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 TreeltKComparable, Vgt
• K key
• V value
• TreeltK, Vgt left
• TreeltK, Vgt 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()

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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 lt 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 lt 0)
• curr curr.left
• else
• curr curr.right
• return null

8
getIterator

• IDictionaryIterator getIterator()
• return new Iterator(root)

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Iterator

• class Iterator
• stackltTreegt rest
• Tree curr null
• Iterator(Tree root)
• rest new stackltTreegt
• 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

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Current

• public string Current()
• return curr.value

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Esercizio 2
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Standard call sequence
local m local m-1 ... local 1
old fp
return addr
arg1 arg2 ... argn
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Coroutine stacks
Coroutine A
Coroutine B
savedSP
savedSP
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Transfer Routine

• transfer(Coroutine c)
• save registers on stack (no SP)
• this.savedSP SP
• SP c.savedSP
• restore registers (except SP)
• return

Why?
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Esercizio 3
17

• class CoroutineA Thread
• public void Run()
• wait() // prepare
• ...
• corB.notify()
• wait() // transfer
• ...

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• public int main(string args)
• CoroutineA corA new CoroutineA()
• coroutineA.start()
• CoroutineB corB new CoroutineB()
• coroutineB.start()
• corA.notify()
• corA.join()

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Esercizio 4
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• coroutine A
• StackltTreegt rest new StackltTreegt()
• 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
• StackltTreegt rest new StackltTreegt()
• 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

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Main

• end1 end2 false
• while (true)
• transfer A
• transfer B
• if (end1) return end2
• if (end2) return end1
• if (leaf1 ! leaf2)
• return false

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Esercizio 6
24
Passaggio per reference

• In Java non esiste il passaggio per reference
• Java does not provide call by reference
• Java navait pas call by reference
• Java hat nicht call by reference

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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

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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

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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 dont support
  copy operations, i.e. file buffers

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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

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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

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Totale esercizi

• 105 righe di codice
• Una settimana di tempo
• 15 righe al giorno

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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)