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Exercise 4: Code Generation using AST

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CMPUT 275 Wi18 – INTRO TO TANGIBLE COMPUT II
Combined LBL Wi18
Exercise 4: Code Generation using AST
The purpose of this exercise is to:
Practice object oriented programming by reading the code for an expression-based calculator with
variables, defined in calculator.py
Understand how the Abstract Syntax Tree is created for an input string by reading the code
in exprparser.py
Understand how the Abstract Syntax Tree is used by a compiler to generate code
1) Abstract Syntax Tree (AST)
You should start with the simple calculator in calculator.py as a study in how to read object
oriented programs. The calculator uses an instance of the expression parser class, ExprParser, to
build an AST for the input string. This is done by calling its parse method. It then evaluates the AST to
produce a numerical result with side effects of setting variables to values in the expression. This is
done by calling the evaluate_ast method of the calculator. Read the code in evaluate_ast to find
out how different node types in the AST are evaluated.
Note: there is nothing to submit for this part; the task is just to understand the code to do the second
part.
2) Code Generation
In this exercise, you will walk over the AST and produce Python code which performs the same
calculations when executed. The module compiler.py contains class definition for Compiler. The
main code generator is a method called compile. You should NOT change this method. Most of the
work is done in the method called compile_ast. This is where all new code you produce (apart from
tests) should be added, and detailed descriptions of the task is in this method. In particular you need
to implement the handling of ‘set’ and ‘apply’ nodes in the AST. You should support assignment (=),
binary (+, -, *, /), and unary operations (neg, sqr) as well as function calls.
You can test your code using comptest.py
python3 comptest.py
and with doctests
python3 comptest.py –test
Examples:
Expression that contains binary and unary operations
Assignment https://eclass.srv.ualberta.ca/mod/assign/view.php…
1 of 4 2018-04-14, 1:01 p.m.
python3 comptest.py
?(1 – 2) – 3 + 4 – – 5
Initial ast: (‘apply’, [(‘-‘, []), (‘apply’, [(‘+’, []), (‘apply’, [(‘-‘, []), (‘apply’,
[(‘-‘, []), (‘const’, [(1, [])]), (‘const’, [(2, [])])]), (‘const’, [(3, [])])]), (‘cons
t’, [(4, [])])]), (‘apply’, [(‘neg’, []), (‘const’, [(5, [])])])])
# code for:
# (1 – 2) – 3 + 4 – – 5
_result = ((((1 – 2) – 3) + 4) – (-5))
print(_result)
Running program …
locals before []
5
locals after [(‘_result’, 5)]
Expression that contains unary operations
python3 comptest.py
?-sqr(2)
Initial ast: (‘apply’, [(‘neg’, []), (‘apply’, [(‘sqr’, []), (‘const’, [(2, [])])])])
# code for:
# -sqr(2)
_result = (-(2 ** 2))
print(_result)
Running program …
locals before []
-4
locals after [(‘_result’, -4)]
Expression that contains a function call
python3 comptest.py
?max(1, 2)
Initial ast: (‘apply’, [(‘max’, []), (‘const’, [(1, [])]), (‘const’, [(2, [])])])
# code for:
# max(1, 2)
_result = max(1, 2)
print(_result)
Running program …
locals before []
2
locals after [(‘_result’, 2)]
Expression that contains assignments
Assignment https://eclass.srv.ualberta.ca/mod/assign/view.php…
2 of 4 2018-04-14, 1:01 p.m.
python3 comptest.py
?(x = 1) + (y = x + 1)
Initial ast: (‘apply’, [(‘+’, []), (‘set’, [(‘x’, []), (‘const’, [(1, [])])]), (‘set’, [
(‘y’, []), (‘apply’, [(‘+’, []), (‘get’, [(‘x’, [])]), (‘const’, [(1, [])])])])])
# code for:
# (x = 1) + (y = x + 1)
_x_1 = 1
_y_1 = (_x_1 + 1)
_result = (_x_1 + _y_1)
# Update and cleanup x
x = _x_1
del(_x_1)
# Update and cleanup y
y = _y_1
del(_y_1)
print(_result)
Running program …
locals before []
3
locals after [(‘_result’, 3), (‘x’, 1), (‘y’, 2)]
Implementation strategy:
First get code generation working for expressions that do not involve any variable assignment
operations (e.g. 2 + 3). Then worry about how to deal with assignment statements.
Summary of the task:
You have to:
1. Add code to compile_ast to make it run properly
2. Add at least five docstring tests to tests
Code Submission
Submit only the file compiler.py, do NOT zip it. You must create at least five more strings in the
expression language to test on. These should be added as a docstring to the tests function
in compiler.py. We will run these tests via
python3 compiler.py
We also use comptest.py to test your code.
Submission status
Ex4.zip
Attempt number This is attempt 1 ( 1 attempts allowed ).
Submission status Submitted for grading
Assignment https://eclass.srv.ualberta.ca/mod/assign/view.php…
3 of 4 2018-04-14, 1:01 p.m.
Feedback
Grading status Graded
Due date Monday, 5 March 2018, 11:55 PM
Time remaining Assignment was submitted 2 hours 45 mins early
Last modified Monday, 5 March 2018, 9:09 PM
File submissions
Export to portfolio
compiler.py
Submission comments Comments (0)
Grade 93.50 / 100.00
Graded on Saturday, 17 March 2018, 4:05 PM
Graded by Taher Jafferjee
Feedback comments Correctness
Your code passed 24 of 26 tests, and your grade on correctness is 92.31
Style
Your mark on style is 100%
Weight Total
93.50%
Assignment https://eclass.srv.ualberta.ca/mod/assign/view.php…
4 of 4 2018-04-14, 1:01 p.m.

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