CS 1112: Fall 2020

Class 18, Wednesday, October 10

Knowing what you know

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

Look both ways

Back and ahead

Agenda

Chronicle what we know or should know

Note

Please do not send to TAs using their personal email addresses.

You are not allowed to post code to Piazza.

Review session Thursday night from 5:00 to 6:30 in Rice 130.

Examples

TBD.py – to be determined during class.

Test logistics

Is next class.

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

You will have access to the module info, info sheet, and the gotchas we recently discussed, 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 laptop.

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.

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.

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.

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.

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.

`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.

`range`

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

`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).

`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 statement_n 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.

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.

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()).

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.

Web processing

Suggest import local module url.

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

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

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.

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

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 course documentation for module random.

Epistle on syntax errors and exceptions.

Resources from previous semesters are available.

Class 18, Wednesday, October 10

Knowing what you know

Look both ways

Agenda

Note

Examples

Test logistics

Chrestomathics

Development and translation

How Python works

Values and types

str

range

list

for loops

Producing values

Functions and methods

Library

Web processing

Errors

Gotchas

To do

`str`

`range`

`list`

`for` loops