Soluzione MidTerm 2005

1. Soluzione MidTerm 2005 Giuseppe Attardi

2. Esercizio 1

3. Albero binario di string class Tree { string key; string value; Tree left; Tree right; public Tree(string k, string v) { key = k; value = v; left = null; right = null; }

4. Albero binario generico class Tree<K:Comparable, V> { K key; V value; Tree<K, V> left; Tree<K, V> right; public Tree(K k, V v) { key = k; value = v; left = null; right = null; } }

5. class Dictionary class Dictionary : IDictionary { class Tree { ... } Tree root; public Dictionary() { root = null; } public bool insert(string k, string v); public string get(string key); IDictionaryIterator getIterator(); }

6. insert public bool insert(string k, string v) { if (k == null) return false; if (root == null) { root = new Tree(k, v); return true; } Tree curr = root; while (true) { int r = curr.key.CompareTo(k); if (r == 0) return false; if (r < 0) { if (curr.left == null) { curr.left = new Tree(k, v); return true; } else curr = curr.left; } else { ... } } }

7. get public string get(string k) { Tree curr = root; while (curr != null) { int r = curr.key.CompareTo(k); if (r == 0) return curr.value; if (r < 0) curr = curr.left; else curr = curr.right; } return null; }

8. getIterator IDictionaryIterator getIterator() { return new Iterator(root); }

9. Iterator class Iterator { stack<Tree> rest; Tree curr = null; Iterator(Tree root) { rest = new stack<Tree>; if (root != null) rest.push(root); }

10. MoveNext public bool MoveNext() { if (rest.empty) return false; curr = rest.pop; if (curr.right != null) rest.push(curr.right); if (curr.left != null) rest.push(curr.left); return true; }

11. Current public string Current() { return curr.value; }

12. Esercizio 2

13. sp fp Standard call sequence local m local m-1 ... local 1 old fp return addr arg1 arg2 ... argn

14. transfer routine saved regs transfer routine saved regs old fp old fp return addr return addr sp fp fp Coroutine stacks Coroutine A Coroutine B savedSP savedSP

15. Transfer Routine transfer(Coroutine c) { save registers on stack (no SP) this.savedSP = SP; SP = c.savedSP; restore registers (except SP) return; } Why?

16. Esercizio 3

17. class CoroutineA : Thread { public void Run() { wait(); // prepare ... corB.notify(); wait(); // transfer ... } }

18. public int main(string[] args) { CoroutineA corA = new CoroutineA(); coroutineA.start(); CoroutineB corB = new CoroutineB(); coroutineB.start(); corA.notify(); corA.join(); }

19. Esercizio 4

20. coroutine A { Stack<Tree> rest = new Stack<Tree>(); rest.push(tree1); while (true) { if (rest.empty) { end1 = true; transfer Main; return; } Tree curr = rest.pop; if (curr.left == null && curr.right == null) { leaf1 = curr.value; transfer Main; } else { if (curr.right != null) rest.push(curr.right); if (curr.left != null) rest.push(curr.left); } } }

21. coroutine C { Stack<Tree> rest = new Stack<Tree>(); rest.push(tree2); while (true) { if (rest.empty) { end2 = true; transfer Main; return; } Tree curr = rest.pop; if (curr.right != null) rest.push(curr.right); if (curr.left != null) rest.push(curr.left); leaf2 = curr.value; transfer Main; } }

22. Main end1 = end2 = false; while (true) { transfer A; transfer B; if (end1) return end2; if (end2) return end1; if (leaf1 != leaf2) return false; }

23. Esercizio 6

24. Passaggio per reference • In Java non esiste il passaggio per reference • Java does not provide call by reference • Java n’avait pas call by reference • Java hat nicht call by reference

25. Language Specific Variations • Pascal: Call by Value is the default, the keyword var denotes Call by Reference • Fortran: all parameters passed by Reference • Smalltalk, Lisp, Java: actual parameter is already a reference to the object, always passed by Value • C: always passed by Value

26. Ada Parameter Modes Three parameter passing modes • in • Passes information from the caller to the callee, can read but not write • Call by Value • out • Passes information from the callee to the caller, can write but not read • Call by Result (formal parameter is copied to actual parameter when subroutine exits) • inout - passes information both directions

27. C++ Parameter Modes • C passes pointers as addresses, must be explicitly dereferenced when used • C++ has notion of references • Parameter passing: void swap (int &a, int &b) • Variable References: int &j = i; • Function Returns: for objects that don’t support copy operations, i.e. file buffers

28. C# Parameter Modes • in • Passes information from the caller to the callee, can read but not write • Call by Value • out • Passes information from the callee to the caller, can write but not read • Call by Result (formal parameter is copied to actual parameter when subroutine exits) • in out – Call by Value-Result • ref • Call by Reference

29. Function Returns • Some languages restrict return types • Algol 60, Fortran: scalars only • Pascal, Modula: scalars or pointers only • Most imperative languages are flexible • Return statements specify a value and also cause the immediate termination of the subroutine

30. Totale esercizi • 105 righe di codice • Una settimana di tempo • 15 righe al giorno

31. MoveNext public bool MoveNext() { while (true) { if (curr == null) { if (rest.empty) return false; else { curr = rest.pop; return true; } curr = curr.left; if (curr.right != null) rest.push(curr.right); } }