CPSC120A
Fundamentals of Computer Science I

Lab 19

Strings

Practice 1

Write a function called append_exclamation(a_phrase), which takes a string as a parameter. This function should return a string, which is simply a_phrase with an exclamation point appended to the end. 

















  
Practice 2




  Write a function called get_extents(a_phrase), which
  takes a string as a parameter.  This function should return a string, which
  is the first character of a_phrase followed by the last character
  of a_phrase.  If the string is not long enough, return an
  empty string.


  

    
  

  

    
  

  





String Strip



  One of the biggest security vulnerabilities that web developers are
  constantly fighting is known as an SQL injection attack.
  In this attack, users of a website format input in such a way that
  their input gets interpreted as code in the web developers systems.
  The easiest mechanism for preventing such an attack is to remove the
  offending characters before they get used in inappropriate ways.




Details



  Write a function
  called strip_non_alpha(offending_string) in a file
  called input_sanitation.py.  This function takes a string
  as a parameter, and returns a copy of the input string with all
  non-alphabetic characters removed.




  Make sure you test your program well.  How many test cases do you
  need?  Make sure you follow all of the course's coding conventions.




Sample Test Cases



  Note: Quotation marks included in output for clairly only.
  Your program should not output the quotation marks.




  
    

      Function Parameters
      Expected Output
    

  
  
    

      "Hello, World"
      "HelloWorld"
    

    

      "Scotty    "
      "Scotty"
    

    

      "!@#$"
      ""
    

  




Hint



  
  

    
    
      
  • 
	
    
	  You are going to need to use the accumulator pattern for
	  this activity.  This is because Strings are immutable
	  datatypes, so you need to make a copy of the input string.
	  The accumulator for strings simply appends characters to the
	  end of the string using the + operator.
	
    
      
    • 
      
  • 
	
    
	  Recall that everything in the computer can be represented as
	  an integer.  The way that characters are represented as a
	  number follows the ASCII standard.  You can get the ASCII
	  value of a character by using the ord function.
	
    
      
    • 
      
  • 
	
    
	  An alphabetic characters ASCII value is between the ASCII
	  values for 'a' and 'z' or 'A' and 'Z'.
	
    
      
    • 
  








Challenge



In the real world, the true offending characters are double and single
quotes.  Write a new function
called escape_strings(offending_string) that takes as
input a single string, and returns a copy of the string with all
single or double quotes "escaped".  Recall that a quote can be escaped
in a string by pre-pending it with a backslash (\" or
\').





Password Generation



  Passwords are possibly the most important way that individuals can
  ensure their own safety on the Internet.  However, it is also
  typically the easiest thing for a hacker to figure out, or to get
  their hands on.  This is simply because most Internet users choose
  incredibly weak passwords.  The easiest way to get around this issue
  to to use a randomly generated password.




Details



  Write a function called generate_password(size), which
  takes a positive integers as a parameter.  Your program should
  return a string of lower case letters of the specified size.  This
  function should randomly choose the lower case letters in the
  returned string.





  Your program should ask the user for a password length, and print to
  the terminal the password you generate.





  Make sure you test your program well.  How many test cases do you
  need?  Make sure you follow all of the course's coding conventions.




Example Execution


$ python3 password_generation.py
How long do you want your password? 5
neolp
$ python3 password_generation.py
How long do you want your password? 10
wigmnhgqwn




Hint



  
  

    
    
      
  • 
	
    
	  You can use the random module to generate
	  random numbers.  Since we know we are generating lower-case
	  characters, you need to generate a random number in the
	  range of the ASCII value for 'a' to the ASCII value for 'z'.
	
    
      
    • 
      
  • 
	
    
	  Use the chr function to convert an integer for
	  an ASCII character into the desired character.
	
    
      
    • 
      
  • 
	
    
	  Since you know how many characters you want,
	  you should use a for loop for this
	  activity.  You are also using the accumulator pattern again,
	  like in the previous activity.
	
    
      
    • 
  








Challenge



  Even random strings of lowercase characters are pretty easy to
  break.  A better program would randomly decide to include some
  uppercase letters as well.  Alter your function so that it will
  sometimes include uppercase letters in the generated passwords.
  Make sure the parameter used to determine how often an uppercase
  character is used is well documented and easily altered.





Submission


Please show your source code and run your programs for the instructor or lab assistant. Only a programs that have perfect style and flawless functionality will be accepted as complete.