.
Programming allows you to tell a computer what you want.
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Programming Languages
A programming language is a formal language that is used to instruct a
computer how to perform tasks
There are many different programming languages. Each one is designed
to suit different applications and users:
• High performance?
• Portable?
• Flexible?
• Rapid development?
• Graphical?
Which programming language?
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Levels of Programming Languages
High-level program
Low-level program
Executable Machine code
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class Line2D:
def __init__(self, x1, y1, x2, y2):
self._x1 = x1
self._y1 = y1
self._x2 = x2
self._y1 = y2
mov rax, 1
mov rdi, 1
mov rsi, message
mov rdx, 13
syscall
mov rax, 60
xor rdi, rdi
0010100010001000100111100101000100010001
0100010100010010001111110010101010010010
0100000010011110100100100100010101001010
1001001010001111111111111100000001010101
0010000000001010101000100100100100010010
1001001010010101001010100010001010010010
1001111100010010010100100000000000000000
Example of Language Types
More sophisticated, restricted-purpose languages
• MATLAB, R, LaTeX
High-level multi-purpose languages
• Python, Ruby, Perl
Mid-level multi-purpose languages, enabling greater flexibility and
performance
• Java, C/C++, C#
Low-level machine code
• Assembly language
Web development languages
• JavaScript, XML, HTML, PHP
Graphical languages
• Scratch, FME
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Language Paradigms
A programming paradigm is a style, or “way” of programming.
Paradigms:
• Imperative
– Results are achieved by ordered change of state
• Functional
– … by a series of function applications
• Logic
– … by declaring a question about a system of facts and rules
• Object-oriented
– … by grouping instructions and states together
• Hybrid
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Compiled vs. Interpreted Languages
Programs written into compiled languages are first compiled (translated)
into machine code:
Programs written into interpreted languages are interpreted into machine
instructions:
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Interpret
Compile Execute
What Language is the Best?
Python. (Next Slide)
Java. OOP, “compiled”, runs on JVM
C. compiled, fast, (too) flexible
C++. OOP, like C
R. OOP, functional, interpreted, slow
JS. OOP, interpreted, runs on browsers
…
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Python
• Python was created by Guido van Rossum in 1989.
• Python is quick:
– To learn
– To write
– To read
– To debug
– To maintain
• Python, today, has a large community of enthusiasts,
who create useful packages for data science, statistics,
machine learning, etc.
• Python is multi-paradigm:
imperative, object-oriented and functional.
• Python is an interpreted language.
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Python for Geospatial
• Lots of libraries to work with geographic data
• Used for scripting and plugin building in QGIS and ArcGIS
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Examples of Mapping in Python
Python can be used for mapping directly
matplotlib is a powerful library for data visualization
Many libraries for dealing with geospatial data:
• GeoPandas
• shapely
• rasterio
• pyproj
• PySAL
• …
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Examples of Mapping in Python (2)
These two visualizations were made using Python alone (no GIS software)
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PYTHON
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Hello, World!
1. Open the python interpreter or
… insert a code cell in a Jupiter notebook
… create a new python script in an IDE
2. Write
print("Hello, World!")
3. Hit return or
… press the play button/shift+return
… run the script
4. Observe your computer greeting the World!
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Our Python (Imperative) Journey
• Types
• Operators
• Variables
• Simple I/O
• Branching
• Loops
• Program Planning
• Strings
• Tuples
• Lists
• Dictionaries
• Files
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Types (I)
Integers (whole number):
• Named int
• Range (−∞; ∞)
• You can express integers in various bases:
print(58, 0b111010, 0o72, 0x3A)
Floating-Point Numbers (real numbers)
• Named float
• Range −∞; ∞ (virtually)
– if greater than 1.79e308 it becomes inf
– if smaller than 5e-324 it becomes 0
print(3.1415)
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Types (II)
Complex Numbers:
• Named complex
• Like float
• Use .real or .imag to extract the real or imaginary parts
print(1+5j, 1+5j.real, 1+5j.imag)
Boolean:
• Named bool
• Values True and False
print(True, False)
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Types (III)
Strings (sequences of character data):
• Named str
• As long as you wish
• Delimited by "" or '' (but please be consistent)
• Escape with \ to insert ' or " in your string
print("I am a string.")
• Use """ to write escaped strings:
print("""This is a
string that spans
across several lines""")
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Operators (I)
With numeric types, arithmetic operators:
WARNING: Strange results when applied to Boolean types
Operator Description Example Result
+ Addition 13+2 15
- Subtraction 13-2 9
* Multiplication 13*2 36
/ Division 13/2 6.5
% Modulus 13%2 1
// Floor Division 13//2 6
** Exponent 13**2 169
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Operators (II)
With numeric or Boolean types, comparison operators:
Operator Description Example Result
== Equality 13==2 False
!= Inequality 13!=2 True
> Greater 13>2 True
< Less 13<2 False
>= Greater or Equal 13>=2 True
<= Less or Equal 13<=2 False
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Operators (III)
With Boolean type, logical operators:
WARNING: Strange results when applied to numeric types
Operator Example Result
and True and False False
or True or False True
not not True False
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Operators (IV)
With String type, string operators:
What happens if to the slice operator you pass a negative number?
Operator Description Example Result
+ Concatenation "hello" + "world" "helloworld"
* Repetition "hello" * 5
in Membership "hello" in "world" False
not in Not Membership "hello" not in "world" True
[] Slice "hello"[1] "e"
[:] Range Slice "hello"[0:2] "he"
"hellohellohellohellohello
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So far
You can use Python as a powerful calculator
By combining data types and operators,
you can write:
12 * 10 + 32 % 2 / 2 - 78
print("hello"[-4] + "and "[1:4] + "ehthe "[2:6] + "world")
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Variables
• A variable provides a way to label and access information
• You can create a variable by typing:
name_of_the_variable = "and its value"
• Once created, you can access its content by using it as data
name_of_the_variable * 5
name_of_the_variable[2:4]
print(name_of_the_variable)
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Naming Variables
• Names of variables are case sensitive and cannot start with a
number. They can contain letters, numbers, and underscores.
bob Bob _bob _2_bob_ bob_2 BoB
• There are some reserved words:
and, assert, break, class, continue, def, del, elif, else,
except, exec, finally, for, from, global, if, import, in,
is, lambda, not, or, pass, print, raise, return, try, while
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Simple I/O
• You have already seen print in action
• It prints to screen the content of a variable or the result of a calculation
• “print” is a built-in function
• You can pass multiple strings, these will be concatenated with a space
• To read from keyboard you can use the input function
• It waits for the user to type something on the console until the user
presses enter
• Then it returns the string value inputted by the user
• To convert it to a digit use the function int (or float).
user_input = input("Please type something: ")
print("User input:", user_input)
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Converting Types
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• The built-in str function converts a data type into a string
• You can convert a string to an integer using int
• You can convert a string to a floating-point number with float
• Example of conversion of an integer to a string:
x = 3
print("x is equal to " + str(x))
• Example of conversion of a string to a float:
pi = input("insert pi ")
print(pi) # "3.14"
pi = float(pi)
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So far
You can use Python as a more powerful calculator
h0 = "rabbit"
h1 = h0
h0 = 5
print(h0, h1)
Where is the rabbit?
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Flow Control
• Control statements are the building blocks of computer programs
• Allow application of conditions on actions
• Allow repeated execution of actions on multiple data
• Three key types of flow control
– if condition statements
– while loops
– for loops
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If Statement (I)
If the condition is verified the block below the if statement is executed
if x == 42:
print("x is equal to 42")
Note:
• Use of indentation for blocks
• Colon (:) after Boolean expressions
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if
END
START
ATrue
False
If Statement (II)
If the condition is not verified the else block is executed
if x == 42:
print("x is equal to 42")
else:
print("x is equal to something else")
Note:
• Use of indentation for blocks
• Colon (:) after Boolean expressions
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if
END
Start
A
True
B
False
If Statement (III)
If you have multiple conditions to check, you can use the elif block. If the if
condition is not verified and the elif block is verified then execute the elif
block.
if x == 42:
print("x is equal to 42")
elif x == "hello":
print("x is equal to 'hello'")
else:
print("x is equal to something else")
Note:
• Use of indentation for blocks
• Colon (:) after Boolean expressions
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if
END
START
A
True
elif
False
C B
TrueFalse
IMPORTANT: Whitespace
• Whitespace is meaningful in Python: especially indentation and
placement of new lines (i.e. press Enter on keyboard).
• Use a newline to end a line of code.
• Use \ when must go to next line prematurely.
• No braces { } to mark blocks of code in Python…
Use consistent indentation instead.
• The first line with more indentation starts a nested block
• The first line with less indentation is outside of the block.
• A colon appears at the start of a new block
• Convention is 4 spaces or 1 tab, but don’t mix them up!
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IMPORTANT: Whitespace
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Whitespace Example
• No braces { } to mark blocks of code in Python…
Use consistent indentation instead.
• The first line with more indentation starts a nested block
• The first line with less indentation is outside of the block.
• Convention is 4 spaces or 1 tab but can be any number as long is it is
consistent; don’t mix them up!
if type(x) == int: # Level 1
print(x, "is an integer") # Level 2
if x == 0: # Level 2
print(x, "is zero") # Level 3
else: # Level 1
print(x, "is not an integer") # Level 2
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Exercise (10 minutes)
Write a program that requests the user to input a temperature in degree Celsius
and prints out a suitable message according to the temperature state below:
• “Freezing” when the temperature is below zero;
• “Very cold” when it’s between 0 and 10 (not included);
• “Cold” when it’s between 10 and 20 (not included);
• “Normal” when it’s between 20 and 30 (not included);
• “Hot” when it’s between 30 and 40 (not included);
• “Very hot” when it’s above 40.
You are allowed to use variables, operators and if statements.
Function reminder:
input("message") prints the message and waits the user for an input
float to convert strings to floating-point numbers
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Solution
print("Temperature to Perceived Temperature Converter (0.1)")
temp = input("What temperature is outside? ") # This returns a string
temp = float(temp) # This converts it to a float
perceived_temp = ""
if temp < 0:
perceived_temp = "freezing"
elif temp < 10:
perceived_temp = "very cold"
elif temp < 20:
perceived_temp = "cold"
elif temp < 30:
perceived_temp = "normal"
elif temp < 40:
perceived_temp = "hot"
else:
perceived_temp = "very hot"
print("Your temperature today is", perceived_temp)
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Exercise (10 minutes)
Write a program that ask the user for 3 angles in degrees and checks, if
possible, which triangle you can build with them:
• Equilateral triangles have all angles equal;
• Isosceles triangles have 2 angles equal;
• Scalene triangles have all of them are different.
Rules are the same of the previous exercise.
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Solution
print("Angles Checkers for Triangles (0.1)")
angle1 = float(input("1st angle: "))
angle2 = float(input("2nd angle: "))
angle3 = float(input("3rd angle: "))
if angle1 + angle2 + angle3 != 180:
print("You can't build any triangle out of these angles!!!")
elif angle1 == angle2 and angle2 == angle3:
print("You can build an equilateral triangle")
elif angle1 == angle2 or angle2 == angle3 or angle3 == angle1:
print("You can build an isosceles triangle")
else:
print("You can build a scalene triangle")
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x = 34 - 23 # A comment.
y = "Hello" # Another one.
z = 3.45
if z == 3.45 or y == "Hello":
x = x + 1
y = y + "World" # String concatenation
print(x)
print(y)
1. A comment
2. Variable assignment
3. Whitespace
4. A mathematical calculation
5. A control statement
6. Function call
Summary
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See you on Thursday for the Practical
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Christopher Ingold
Building G20