20-CS Programming Languages Fall 2016

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1 20-CS Programming Languages Fall 2016 Answer questions 1-6 for exam score, question 7 for brownies Be sure to put your name on the paper in the space provided! Name Signature 1. Java, the language, is not type safe but the latest version contains a feature that allows a developertocreaterelativelytypesafecode. Whatisthatfeatureandgiveanexampleofitsuse? 2. Show how a developer may still be able to create type unsafe code even if the above feature is used to the fullest.

2 3. For each of the code snippets below, is there a safety problem? If not say why not. If so, say what the problem is (type, thread or memory safety), express the danger in a sentence or two, and suggest how the C++ language might be changed to fix it? Consider each independently and state what the expected output is (or whether the code crashes). Note: #include lines are omitted to save space. class A { private: int a,b,c; public: A (int x) { a = x; b=x+20; c=x+31; int get() { return c; ; int main () { A *a = new A(125); *((int*)a+2) = 876; cout << a->get() << "\n"; Problem (circle one)? Yes No Output (or crash): Explain: class A { public: char ch; A (char x) { ch = x; ; int main () { A *a = new A( c ); cout << a->ch << " " << (int)(a->ch) << " " << *((double*)&(a->ch)) << "\n"; Problem (circle one)? Yes No Output (or crash): Explain:

3 The following Scheme can be used to produce a list of the first n prime numbers where n is a parameter to the procedure take. (define int-builder (lambda (x) (cons x (lambda () (int-builder (+ 1 x)))))) (define filter-out-mults (lambda (num s) (if (= (modulo (car s) num) 0) (filter-out-mults num ((cdr s))) (cons (car s) (lambda () (filter-out-mults num ((cdr s)))))))) (define sieve (lambda (s) (cons (car s) (lambda() (sieve (filter-out-mults (car s) ((cdr s)))))))) (define take (lambda (n s) (if (or (= n 0) (null? s)) () (cons (car s) (take (- n 1) ((cdr s))))))) 4(a) For small n this works great. For n large, something goes wrong - what is that? 4(b) Write a line of Scheme code, using the above procedures, that evaluates to a list of the first 1000 prime numbers. 4(c) If you remove the (lambda () terms and their matching parens, and if you change ((cdr s)) to (cdr s), and if you make adjustments to filter-out-mults and sieve to account for finite lists, and if you use the following replacement for int-builder: (define int-builder (lambda (x) (if (= x 1) () (append (int-builder (- x 1)) (list x))))) youwill havecodethatcan producethesameoutputastheabove. Whichofthetwoprograms would you consider to be superior (top or bottom) and why?

4 Consider the following Scheme code: (define call/cc call-with-current-continuation) (define showit (lambda (a b) (begin (display a) (display b) (display " ")))) (define f (lambda (n) (let ((p (call/cc (lambda (k) k)))) (begin (showit f= n) p)))) 5(a) what does ((f 2) (f 4)) produce? 5(b) what is the value of ((f 2) not)?

5 Consider the following C++ code: #include <iostream> using namespace std; int i, j; int array[10]; int x() { return array[i]; int y() { return array[j]; void swap (int (*x)(), int (*y)()) { int t = array[x()]; array[x()] = array[y()]; array[y()] = t; int main () { i = 0; j = 3; for (int k=0 ; k < 10 ; k++) array[k] = k+1; swap(x, y); for (int k=0 ; k < 10 ; k++) cout << array[k] << " "; cout << "\n"; 6(a) Would you say this code simulates passing values to swap by value, value-result, reference, or name? Why do you say this? 6(b) What is the output of the program?

6 7. An undirected Graph is a collection of objects, called vertices, and a collection of pairs of vertices, called edges. Information may be carried on the vertices or on edges, or on both. Such information depends on the problem that the graph is to represent. Examples of such problems are: Minimum Spanning Tree: Given a connected, undirected graph where only edges carry information and that information is just a number called weight, the problem is to find a subset of the edges that connects all the vertices together, without any cycles, and with the minimum possible total edge weight. Shortest Path: Given a connected, undirected graph where only edges carry information and that information is just a number called weight, and where two vertices are marked as distinguished with the names source and destination, the problem is to find the path between source and destination such that the sum of the edge weights along that path is no greater than the sum of weights of edges along any other path between source and destination. Biconnected Components: A biconnected component of a given connected, undirected graph is a largest subset of vertices such that at least two paths connect every pair of vertices in the subset. The problem is to find all the biconnected components of a given graph. Three Coloring a Graph: Given an undirected graph, can the vertices be assigned colors from a palett of three colors such that for every edge in the graph the vertices of that edge are colored differently? State what components should go into making a Graph class in C++ or Java so that implementations of solutions to all of the problems above can be based directly on a graph data structure. Be sure to indicate any additional classes that would be needed and express components that those classes would have.