Class 17, Friday, February 23

Knowing what you know

Speak to Chiara — Ask questions, provide answers — Do not waste your time

Though the past is past — It is still an augury — Of future tidings

Looking backward now – I see the future is bright – Because I learned so much

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Look both ways

• Back and ahead

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Agenda

• Chronicle what we know or should know

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Examples

• TBD.py — to be determined during class.

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Test logistics

• Is next class.

• In the normal classroom for the normal time (2–3:15 pm).

• You will have access to the modules and the info sheet, but nothing else from our website or other websites. Access to those other elements of the web site will not be available until after everyone completes the test.

• The test will include some written and some coding parts, so bring both a pencil (or pen) and a computer.

• You are not allowed to access past code (yours or ours) during the test.

• You are not be allowed to use PyCharm for the paper part of the test.

• Class personnel cannot help you debug your answers.

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Chrestomathics

• An algorithm is doable structured communication without ambiguity.

• A program is an algorithm written in a language that can be carried out on a computer.

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Development and translation

• Python is an interpretive programming language designed for automating tasks.

• There are compilers that can be downloaded for most computers to execute Python programs.

• There are IDEs like PyCharm for computers to develop Python code.

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How Python works

• Executes one line at a time, in order from top to bottom (except for repeats some lines).

• Evaluates literals, expressions, variables, function, and method invocations according to operator precedence.

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Values and types

• All values have a type and an identity. Built-in functions type() and id() return respectively the type of a value and its identity. The identity of a variable is a reference to where its value is stored in computer memory.

• The simple value (primitive) types are int, float, and bool. Primitive type values have no behaviors.

• The values (objects) of types such as str and list do have behaviors. Behaviors are implemented as methods (message functions). A method function invocation tells its object to carry out a behavior. For example, if s is a string, then s.lower() returns a new copy of s where are letters in s are in lower case form in the copy.

• Literal values are composed of explicit strings, numbers, and logical values like 3, 0.1415, True, [ 2, 'abc', 5.85 ].

• Operators like +, /, //, %, and ** evaluate their operands and return the value of the corresponding mathematical operation save Python limitations on decimal representations.

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str

• Once created str values are immutable character sequences (there is no function, method, or operator that can modify an existing string).

• Escape sequences, like '\n' represent the new-line character.

• Operators [ i ] and [ i : j ] retrieve substrings and + to concatenate.

• Built-in function len() to find how many characters there are

• Methods like find() and upper() return values.

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range

• range( i , j ) is the sequence of integer values from i to j-1.

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list

• Literal values like [] and [ 1, 'two', [ 3, 3.0 ] ].

• Operators [ i ] and [ i : j ] to retrieve or modify elements or sublists; + to concatenate; and in to check if a value is within the list can be a range or a list.

• Functions like len(), max(), and sum() to analyze the contents of a list.

• Method functions like index() return values.

• Method functions like append() and sort() that modify the list in place (and return None).

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for loops

• Repeat actions by iterating over a sequence of values. The sequence is typically list or a range, but can also be a string. Consider

for item in sequence :

   statement₁

   ...

   statement_n

here

• Iterator variable item takes on each of the values of sequence in turn. For each of those values, action1, ..., and actionn are repeated.

• A for loop inside another for loop is called a nested loop.

• for loops can be used to build up (accumulate) a result.

• If the result is to be a sum, then the accumulator is initialized before the loop to 0 and increased within the loop using numeric operator +.

• If the result is to be a string, then the accumulator is initialized before the loop to '' and added to within the loop using string operator +. Observe that '' is a string of length 0 and different from the string ' ' which is a string of length 1 that consists of a single splace.

• If the result is a list, then the accumulator is initialized before the loop with [] and added to within the loop using append().

• If the result is to be a product, then the accumulator is initialized before the loop with 1 and increased within the loop using numeric operator *.

• If the result is to a check for sameness (i.e., all values have a property), then the accumulator is initialized before the loop with True and updated within the loop using Boolean operator and.

• If the result is to a check for occurrence (i.e., a value(s) have the property), then the accumulator is initialized before the loop with False and updated within the loop using Boolean operator or.

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Producing values

• Values are produced whenever Python evaluates an expression.

• Anytime an expression is encountered it is evaluated.

• Anything following an assignment operator is an expression.

• Any argument to a function or method is an expression.

• The operand(s) for an operator are expression(s).

• An expression can be composed from literals, variables, operators, and sub-expressions.

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Functions and methods

• A built-in function invocation does not have an object to which its actions are targeted; e.g., max( a, b ) returns the larger of values a and b.

• A method is named piece of code that manipulates the object to which it is targeted (dotted); e.g., in invocation s.capitalize() method capitalize() examines s to produce and return a copy of string s with the first character capitalized and the other characters in lowercase.

• Whenever a function or method is invoked it, returns a value, even if that value is just None (like it is for print() and a.append()).

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Library

• A library is a modue of related functions and values that must be imported to gain access to them.

• Standard library math provided access to a wide variety of mathematical computations and values; e.g., math.sqrt(), math.sin(), and math.pi.

• Standard library random provides ability to generate random values. The most important functions for us are

• random.randrange( a, b ) returns a random integer from a to b-1.

• random.choice( s ) returns a random value from sequence s.

• random.shuffle( s ) returns a new sequence whose values are a scrambling of the values of sequence s.

• random.seed( s ) uses s to set up the initial configuration of the Python random number generator.

• Local library url provides web access

• url.get_contents( link ) returns the contents of the web page at url link.

• url.get_dataset( link ) returns the contents of the web page at url link as as a dataset sheet whose cell values are all strings.

• url.get_and_parse_dataset( link ) returns the contents of the web page at url link as as a dataset sheet whose cell values are either string, int, float, or bool as appropriate.

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Manual CSV processing (suggest use url.get_dataset() and url.get_and_parse_dataset())

• Specify base folder

• Determine date filename

• Specify data source

• Get the data from the web

• Strip the data

• Convert data from string form into list form by splitting on '\n'

• Determine number of rows

• Split each row into a listing using ',' as the separator

• Massage data as necessary

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Errors

• Syntax errors are illegally formed Python instructions. This web page epistle might prove useful in dealing with syntax errors.

• Semantic errors are actions not doing what is intended.

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Gotchas

• Ignoring the Bill Murray rule of life — baby steps to obtain your goals

• Forgetting the Law of Cole

• Not reading the complete description before starting to problem solve

• Ignoring the input order

• Not parsing (converting) the input into the needed values

• Not converting a string to a number using int() or float() or to a Boolean value using bool().

• Output expectations

• Improper accumulation

• Combining a string and a number using the + operator

• Using only the first or last value of a range or sequence to compute result

• Not checking whether your solutions match given test run expectations

• Not mindfully writing and reading your solution — you must understand what you are writing

• Make your computational journey clearer and easier to mindfully read

• Corollary: use print statements when developing code to document computational progress and comment out those statement before submitting

• Ignoring the Lamas rule of life — looking good makes a life complete

• Consistent use of whitespace around operators and blank lines make mindfulness easier to obtain.

• Functions use parentheses; lists use brackets. Remember this

• Functions must always have parentheses. Remember this

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To do

• Review class artifacts

• Review standard documentation for built-in functions.

• Review course documentation for module str.

• Review course documentation for module list.

• Review course documentation for local module url.

• Review standard documentation for module math.

• Review ccourse documentation for module random.

• Epistle on syntax errors and exceptions.

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