Class 18 — Monday March 15

Knowing what you know

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

Always be mindful. — Make sure what you are doing, — Is what you intend.

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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

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Recent Test 1 solutions

• Fall 2020.

• Spring 2020.

• Fall 2019.

• Spring 2019.

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Review module documentation

• Built-in functions.

• Module str.

• Module list.

• Module math.

• Module random: our only interest are its functions seed(), randrange(), and choice().

• Module url.

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Epistles

• Things to remember

• Syntax errors and exceptions.

• Accomplishments.

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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 ] can retrieve substrings.

• Operator + produces a new sring formed by concatenating its operands.

• 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 ] can retrieve or modify elements or sublists

• Operator + with list operands produces a new list by concatenating the elements of its list operands.

• Operator + cannot be used when only one element is a list.

• Operatop in checks 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 function append() modifies the list in place (and returns None).

• For a lot more discussion go here.

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for loops

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

for item in sequence :

   statement₁

   statement₂

   ...

   statement_n

In the above, iterator variable item takes on each of the values of sequence in turn. For each of those values, statement1, statement2, ..., and statementn 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 *.

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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 resource indicated by web location link.

• url.get_dataset( link ) returns the contents of the web resource indicated by web location link as a dataset sheet whose cell values are either string, int, float, or bool as appropriate.

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Web processing

• Suggest redownloading local module url.py.

• Use url.get_contents() to get the contents of a web file.

• Use url.get_dataset() to get the contents of a web dataset.

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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's Rule of Life — baby steps to obtain your goals.

• Ignoring the Fernando Lamas' Rule of Life — looking good makes a life complete.

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

• 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

• Mindfullness makes your computational journey clearer and easier

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

• Functions use parentheses; lists use brackets. Remember this.

• Functions must always have parentheses. Remember this.

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Datasets

• A dataset is a collection of informations arranged as list of rows, where each row is a list of cells (dataset value).

• The basic way of accessing the rows in the dataset follows.

dataset = ... # get the dataset

... # set up dataset processing (if needed)

for row in dataset : # consider rows one by one

  # process current row of the dataset

  ... # process the row

... # finish off dataset (if needed)

• The basic way of accessing the individual cells in a dataset is row by row.

dataset = ... # get the dataset

... # set up dataset processing (if needed)

for row in dataset : # consider rows one by one

  # process current row of the dataset

  ... # prepare to process the cells (if needed)

  for cell in row : # consider cells of the row one by one

  # process the current cell for the row

  ... # process the cell

  ... # finish off row (if needed )

... # finish off dataset (if needed)

• To create a copy of a column of a dataset

dataset = ... # get the dataset

c = ... # get the column

column_copy = [] # need a cell accumulator

for row in dataset : # consider rows one by one

  # get cell from row' column c

  cell = row [ c ] # get the cell in column c

  column_copy.append( cell ) # copy the cell into accumulator

... # analyze the column

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🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.