topic 2: simple programming / textual analysiscsgngai/comp1d04/lectures/7_simplepr… · textual...

COMP1D04: From Scratch to Apps: Foundations of Computational Thinking and Literacy for Problem Solving

Topic 2: Simple Programming /

Textual AnalysisGrace Ngai

Much of this course and lots of the lecture notes were inspired by or derived from Brown University’s CS931. Our thanks go to Prof

Shriram Krishnamurthi and Hammurabi Mendes for their kind permission in allowing us to use their materials.


Up to now…Define the problem Find the Data

Write the Instructions

Solution

Computer (Google Docs)

XML data from Legco records

Make a big spreadsheet table

CSV format

Use X’s and Y’s votes to compare how similar they are to each other.


Textual AnalysisDefine the problem Find the Data


Solution

Computer

Build a concordance of a text - Locations of words - Frequencies of words


What is a Concordance?• An alphabetical index of all words in a text

Word LocationsApple 43, 57, 90, 103Boy 3, 6, 7, 8, 9Cat 33, 75, 90…

• Useful for understanding patterns of usages of words


Google n-gram viewer• What happens when we get to analyse lots and lots

and lots and lots of books, newspapers, etc?


https://books.google.com/ngrams

https://books.google.com/ngrams


Textual AnalysisDefine the problem Find the Data


Solution

Computer

Build a concordance of a text - Locations of words - Frequencies of words

Word frequencies over time Author of texts Bias of authors (e.g. liberal media bias) Worldwide trends (e.g. oil vs. gold) …

Python


Python

piebald ball python, http://snakes-2013.blogspot.hk/2013/01/piebald-ball-python.html

http://snakes-2013.blogspot.hk/2013/01/piebald-ball-python.html


Python• Also, a programming language

• Gives the computer instructions

• Just like a real language — has a grammar (syntax)

• I will back to school today.

• I will go back to school today.

• Also has meaning (semantics)

• I saw John in the park with the telescope.


Python Programming• Writing a Python program = Writing a program in the

Python language = Writing instructions for the computer in the Python language

• Interpreted mode: Interpreter runs the Python instructions line by line

• Reads in what you typed

• “Translates” to something the computer understands

• Executes it on the computer.


Python Interpreter — IDLE


Introduction to Python• Expressions are inputs that are evaluated by the computer

• Expressions return an answer (or an output)

>>> 4+2 6 >>> 4-2 2 >>> 4*2 8 >>> 4/2 2.0

Try these expressions in the IDLE window

IDLE prompt: Indicates that IDLE is ready for user input

input from user

output from computer

1. Expressions2. Assignments

a. Variables 3. Data Types

a. Integers b. Floats c. Strings d. Lists


Introduction to Python

We did the same thing in Google Sheets Assigned the value 4 to cell A1

• Assignments do not have an output. • They store values in the computer memory for future use.

1. Expressions 2. Assignments




Introduction to Python

We did the same thing in Google Sheets Renamed A1 to “myNumber”

• Assignments do not have an output. • They store values in the computer memory for future use.


a. Variables3. Data Types



Introduction to Python• Assignments do not have an output. • They store values in the computer memory for future use.

>>> myNumber = 4 >>> 1. Expressions

2. Assignmentsa. Variables

3. Data Types a. Integers b. Floats c. Strings d. Lists



variable expression

Computer Memory

Variable Name Value

myNumber 4




>>> myNumber = 4 >>>



>>> myNumber + 1 5 >>> (myNumber+2)*3 18

We can now use myNumber in expressions

Computer Memory

Variable Name Value

myNumber 4




>>> myNumber = 4 >>>


Naming Variables• We can name variables anything we like in Python.

• As long as we follow some rules

• Can only have letters, numbers and underscores (no spaces, no special characters — no “-“, “!”, “~”)

• Must start with a letter

• Some words aren’t allowed (reserved words)

• And good if we follow some conventions

• Usually starts with a lowercase letter

• Usually describes the value that it is storing.




>>> myNumber = 4 >>> myGPA = 3.3 >>> numOfEggs = 5


Reserved Words• Python allows us to name our variables almost anything we wish.

• But some words have special meaning in Python and we cannot use them.

• They are called reserved words.

and del for is raise

assert elif from lambda return

break else global not try

class except if or while

continue exec import pass yield

def finally in print


Introduction to Python• Python has different types of data



a. Integersb. Floatsc. Strings d. Lists

>>> type(4) <class ‘int’> >>> type(4*2) <class ‘int’> >>> type(4+2) <class ‘int’> >>> type(4/2) <class ‘float’> >>> myGPA = 3.3 >>> type(myGPA) <class ‘float’>

• There are two kinds of numbers in Python.

• Integers are numbers without decimal places.

• Floats are numbers that have decimal places.

• Python decides whether the result of a numerical calculation is an integer or a float based on some rules.


Stuff we can do with numbers

• Python allows us a number of mathematical operations on numbers:

Operator Operation+ add- subtract* multiply/ divide** power% remainder

abs() absolute valueint() integral part onlyround() round to nearest whole number

>>> print(3+4)7>>> print(3*4)12>>> print(3-4)-1>>> print(3/4)0.75>>> print(10/3)3.3333333333333335>>> print(10%3)1>>> print(abs(5))5>>> print(abs(-3.5))3.5>>> print(int(5))5>>> print(int(3.5))3>>> print(round(3.5))4





a. Integers b. Floats c. Stringsd. Lists

>>> “hi” ‘hi’ >>> word = “Hi there” >>> word ‘Hi there’

• Strings are sequences of characters (letters, numbers, spaces, special symbols, etc) surrounded by quotes.


Introduction to Python• We must put quotes around a string (otherwise Python doesn’t

know that it’s a string and doesn’t know what to do with it.




>>> "hi" 'hi' >>> hi Traceback (most recent call last): File "<pyshell#13>", line 1, in <module> hi NameError: name 'hi' is not defined >>>

anything in red in IDLE is not a good sign.


Introduction to Python• We can do some arithmetic on strings, too!




>>> x = "hi" >>> x + x ‘hihi’ >>> x ‘hi’ >>> y = x + x >>> y ‘hihi’ >>> z = “there” x+y+z ‘hihithere’ >>> x + “ “ + y + “ “ + z ‘hi hi there’

• The + operator concatenates (joins) strings together.


Introduction to Python• We can do some arithmetic on strings, too!




>>> x = "hi" >>> x * 3 ‘hihihi’ >>> y = “there” >>> (x+y)*3 ‘hitherehitherehithere’ >>> x ‘hi’ >>> y ‘there’

• The * operator repeats strings.

• Note that using variables in arithmetic operations do not change the value stored inside the variable!






>>> [5, 10, 15] [5, 10, 15] >>> myList = [5, 10, 15] >>> myList [5, 10, 15] >>> stringList = [“hi”, “there”] >>> stringList [‘hi’, ‘there’]

• Lists are an ordered collection of items (values, “things”)

• Individual elements in a list are called elements


Introduction to Python• We can do some arithmetic on lists

• + concatenates, * repeats

>>> x + y [5, 10, 15, ‘hi, ‘there’] >>> x * 3 [5, 10, 15, 5, 10, 15, 5, 10, 15]

• We can only add lists to lists!>>> x + 3 Traceback (most recent call last): File "<pyshell#18>", line 1, in <module> x + 3 TypeError: can only concatenate list (not "int") to list





Introduction to Python• We can get an element from a list

• Called indexing into a list • i is the index• myList[i] gets the i-th element from the list.

• Usually pronounced “my-list-sub-i” (like myListi) • The first element of a list is at index zero!

>>> myList = [5, 10, 15] >>> myList[0] 5 >>> myList[1] 10 >>> myList[2] 15

• What would do?>>> myList[1] = 4





Introduction to Python• We can also get a range from a list

• myList[i:j] • i is the index of the first element, j is the index just after the last element• Or: i is the start index (inclusive), and j is the end index (exclusive) • Also called slicing a list

>>> myList [5, 10, 15] >>> myList[0:2] [5, 10] >>> myList[1:3] [10, 15] >>> newList = [1, 1, 2, ”three”, 5, 8, “thirteen”, “cat”] >>> newList [1, 1, 2, ‘three’, 5, 8, ‘thirteen’, ’cat’] >>> newList[2:6] [2, ‘three’, 4, 8] >>> newList [1, 1, 2, ‘three’, 5, 8, ‘thirteen’, ’cat’]





Introduction to Python• We can also use indices and slices on strings




>>> myString ‘Hong Kong Polytechnic University’ >>> myString[0] ‘H’ >>> myString[5] ‘K’ >>> myString[4] ‘ ' >>> myString[0:4] ‘Hong’ >>> myString[0:8] ‘Hong Kon’


Introduction to Python• Review: Assignments




Command MemoryVariable Name

Value

x = 1 x 1

myNumber = 5/2 x 2.5

myString = “Hong Kong” myString ‘Hong Kong’

myList = [1, 3, 5, 7, 9] myList [1, 3, 5, 7, 9]

newList = [44, -1, 66, 29, “cat”] newList [44, -1, 66, 29, “cat”]

blankString = “” blankString ‘’

newList[1] = 55/5 newList [44, 11.0, 66, 29, “cat”]


So far…• Expressions

• Evaluate input and returns some output

• Variable assignments: <variable> = <expression>

• Store the value of the expression in the variable

• There is always a equals sign in an assignment

• Variables can be named many things, as long as we follow rules.

• Variables can hold many types of data: numbers, strings, lists

• List assignments: <listVariable>[<index>] = <expression>


Relating back to Google Sheets…

• Google sheets has different types of data:

• Numbers (start with a digit, or a -)

• Strings (non-digit start, or start with a ‘)

• Formulae (start with =)

• Ranges (B2, B2:B4, B2:D5)

• Errors (#N/A)

• Blanks


Sequential Operation• Remember how in Google sheets, if we put “=A1” in B2,

and we changed the value in A1, B2 automatically updates?

• That doesn’t work for Python.

• In Python, the programming statements (the commands) are operated sequentially.

• Each statement is executed once, from top to bottom.

• We assigned a value to y once (y = x + 4). At the point when that assignment happened, the value of x was 3. So the value that gets put” into y is 3 + 4 = 7.

• Even though we change the value of x afterwards, the value of y doesn’t change.

79.5Hello, Grace77>>>


More sequential operation• When we say y = x + 4

• Python evaluates the RHS of the equation first.

• If the RHS has variables, Python looks up the values from the memory table.

• RHS = x + 4 = 3 + 4 = 7

• Then Python looks at the LHS

• LHS of an equals sign in Python can only have one variable.

• If there is not already an assigned value in the memory table for the variable on the left, Python makes space for it.

• The RHS value is then stored into the memory table.


Assignments

• Evaluate RHS: 5

• There is no entry in the table for x: create one.

• Put the value of the RHS into the memory table as the “value” for x.

Name Value

x = 5

Name Value

x

Name Value

x 5


Assignments (cont’d)• Supposing the next statement is:

• Evaluate RHS: 13

• There’s already a entry for x: do nothing.

• Put the value from the RHS into the value for x

x = 13

Name Value

x 5

Name Value

x 5

Name Value

x 13


What if we have x = x + 1• If x is already defined (let’s say that the

current value of x is 13)

• Evaluate RHS: x + 1

• Look up x in the memory table: get 13

• Add 1 to 13, get 14

• There’s already a memory spot for x: do nothing

• Put the value of the RHS into the “value” for x

Name Value

x 13

Name Value

x 13

Name Value

x 14


Another example for x = x + 1

• This time, x is not previously defined (i.e. we have never assigned anything to it in this program, or since we opened IDLE)

• Evaluate RHS: x + 1

• Lookup x — can’t find it!

• ERROR!!!!!!

Name Value


Some details• Evaluate:

• Compute

• Look up variable names to find values

• Do math or string or list operations in the order specified to combine these values and get a result.


More details• We saw indexing and slicing before

• Some shortcuts:

• Leaving out the begin or end of a slice means “stuff from the beginning” or “stuff including and after” respectively

• myList[:3] — everything in myList from the beginning up to but not including myList[3]

• myList[2:] — everything in myList including and after myList[2]

>>> myList = [1, 3, -1, -9]>>> myList[2:][-1, -9]>>> myList[:1][1]>>> myList[:3][1, 3, -1]


More details• The same tricks that work on lists also work on

strings>>> myString = "Hong Kong Polytechnic University">>> myString[:5]'Hong '>>> myString[23]'n'>>> myString[:23]'Hong Kong Polytechnic U'>>> myString[19]'i'>>> myString[19:]'ic University'>>> myString[19:23]'ic U'


One last trick (for the time being)

• We have also seen how we can have lists of strings.

• How can we say “I want the first letter of the second string in the list?” • Index first, to get the element

(which will be a string) • Then index (or slice) into the

string • Remember to use a [] for each

time we index or slice!

>>> myWords = ["Hong", "Kong", "Polytechnic", “University"]>>>>>> myWords[0]'Hong'>>> myWords[0][0]'H'>>> myWords[0][0:2]'Ho'>>> myWords[3]'University'>>> myWords[3][10:]''>>> myWords[3][6:]'sity'>>>

Do Activity 2-1 Task 1


Interactive Python• We have tried the interactive mode on IDLE


Non-interactive Python — Python program


Program Files

• We can save our code in a Python program file.

• Python programs have the “.py” extension

• e.g. example.py, HelloWorld.py

Finish Task 2

topic 2: simple programming / textual analysiscsgngai/comp1d04/lectures/7_simplepr… · textual...

Documents