Lab 9 COMP9021 solution

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1 Using linked lists to represent polynomials
Extend the program that implements a class Polynomial from the previous lab to implement the functions __add__(), __sub__(), __mul__() and __truediv__(). Next is a possible interaction.
\$ python … from polynomial import * poly_6 = Polynomial(’-2x + 7x^3 +x – 0 + 2 -x^3 + x^23 – 12x^8 + 45 x ^ 6 -x^47’) print(poly_6) -x^47 + x^23 – 12x^8 + 45x^6 + 6x^3 – x + 2 poly_7 = Polynomial(’2x^5 – 71x^3 + 8x^2 – 93x^4 -6x + 192’) poly_8 = Polynomial(’192 -71x^3 + 8x^2 + 2x^5 -6x – 93x^4’) poly_9 = poly_7 + poly_8 print(poly_7) 2x^5 – 93x^4 – 71x^3 + 8x^2 – 6x + 192 print(poly_8) 2x^5 – 93x^4 – 71x^3 + 8x^2 – 6x + 192 print(poly_9) 4x^5 – 186x^4 – 142x^3 + 16x^2 – 12x + 384 print(poly_7 * poly_7) 4x^10 – 372x^9 + 8365x^8 + 13238x^7 + 3529x^6 + 748x^5 – 34796x^4 – 27360x^3 + 3108x^2 – 2304x + 36864 print(poly_7) 2x^5 – 93x^4 – 71x^3 + 8x^2 – 6x + 192 print(poly_7 – poly_7) 0 print(poly_7) 2x^5 – 93x^4 – 71x^3 + 8x^2 – 6x + 192 print(poly_9 / poly_7) 2 print(poly_9) 4x^5 – 186x^4 – 142x^3 + 16x^2 – 12x + 384 print(poly_7) 2x^5 – 93x^4 – 71x^3 + 8x^2 – 6x + 192 poly_10 = Polynomial(’-11x^4 + 3x^2 + 7x + 9’) poly_11 = Polynomial(’5x^2 -8x – 6’) poly_12 = poly_10 * poly_11 print(poly_12) -55x^6 + 88x^5 + 81x^4 + 11x^3 – 29x^2 – 114x – 54 print(poly_12 / poly_10) 5x^2 – 8x – 6 print(poly_12 / poly_11) -11x^4 + 3x^2 + 7x + 9 poly_13 = poly_6 * poly_7
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print(poly_13 / poly_6) 2x^5 – 93x^4 – 71x^3 + 8x^2 – 6x + 192 print(poly_13 / poly_7) -x^47 + x^23 – 12x^8 + 45x^6 + 6x^3 – x + 2
2 R Using a stack to evaluate fully parenthesised expressions
Modify the program postfix.py from the 9th lecture so that a stack is used to evaluate an arithmetic expression written in inﬁx, fully parenthesised, and built from natural numbers using the binary +, -, * and / operators. Fully parenthesised means that all expressions of the form e + e’, e – e’, e * e’ and e / e’ are surrounded by a pair of parentheses, brackets or braces. Of course a simple solution would be to replace all brackets and braces by parentheses and call eval(), but here we want to use a stack. Hint: think of popping when and only when a closing parenthesis, bracket or brace is being processed. Here is a possible interaction:
\$ python … from exercise_2 import * expression = FullyParenthesisedExpression(’2’) expression.evaluate() 2 expression = FullyParenthesisedExpression(’(2 + 3)’) expression.evaluate() 5 expression = FullyParenthesisedExpression(’[(2 + 3) / 10]’) expression.evaluate() 0.5 expression = FullyParenthesisedExpression(’(12 + [{[13 + (4 + 5)] – 10} / (7 * 8)])’) expression.evaluate() 12.214285714285714
3 Introduction to context free grammars
A context free grammar is a set of production rules of the form
symbol_0 –- symbol_1 … symbol_n
where symbol_0, …, symbol_n are either terminal or nonterminal symbols, with symbol_0 being necessarily nonterminal. A symbol is a nonterminal symbol iﬀ it is denoted by a word built from underscores or uppercase letters. A special nonterminal symbol is called the start symbol. The languagegenerated bythegrammaristhesetofsequencesofterminalsymbolsobtainedbyreplacing
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a nonterminal symbol by the sequence on the right hand side of a rule having that nonterminal symbol on the left hand side, starting with the start symbol. For instance, the following, where EXPRESSION is the start symbol, is a context free grammar for a set of arithmetic expressions.
EXPRESSION — TERM SUM_OPERATOR EXPRESSION EXPRESSION — TERM TERM — FACTOR MULT_OPERATOR TERM TERM — FACTOR FACTOR — NUMBER FACTOR — (EXPRESSION) NUMBER — DIGIT NUMBER NUMBER — DIGIT DIGIT — 0 … DIGIT — 9 SUM_OPERATOR — + SUM_OPERATOR — MULT_OPERATOR — * MULT_OPERATOR — /
Moreover, blank characters (spaces or tabs) can be inserted anywhere except inside a number. For instance, (2 + 3) * (10 – 2) – 12 * (1000 + 15) isanarithmeticexpressiongeneratedbythe grammar. Verify that the grammar is unambiguous, in the sense that every expression generated by the grammar has a unique evaluation. Write down a program that prompts for an expression, checks whether it can be generated by the grammar, and in case the answer is yes, evaluates the expression, following this kind of interaction:
\$ python exercise_3.py Input expression: 2 The expression evaluates to: 2 \$ python exercise_3.py Input expression: 2 * 2 The expression evaluates to: 4 \$ python exercise_3.py Input expression: (2 + 3) * (10 – 2) – 12 * (1000 + 15) The expression evaluates to: -12140 \$ python exercise_3.py Input expression: 2 + +3 Incorrect syntax