CSE307 - Principles of Programming Languages -  Homework Assignment 01 - Solved

Python Basics and Parsing with Python




 
 
Solve the following problems in Python. Each problem  is worth five (5)  points each. You may use regular expressions in your solutions to the problems.

Problem 1:



 
 
Write a program  that reads a line via  input() and determines  if the input is  a legal identifier. The program should print only one output: True or False, and then exit.

A Python identifier starts with a letter  (A    Z, or a   z) or  an

underscore (_), followed... by zero or more letters, underscores, or digits (0 - 9).



 
 


 
 
Legal Python identifiers cannot be Python keywords. Python keywords are reserved and so cannot be used as constants, variables, function names, class names,  or as any other  identifier in Python.



 
 
Note: Starting an identifier with a single leading underscore indicates that the identifier is private. Starting an identifier with two leading underscores indicates a strongly private identifier. If the identifier also ends with two trailing underscores, the identifier is a language-defined special name.

You are not permitted to use the following library methods for this problem: -keyword.iskeyword()

 -str.isidentifier()



 
 
The point of this problem  is to work out how one would  implement such a method by manually parsing the input.



 
 
You can easily.. view a list of all Python keywords by using the built-in "help"

function.



 

[GCC 5.4.0 20160609] on linux

Type "help", "copyright", "credits" or "license" for more information. >>> help()

Welcome to Python 3.5's help utility!

If... this is your first time using Python, you should definitely check out



 
 
the tutorial on the Internet at http://docs.python.org/3.5/tutorial/.

Enter.. the name of any module, keyword, or... topic to get help on writing

Python programs and using Python modules.... To quit this help utility and 
return to the interpreter, just type "quit".

To.... get a list of available modules, keywords, symbols, or topics, type

"modules", "keywords", "symbols", or "topics". Each module also comes with a one-line summary  of what it does; to list the modules whose name or summary contain a given string such as "spam", type "modules spam".

help> keywords



 
 
Here  is a list of the  Python keywords. Enter any keyword to get more help.

False               def.............. if                raise

None                   del                 import...........  return

True                 elif              in..............  try

and               else............. is                while

as............... except             lambda............. with

assert              finally   nonlocal.......... yield

break                  for  not

 class                  from  or

continue.......... global........... pass

help>

Problem 2:


 
 


 
 
Write a program  that reads a line via  input() and determines  if it is a  legal number. The program should print only one output: int, float, or None and then exit.



 
 
Your program must manually parse the inputs. Do not merely pass the strings to the type conversion functions and catch the exceptions.



 
 


 
 
In Python 3, there is effectively no limit to how long an integer value can be. Of course, it is constrained  by the amount of memory your  system has, as are all things , but beyond that an interger can be as long as your need it to be. The following strings can be prepended to an integer value to indicate a base other than 10:



 
 


 
 


 
 
0b (zero + lowercase letter 'b') Base 2 (binary) 
0B (zero + uppercase letter 'B') Base 2 (binary)



 
 


 
 
0o (zero + lowercase letter 'o') Base 8 (octal) 
0O (zero + uppercase letter 'O') Base 8 (octal)



 
 


 
 
0x (zero + lowercase letter 'x') Base 16 (hexadecimal) 
oX (zero + uppercase letter 'X') Base 16 (hexadecimal)

Values with these prefixes are legal numbers (integers) in Python.



 
 
The float type in Python designates a floating-point number. Float values are specified with a decimal point.

https://docs.python.org/3/tutorial/floatingpoint.html

Optionally, the character 'e' or 'E' followed by a positive or negative integer may be appended to specify scientific notation.

 Example: 4.2e-4 = 0.00042



 
 
Float values are 64-bit "double-precision" values, according to IEEE 754 standard ( https://en.wikipedia.org/wiki/IEEE_754  ). In that case, the maximum value a floating-point number can have is approximately 1.8 x 10^308. Python will indicate a number greater........................................ than that by the string "inf".

The closest a nonzero number  can be to zero is approximately 5.0 x 10^-324. Anything closer  to zero than that is  effectively zero.



 
 


 
 
Floating point numbers are represented internally as binary (base-2) fractions. Most decimal fractions cannot be represented exactly as binary fractions, so in most cases the internal representation of a floating-point number is an approximation of the actual value. In practice, the difference between the actual value and the represented........................................................ value is very small and should not usually 
cause significant problems.

Problem 3:



 
 


 
 
3. Write a program  that reads two lines (two sequential calls to input()) and then determines if the combined result is a legal string enclosed within single or double quotes. The program should print only one output: True or False, and then exit.



 
 
The following  is a table of  escape sequences which cause Python to suppress the usual special interpretation of a character in a string:



 
 
\'                  Literal single quote (') character