python代写 CMPSC-132: Programming and Computation Homework 2

程序代写 CS代考 / Python代写代考

Homework 2
Due Date: 09/21/2018, 11:59PM 100 pts

Instructions:

CMPSC-132: Programming and Computation II

Fall 2018

  • –  The work in this assignment must be completed alone.
  • –  Use the starter code provided on this CANVAS assignment. Do not change the function names or given started code on your script
  • –  The file name must be HW2.py (incorrect name files will get a -10 point deduction)
  • –  When any function returns an error, it must be a string containing “error”
  • –  A doctest is provided as an example of code functionality. Getting the same result as the

    doctest does not guarantee full credit. You are responsible for debugging and testing your

    code with enough data.

  • –  Do not include test code outside any function in the upload. Printing unwanted or ill- formatted data to output will cause the test cases to fail. Remove all your testing code before uploading your file. Do not include the input() function in your submission.

    Goal:

    Write the function calculator(expr), where expr is a string. This function will compute the arithmetic expression given in expr. The arithmetic expression is a string of operands and operators that may include numeric values, four arithmetic operators (+, -, /, *) and extra spaces. An example of such expression is “-4.75 * 5 – 2.01 / 3 * 7 + 2 ”

    Notes:

– For this assignment you can return an error message if 2 consecutive operators are found (‘ 2 +-3 / 8’ for example)

  • –  In the starter code provided on CANVAS, there are 4 additional functions (partially written) that will help calculator(expr) to evaluate the expression. Try to understand all the variables given in the calculator(expr) code provided.
  • –  Except for exeOpr, you must code the empty segments so the five functions work completely.

    Function requirements:

  •   The function must return the computed value if expr is a correct formula, otherwise it must

    return an error message.

  •   When any function returns a numeric value, it must be float
  •   Do not use exec or eval function. You will not receive credit if your program uses any of the

    two functions anywhere

  •   The five functions provided in the starter code must work

    Grading Notes:

– calculator(expr) [60 pts]: The grading script will feed 4 randomly chosen test inputs, each for

15 points. One of them will be an input that should cause an error such as “4 * / 2 + 5” or “2 + 3 5”, whose expected returned value is an error message.

Complete case analysis: Any case is included and executed exactly once

If the maintenance of pos and opr is all the same for each case, you can do it outside the whole if-statement

If you know the current step is over, you can break or return to exit the loop

Make a complete case analysis (operator found and current mode)

initialization
   get the first operator and number before it

newNumber, newOpr, oprPos by getNextNumber
continue the initialization for operation precedence by if-elif-elif-…-else

statements
     if newOperator is None then

          return newNumber
     elif .........

  pos  =  oprPos+1
  opr  =  newOpr

while True:
  get netNumber, newOpr, oprPos
  if newNumber is None or ...

  elif newOpr is None or ...
  elif newOpr=="+"...
  elif...
  • –  findNextOpr(txt) [20 pts]: 2 randomly chosen test inputs checking the correct returned values.
  • –  isNumber(txt) [10 pts]: 2 randomly chosen test inputs checking the correct returned values.

    – getNextNumber(expr, index) [10 pts]: 1 randomly chosen test input checking the correct returned value.

    Deliverables:

 Include all the functions in your script named HW2.py. Submit it to the HW2 CANVAS assignment before the due date

Starter code appendix:

calculator(expr):
       input check

– A while True loop will execute when evaluating only valid expressions. This loop must update the values of pos and opr after an operation is performed, where:

o pos = current position (Every time pos increases, it must be right after the current operator)

o opr = the most recent operator
For the example, when calling calculator(“4.25 * 5 – 2.01 / 3 * 7 + 2”), in the next step opr=”*” and pos is right after it

pos

4.25 * 5 – 2.01 / 3 * 7 + 2

opr

The loop may exit with a break or return statement.

Overall functionality:

Your program should not go back and forth because of the operator precedence. To keep a linear time algorithm (we will learn its exact meaning later), code must scan from the left to right, and your calculation should be done in place.

returns the operator position in expr[pos:]

findNextOpr(txt)

– Receives expr[pos:] calculator(“ 3*4 – 5 ”)

expr = “ 3*4 – 5 ”, where:

expr[0] = space expr[1] = 3 expr[2] = * expr[3] = 4 expr[4] = space expr[5] = – expr[6] = space expr[7] = 5 expr[8] = space

returns a

ccalculator

call

returns next number, next operator and operator position

getNextNumber

call boolean value

isNumber

call cfindNextOpr

initially, pos = 3, thus, findNextOpr receives expr[pos:] = “4 – 5 ”=txt, where:

expr[0] = 4 expr[1] = space expr[2] = – expr[3] = space expr[4] = 5 expr[5] = space

  • –  It returns the position of the next operator (+, -, *, /) in txt, -1 if no operator was found. In the above, it is 2
  • –  Internal process
    o Check the positions of all the four operators. o If exists, return their minimum
    o Otherwise return -1
  • –  Tip: you can use the string.find method or your own custom while loop

isNumber(txt)

  • –  It returns True if txt is a string convertible to float, otherwise False. Note that “ -25.22222 “ is a string convertible to float but “ -22 33 “ and “122 ; 45” are not.
  • –  Internal process

o The string must consist of 0 to 9 and at most 1 period.
o If so, the function should return True, otherwise False (an easy way to check if str to

float is possible is with a try-except block)

getNextNumber(expr, pos)

calculator(“ 3*4 – 5 ”)

expr = “ 3*4 – 5 ”, where:

expr[0] = space expr[1] = 3 expr[2] = * expr[3] = 4 expr[4] = space expr[5] = – expr[6] = space expr[7] = 5 expr[8] = space

initially, pos = 3

  • –  It returns newNumber=4, newOpr=”-“ and oprPos=5 o newOpr is the leftmost operator in expr[pos:] o oprPos is its position in expr, not in expr[pos:]

     If no such operator, return None for both o newNumber is the number in expr[pos:oprPos]

     If there is no single number in it, return None

  • –  It MUST use the functions findNextOpr and isNumber effectively, otherwise, no credit is

    given to this function
    How do you calculate everything in one linear scan on expr?

    pos

    4.25 * 5 – 2.01 / 3 * 7 + 2

    opr

– When 4.25*5 is done, you have calculated 4.25*5=21.25. Save it.
o Newly start 2.01/3, then multiply it by 7 to get 4.69.
o Retrieve 21.25 and perform 21.25-4.69, repeat until you are done with the expression

Debugging is important!

  • –  Check every function individually first
    o Input some parameters and print the returned value(s) at the bottom
  • –  Use the Python debugger discussed on Module 2 to help you debug your code
  • –  When checking calculator(expr), first try simple inputs such as expr = “ 2 + 3 * 4.0 ”, then

    gradually make it more complicated by trying “ -2.0 + 3 * 4.0 ”.
    o Use Python’s unittest module to run extensive cases on your code

    Examples:

    >>> calculator("   -4 +3 -2")
-3.0
>>> calculator("-4 +3 -2 / 2")
-2.0

    >>> calculator(“-4 +3 – 8 / 2″)
    -5.0
    >>> calculator(” -4 + 3 – 8 / 2″)
    -5.0
    >>> calculator(“23 / 12 – 223 + 5.25 * 4 * 3423”)
    71661.91666666667
    >>> calculator(“2 – 3*4”)
    -10.0
    >>> calculator(“4 3 +2”)
    ‘input error line B: calculator’
    >>> calculator(“4++ 3 +2”)
    ‘error message’ # message must contain the word error, all lowercase

Related Posts