# 21 - Click to edit Master subtitle style Chapter Twenty One...

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Unformatted text preview: Click to edit Master subtitle style Chapter Twenty- One Modern Programming Languages, 2nd ed. Cost Models Chapter Twenty- One Modern Programming Languages, 2nd ed. 11 Chapter Twenty- One Modern Programming Languages, 2nd ed. Which Is Faster? ■ Every experienced programmer has a cost model of the language: a mental model of the relative costs of various operations ■ Not usually a part of a language specification, but very important in practice Chapter Twenty- One Modern Programming Languages, 2nd ed. 22 Y=[1|X] append(X,[1],Y) Chapter Twenty- One Modern Programming Languages, 2nd ed. Outline ■ A cost model for lists ■ A cost model for function calls ■ A cost model for Prolog search ■ A cost model for arrays ■ Spurious cost models Chapter Twenty- One Modern Programming Languages, 2nd ed. 33 Chapter Twenty- One Modern Programming Languages, 2nd ed. The Cons-Cell List ■ Used by ML, Prolog, Lisp, and many other languages ■ We also implemented this in Java Chapter Twenty- One Modern Programming Languages, 2nd ed. 44 ?- A = , | B = .(1,), | C = .(1,.(2,)). A = , B = [1], C = [1, 2]. A : B : 1 C : 1 2 Chapter Twenty- One Modern Programming Languages, 2nd ed. Shared List Structure Chapter Twenty- One Modern Programming Languages, 2nd ed. 55 ?- D = [2,3], | E = [1|D], | E = [F|G]. D = [2, 3], E = [1, 2, 3], F = 1, G = [2, 3]. F : E : 1 D : 2 3 G : Chapter Twenty- One Modern Programming Languages, 2nd ed. How Do We Know? ■ How do we know Prolog shares list structure—how do we know E=[1|D] does not make a copy of term D ? ■ It observably takes a constant amount of time and space ■ This is not part of the formal specification of Prolog, but is part of the cost model Chapter Twenty- One Modern Programming Languages, 2nd ed. 66 Chapter Twenty- One Modern Programming Languages, 2nd ed. Computing Length ■ length(X,Y) can take no shortcut—it must count the length, like this in ML: ■ Takes time proportional to the length of the list Chapter Twenty- One Modern Programming Languages, 2nd ed. 77 fun length nil = 0 | length (head::tail) = 1 + length tail; Chapter Twenty- One Modern Programming Languages, 2nd ed. Appending Lists Chapter Twenty- One Modern Programming Languages, 2nd ed. 88 ?- H = [1,2], | I = [3,4], | append(H,I,J). H = [1, 2], I = [3, 4], J = [1, 2, 3, 4]. H : 1 2 I : 3 4 J : 1 2 ■ append(H,I,J) can also be expensive: it must make a copy of H Chapter Twenty- One Modern Programming Languages, 2nd ed. Appending ■ append must copy the prefix: ■ Takes time proportional to the length of the first list Chapter Twenty- One Modern Programming Languages, 2nd ed. 99 append(,X,X). append([Head|Tail],X,[Head|Suffix]) :- append(Tail,X,Suffix). Chapter Twenty- One Modern Programming Languages, 2nd ed....
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## This note was uploaded on 09/30/2011 for the course CS 6371 taught by Professor Hamlen during the Spring '11 term at University of Texas at Dallas, Richardson.

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21 - Click to edit Master subtitle style Chapter Twenty One...

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