Programming and Computation II Homework 2

CMPSC-132: Programming and Computation II
Homework 2

100 pts
Instructions:
- 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 illformatted 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.
- 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
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…
- 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
4.25 * 5 - 2.01 / 3 * 7 + 2
The loop may exit with a break or return statement.
Make a complete
case analysis
(operator found
and current mode)
pos
opr
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
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.
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
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
ccalculator
isNumber cfindNextOpr
getNextNumber
call
call
call
returns the operator
position in expr[pos:] returns a
boolean value
returns next number, next
operator and operator position
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?
4.25 * 5 - 2.01 / 3 * 7 + 2
- 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
pos
opr
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