Some basic Python, starting Oct 22, 2018 for UBC course Math 441:

Copyright Joel Friedman, 2018

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--Some basic python commands: variables, types, file opening/closing, functions, etc.
--Some basic Gurobi commands (especially addConstrs( ... ) )
--Look at some examples in 
    http://www.gurobi.com/documentation/8.0/examples/a_list_of_the_gurobi_examp.html

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Specific Python words/commands you should know (aside from the basics about
variables and assignment):
  quit(), help(),
  print(), open(), close(), while, for, range(),
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SOME RESOURCES:
The following websites seem to have good info on Python

General info:

https://www.python.org
https://wiki.python.org/moin/SimplePrograms
https://www.w3schools.com/python/
https://www.pythonforbeginners.com

Some Python basics:

http://hetland.org/writing/instant-python.html

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WARNING -- You have to indent (with TAB key, it seems)

e.g.,

x = 5
while x > 0 :
print(x)      --- now python complains that it was expecting indentation

x = 5
while x > 0 :
	print(x)
	x = x-1

--- now python does what you want

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HELP: Typing help() into Python brings you to its interactive help shell.
Typing the RETURN-key (sometimes ENTER-key) will get you back to python.

>>> help()

Welcome to Python 3.7's help utility!
[Followed by a long message with some instructions]

help>

Now if you hit the RETURN-key:

You are now leaving help and returning to the Python interpreter.
[Followed by some more stuff]

>>>

Exercise: at the help prompt (i.e., help>), try typing (without quotation marks)
the following: "keywords", "modules", "modules functions", "help", "topics",
  "FUNCTIONS"

[These days you might use a search engine, e.g., Google, for help.]

Then try running Gurobi and type "help()".  You will get help specific to
Gurobi...

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OBJECTS:  [This section is to explain why you type commands like:
  m.optimize(), m.printAttr("X") in Gurobi, and in Python, C++, etc.
  This section is extremely abbreviated and imprecise.]

(1) In the old days, you had to set up separate variables and arrays and
declare their type (in most programming languages), e.g.,

  float UBC_student_bill[1000] ;
  char UBC_student_names[1000][20] ;
  int UBC_student_num_of_courses[1000] ;
  int i ;
  float my_salary ;

The first statement above sets up an array of 1000 floating point numbers
which can be referred to as:

  UBC_student_bill[0], UBC_student_bill[1], ..., UBC_student_bill[999]
  UBC_student_bill[i], UBC_student_bill[i+j], etc., assuming i,j are of type 

Typically all such declarations had to appear in the start of a program.
[These days most computer languages allow you to create new variables/arrays
"dynamically", i.e., whenever you want during the time that you run your
program.  This feature introduced a number of problems, such as "memory
leaks"...]


(2) Later, for convenience, many languages allowed programmers to declare
their own types or structures, which bundled together related variables, e.g.,

  struct student { 
    float bill ;
    char name ;
    int num_of_courses ;
  }

To set up an array of 1000 UBC students, plus a variable "current_student"
consisting of a single student, one would use the statement

  struct student UBC_student[1000] ;
  struct student current_student ;

You would refer to the parts of the above students as follows:

  current_student.num_of_courses, current_student.bill, etc.,
  UBC_student[32].bill, UBC_student[i].name, etc., assuming i is of type int

(3) What is known as "object-oriented programming" began with the idea that
structures should have their own built-in programs, and that a large software
projects should divided the program into (1) (mostly) writing programs within 
each structure, and (2) a small amount of communication between the various
structures.  These structures are typically called "classes", and variables
declared to be of such a type is called an "object" (of the class).

So when you type

m.optimize(), m.printAttr("X")

you are running the programs (built by the people at Gurobi):

  optimize  (which takes no input)
  printAttr (which needs a string, such as "X", as input)

which are built into a class (each "instance" or "object" of which forms an
LP/IP/QP/etc. model).  When you write

my_lp = read("myTSPsolver.lp")

the Python/Gurobi reads your file "myTSPsolved.lp" and creates the object
"my_lp" whose class is that of an LP/IP/QP/etc. model.  So typing

my_lp.optimize()

runs the program optimize(), which your object "my_lp" has built into it.

Object-oriented program is designed to (a) divide up tasks so that you can
modify a small part of a program without having to know the entire program,
and (b) make software bugs easier to detect (hopefully if you have a subtle
bug, you can at least tell which class(es) is at fault), (c) etc.

(4) These days there are a number of variants on the idea of object-oriented
programming.  In general, a lot of software engineering is geared towards
large computer programs involving many independent parts and many programmers.

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VARIABLES AND ASSIGNMENT:

Python allows you to introduce new variables whenever you want and does not
force you to declare their type.  Strings are specified in single quotes or
double quotes.  E.g.

>>> a = "Hi there."
>>> a
'Hi there.'
>>> print(a)
Hi there.
>>> print(a+a)
Hi there.Hi there.

Python understands "a+a" to mean two copies of the string  a  , provided that
the variable  a  is a string.

Note that there is a difference between typing "a" and "print(a)" in Python.
There are special characters like "\n" for "new line" (or "carriage return"
or "return").

>>> a = "Hi there.\nHow are you?"
>>> a
'Hi there.\nHow are you?'
>>> print(a)
Hi there.
How are you?

If the variable  a  is an integer, then a + a is the usual addition, and
a ** a is "a to the power a":

>>> a = 34
>>> a + a
68
>>> a ** a
11756638905368616011414050501310355554617941909569536
>>> print(a ** a)
11756638905368616011414050501310355554617941909569536

Notice that "==" tests whether the two sides are equal.

>>> 3 == 4
False
>>> 3 == 2+1

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LOOPS: How to write repetitive tasks in Python.

Above we gave an example using the "while" statement.  Here is another
example, upon typing the two lines:

for i in range(5,8):
	print("The square of the number " , i , " is " , i ** 2)

into Python.  You get the following:

>>> for i in range(5,8):
...     print("The square of the number " , i , " is " , i ** 2)
...
The square of the number  5  is  25
The square of the number  6  is  36
The square of the number  7  is  49

If you forget to type the colon (the ":") at the end of the first statement,
Python complains:

>>> for i in range(5,8)
  File "<stdin>", line 1
    for i in range(5,8)
                      ^
SyntaxError: invalid syntax

If you forget to type the TAB-key before the print statement, Python complains:

>>> for i in range(5,8):
... print("The square of the number " , i , " is " , i ** 2)
  File "<stdin>", line 2
    print("The square of the number " , i , " is " , i ** 2)
        ^
IndentationError: expected an indented block

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FILES: How to read and write to files.

https://www.pythonforbeginners.com/files/reading-and-writing-files-in-python
(this page contains a misprint involving "F" and "f")

a = open("example.txt","r") --- can also use "w","a" (append), "r+"
print a.read()  --- reads all of a

a.close()
a = open("example.txt","r") 
b = a.readline()  --- reads one line at a time
print b
a.close()

c = open("my_new_file.txt","w")  
c.write(b)
c.close()

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STRINGS, LISTS -- Examples in class: In Python type the following
(the "\u" is for unicode, the "\n" is for "new line")

help(STRINGS)

a = "\u4ECA\u5292, \u6211\u5F88\u7D2F\u3002"

print("\n\n" + a + "\n\n" + "Today, I am tired. \n\n\n\n" )

String splitting:
(see https://www.pythonforbeginners.com/dictionary/python-split)

fib = "1,1,2,3,5,8,13,21,Good morning,Good evening,Bye\nbye" # this is a string
fib

fib.split(",")
spl = fib.split(",")
spl[0]
spl[1]
spl[2]
spl[6]
spl[9]

for i in range(0,10)    # python complains because of the missing colon (:)

for i in range(0,11):
	print(spl[i])

for i in range(0,12):  
	print(spl[i])   # our list/array is out of range

for i in range(0,len(spl))  # len(spl) refers to the length of spl
	print(spl[i])

# if there is only one number in range(), then range begins with 0
for i in range(len(spl))   
	print(spl[i])

for s in spl:
	print("Here is the string: " + s)

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CONVERTING STRINGS TO INT, FLOAT, ETC.
This uses int(), float() (which need a string as the argument), and str()

spl[6]

spl[6] * 2

int( spl[6] ) * 2

a = "10.3"

a * a    # this will make Python unhappy

b = float(a)  

b * b

str(100000.3) + str(2222222.3)

3 + "Hi there."    # adding an integer and a string makes Python complain.

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