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Theory of Computation (Fall 2013): Review of Program States & Snapshots; Successor Snapshots; Partially Computable & Computable Functions
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Theory of Computation (Fall 2013): Review of Program States & Snapshots; Successor Snapshots; Partially Computable & Computable Functions

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  • 1. Theory of Computation Review of Program States & Snapshots, Successor Snapshots, Computation, Partially Computable & Computable Functions Vladimir Kulyukin www.vkedco.blogspot.com
  • 2. Outline ● ● ● ● Review of Program States & Snapshots Successor Snapshots Computation Partially Computable & Computable Functions
  • 3. Review of Program States & Snapshots
  • 4. Instructions & Programs • An instruction is either a labeled or unlabeled statement • A program is a finite sequence of instructions • A program with no instructions is called the empty program
  • 5. Definition: A State of Program P • A state of a program P is a set of equations of the form V = m, where V is a variable and m is a number • The set must have exactly one equation for every variable that occurs in P • Corollary 1: The state may have equations for variables that do not occur in P (this is very useful for passing meta information) • Corollary 2: It is irrelevant whether a state can or cannot be achieved by P
  • 6. Example [ A1] IF X 1! 0 GOTO B1 Y  Y 1 Z1  Z1  1 IF Z1!  0 GOTO E1 [ B1] X 1  X 1  1 Y  Y 1 Z1  Z1  1 IF Z1!  0 GOTO C1 [C1] IF X 1! 0 GOTO B1 Z1  Z1  1 IF Z1!  0 GOTO E1
  • 7. Program State Example • The program on the previous slide (in this context, it does not really matter what it does in this context of program states) contains variables X1, Z1, and Y • { X1 = 2, Z1 = 3, Y = 5 } is a legal state • { X1 = 2, X2 = 4, Z1 = 3, Y = 5 } is also a legal state, although it contains a variable that does not occur in the program; however, it does satisfy the definition • { X1 = 2, Y = 5 } is not a state, because it does not define the value of Z1 • { X1 = 2, Z1 = 3, Y = 5, X1 = 5 } is not a state, because it has two equations for X1
  • 8. Formal Definition of Program A Program is a Finite Sequence of Primitive L Instructions 1. Primitive L Instruction 2. Primitive L Instruction 3. Primitive L Instruction 4. Primitive L Instruction 5. Primitive L Instruction … n. Primitive L Instruction
  • 9. Program Snapshots • A program P is assumed to have n ≥ 0 instructions • A program snapshot is a pair (i ,σ) where i is the number of the instruction to be executed and σ is a state of P • The values of i are in [1, n+1]
  • 10. Initial Snapshot Let P be a program in L and let r1 ,..., rm be m given numbers. Let  consist of two sets of equations : 1. X 1  r1 ,..., X m  rm , Y  0; 2. V  0, for any other V in P. Then (1,  ) is the initial snapshot.
  • 11. Terminal Snapshot A snapshot (i,  ) is terminal if i  n  1 and  is a legal program state
  • 12. Successor Snapshots
  • 13. Successor Snapshots ● ● ● Suppose there is a program P whose current state is captured in a snapshot S If we want to answer the question of what will happen next in P, we have to be able to compute the next instantaneous description, i.e., the successor snapshot of the current snapshot If we want to proceed in a deterministic fashion, there must be one and only one successor snapshot
  • 14. Key Successor Snapshot Assumption ● ● Let us assume that every program P, for which we are interested in computing successor snapshots (i.e., emulating), is compiled down to four types of primitive instructions, i.e., all of the macros have been expanded appropriately Recall that the four primitive instructions are:  1) V ← V + 1;  2) V ← V – 1;  3) V ← V;  4) IF V != 0 GOTO L
  • 15. Successor Snapshots ● The computation of the successor shot depends on two things:  the current snapshot  the ● instruction that is about to be executed We will define the successor snapshot function with four cases (four primitive instruction types)
  • 16. Successor Snapshot successor((i,  ))  ( j , ), where i,   is the current snapshot; ( j , ) is the successor snapshot
  • 17. Successor Snapshot: Case 1: Increment The i instruction of P is V  V  1 and  contains the equation V  m. Then j  i  1 and  is obtained from  by replacing the equation V  m by V  m  1. th
  • 18. Successor Snapshot: Case 2: Decrement The i instruction of P is V  V  1 th and  contains the equation V  m. Then j  i  1 and  is obtained from  by replacing the equation V  m by V  m  1 if m  0; if m  0,    .
  • 19. Successor Snapshot: Case 3: NO-OP The i instruction of P is V  V . th Then    and j  i  1.
  • 20. Successor Snapshot: Case 4: Conditional Dispatch th The i instruction of P is IF V  0 GOTO L 1.  contains the equation V  0. Then j  i  1. 2.  contains the equation V  m, where m  0. Then, if there is an instruction of P labeled L, j is the least number such that the jth instruction of P is labeled L. Otherwise, j  n  1.
  • 21. Example 1. [A] 2. 3. 4. [B] 5. 6. 7. IF X != 0 GOTO B Z←Z+1 IF Z != 0 GOTO E X←X–1 Y←Y+1 Z←Z+1 IF Z != 0 GOTO A Suppose the current snapshot is (i, σ), where σ = {X = 4, Y = 0, Z = 0}. If i = 1, the successor snapshot is (4, σ). If i = 2, the successor snapshot is (3,σ'), where σ' = {X = 4, Y = 0, Z = 1}.
  • 22. Computation
  • 23. Formal Definition of Computation A computation of a program P is a sequence of snapshots s1 , s2 ,..., sk of P such that si 1 is the successor of si , i  1,2,..., k  1 and sk is terminal.
  • 24. Remarks on Computation ● ● ● Given a program P and a sequence of snapshots S, it is possible to determine whether S constitutes a computation of P on a given input Computation is finite by definition The finiteness requirement may draw some disagreement, because one can think of meaningful programs that deliberately go into infinite loops (e.g., robot control loops of the type sense – act – sense)
  • 25. What does P compute? • • • Suppose there is a program P Suppose that we give some inputs for P If we start running or simulating P on the inputs, there are two cases: 1. There is a computation, i.e., a sequence of snapshots s1 , s2 , … , sk with sk being a terminal snapshot 2. There is no computation, i.e., the sequence of snapshots is infinite
  • 26. What does P compute? P is said to compute the value of Y at terminal snapshots. (m) Ψ P (r1,r2 ,...,rm ) denotes the value of Y at the terminal snapshot sk and the initial snapshot s1 contains the equations X 1  r1, X 2  r2 , ... X m  rm .
  • 27. Example 1: What does P compute? [A1] [B1] IF X1 != 0 GOTO B1 GOTO E1 X1 ← X1 – 1 Y ← Y+1 GOTO A1
  • 28. Example 1: What does P compute?  ( x)  x (1) P
  • 29. Example 2: What does P compute? [A1] [B1] Y ← X1 Z1 ← X2 IF Z1 != 0 GOTO B1 GOTO E1 Z1 ← Z1 – 1 Y ← Y+1 GOTO A1
  • 30. Example 2: What Does P Compute? Ψ P  x1 , x2   x1  x2 2 
  • 31. Number of Program Inputs Let P be a program of n variables. What is  (m) P (r1 ,..., rm ) if m  n ? Case 1 : m  n. Case 2 : m  n.
  • 32. Case 1: m < n ● ● ● If the remaining inputs are not specified in the input, all of them default to 0 Example: If a program computes a function of 5 variables and only X1, X2, and X3 are specified in the initial snapshot, the values of X4 and X5 are assumed to be 0 In practical terms, X4 = 0 and X5 = 0 are added to the initial snapshot
  • 33. Case 2: m > n ● ● If more inputs are specified than necessary, the extra inputs are ignored Example: If a program computes a function of 2 variables but X1, X2, X3, and X4 have equations in the initial snapshot, the values for X3 and X4 are ignored
  • 34. Example 3 Suppose P is a program that assigns r1 to Y . In other words,  (1) P ( x)  x. Then the following statements are true : 1 . (1) P 2 . ( 2) P ( r1 , r2 )  r1 3 . ( 3) P (r1 , r2 , r3 )  r1 ( r1 )  r1
  • 35. Example 4 Suppose that P is a program such that (  P2 ) ( x, y )  x  y. Then the following statements are true :  (1) P (r1 )  r1  0. (  P3) (r1 , r2 , r3 )  r1  r2 .
  • 36. Partially Computable Functions & Computable Functions
  • 37. Example 1  1 if n  0  n f ( n)    i if n  0   i 1
  • 38. Example 1: C Program fact.c long fact(long n) { if ( n == 0 ) return 1; else return n * fact(n-1); }
  • 39. Example 1: Comments on fact.c ● ● ● We can say that f(n) is computed by the C program in fact.c We can also say that the C program in fact.c computes f(n) The two statements are semantically equivalent:   programs compute functions functions are computed by programs
  • 40. Definition: Partially Computable Functions A partial function g is said to be partially computable if it is computed by some L program P.
  • 41. Definition: Partially Computable Functions g is a partially computable function iff : 1. P r1 , r2 ,..., rm g r1 , r2 ,..., rm    (m) P where P is an L program 2. g r1 , r2 ,..., rm   iff  (m) P r1 , r2 ,..., rm   r1 , r2 ,..., rm ,
  • 42. Example 2  x1  x2 if x1  x2 g  x1 , x2     if x1  x2 
  • 43. Example 2 Y ← X1 Z ← X2 [A] IF Z != 0 GOTO B GOTO E [B] IF Y != 0 GOTO C GOTO B [C] Y ← Y – 1 Z←Z–1 GOTO A
  • 44. Example 2 ● ● g is a partially computable function because there exists a L program, defined in Example 2, that computes it For any two natural numbers x and y,  g(x, y) is the same as the value of Y and the program terminates;  g(x, y) is undefined and the program does not terminate, in which case the value of Y is undefined as well
  • 45. Definition: Computable Functions ● ● ● ● g(r1 , r2 , …, rm) is total if it is defined for all r1 , r2 , …, rm in its domain g is computable if it is both partially computable and total Example: g(x, y) = x + y Remember the following equivalents:   partially computable = partially recursive computable = recursive.
  • 46. Partially Computable vs. Computable Functions ● ● ● ● ● ● f(x) = x f(x, y) = x + y f(x, y) = x * y f(x, y) = x – y All are partially computable on natural numbers Only the first three are computable on natural numbers
  • 47. Reading Suggestions ● Ch. 2, Davis, Weyuker, Sigal. Computability, Complexity, & Languages

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