Programming Abstraction and Methods

ment_eval.py, write the function interpret_statements(filename) that reads and interprets from the file filename a sequence of statements, one statement per line, ignoring lines that either have only space characters and/or a comment. The format of statements is described below. For each statement, your function should print the result of the evaluation. The format of the result is also described below.

If a line contains only space characters and/or a comment, the line should be ignored and no result displayed for it.

A comment starts with the hashtag symbol #. Everything on a line starting with the first hashtag should be ignored.

You should maintain a count of line numbers (the first line is line 1). Lines that are blank or that contain only comments are still counted for purposes of counting line numbers.

A statement can either be an expression statement or an assignment statement.
The format of these is:

An expression statement consists of a single expression. An expression is a sequence of operands separated by operators. One or more spaces separate the operands and operators. An operand is either a variable (that must have been previously defined in an assignment statement) or a number (an integer or floating point literal). An operator is either + or -. Examples of expression statements are:
14.4
23.4 + -19 + 98223
10 – 34 + 12.66 – 28 – 3323.2 + 12
14.3
92 – 13 # here is a comment after an expression
34 + 13#and another comment
x123
14.4 – salary
time + salary – 10 + x123
The value of an expression is just what you would expect. Since the only operators allowed are + and -, the expression is evaluated left to right. You will need to store the value of variables somewhere, and a dictionary is good for this.The result printed for an expression statement should be the line number followed by the expression followed by a equal sign followed by the value of the expression. So, the results displayed by the above examples should be:
Line 1: 14.4 = 14.40
Line 2: 23.4 + -19 + 98223 = 98227.40
Line 3: 10 – 34 + 12.66 – 28 – 3323.2 + 12 = -3350.54
Line 4: 14.3 = 14.30
Line 5: 92 – 13 = 79.00
Line 6: 34 + 13 = 47.00
Line 7: x123 = 19278.00
Line 8: 14.4 – salary = -845.60
Line 9: time + salary – 10 + x123 = 1363.67
The last three results depend on the values of the variables, which are assigned in assignment statements. (And the description of assignment statement follows.)
An assignment statement consists of a variable name followed by an equal sign followed by an expression. (Expressions are described above.)A variable must start with a letter or underscore and be followed by any number of letters, digits, or underscores.Examples of valid assignment statements are:
x123 = 19278
salary = 873 – 13
time = -123.33 + salary – 123 + 23 + 0 + -123 – x123
x123 = x123 + 15 # reassign the value of x123
The result printed for an assignment statement should be the line number followed by the name of the variable being assigned to followed by an equal sign followed by the value assigned to the variable. The “side effect” of evaluating an assignment statement is to store the value of the variable in a dictionary, so that its value can be used in future expression and assignment statements.The results displayed by the above examples should be:
Line 1: x123 = 19278.00
Line 2: salary = 860.00
Line 3: time = -18764.33
Line 4: x123 = 19293.00
Any statement that is not a valid statement is an invalid statement. The result displayed for an invalid statement should be “Invalid statement”. Examples of invalid statements are:
1474 +
820.34 – junk # junk has not been defined
2884 – -1293 + 399 t 266
23.8 – 203.3 +

  • 123.2 – 23.2
    2884.2 + 293 – 23.2 283.3 + +
    34.8 273 – 12
    x =
    x = salary + time = 15
    x$y34 = 17
    19 = 19
    1xyz = 123
    Testing your program
    You can run your program by excuting your statement_eval.py file. Do this by selecting from the VS Code menu View->Command Palette, and then typing in (or selecting if you see it) “Pyton: Run Python File in Terminal.” The program will call your interpret_statements function with the statements.txt file as input. If you want, you can create a new file in the folder containing just the statements you want to test. Then, edit the bottom of statement_eval.py so that it calls your interpret_statements function with your new file.

Once you are convinced that your program runs correctly on all of the test inputs, you should verify this by running the program test_psa1.py. Do this by opening test_psa1.py, and then selecting from the VS Code menu View->Command Palette->“Python: Run Python File in Terminal”. The test program will tell you if your results are correct. (And if not, it will tell you the results that you got wrong.)

You should not submit your program until test_psa1.py tells you that you can submit. Your program will receive no points if your program fails this.

Program design requirements
Here are requirements related to the design of your program. Failure to follow these will not result in rejection of your program, but there will be deductions (as detailed under each item).

Your interpret_statements function should follow the design that we developed in class (the first two classes of the semester). You will still need to translate the algorithm we wrote into Python code, and add exception handling. You will also have to development an algorithm for the evaluate_expression function, and translate it into code, being sure to include exception handling.Neither of your functions should be longer than 30 lines of code, not counting blank lines or comments.Failure to use good procedural abstraction as we talked about in class will result in a penalty of 10 points.
Your program should define a BadStatement exception, and raise this exception any time an invalid statement is encountered. Failure to do this results in a 5 point penalty.
If your program cannot open the statements file, it should catch the resulting exception and print a meaningful message. Failure to do this results in a 5 point penalty.
To decide if a variable name is valid (starts with a letter or underscore, remaining characters either digits, letters, or underscores), you must use Python’s regular expression capability. Read about this at the Python documentation website, https://docs.python.org/3/howto/regex.html. (Regular expressions are a very powerful programming tool, and you will be happy to have been exposed to them.) Failure to do this results in a penalty of 5 points.
You should use descriptive variable names in your program, and use all lowercase letters with underscores separating words. Failure to do this results in a penalty of up to 5 points.
You should appropriately comment your program. It should have a header (this is started for you – be sure to add your names and the date you started it); each function should have a header. I’ve given you the header for the interpret_statements function. Any additional functions you add should have a descriptive header also.You should also comment blocks of code within your functions, explaining what the code is doing. How much commenting to add is a judgement call – you don’t want too much, or too little. If you have a block of code (say up to 10 lines long), put a brief comment before it saying what is about to happen. Then put blank lines between the blocks of code. Don’t comment individual lines of code, unless they are doing something that the reader might not see right away.Failure to comment your program appropriately will result in a penalty of up to 10 points.