1 Lecture 7 Halting Problem –Fundamental program behavior problem –A specific unsolvable problem –Diagonalization technique revisited Proof more complex

2 Definition Input –Program P Assume the input to program P is a single unsigned int –This assumption is not necessary, but it simplifies some of the following unsolvability proof –To see the full generality of the halting problem, remove this assumption –Nonnegative integer x, an input for program P Yes/No Question –Does P halt when run on x?

3 Example Input Program with one input of type unsigned int bool main(unsigned int Q) { int i=2; if (Q == 0) return false; while (i<Q) { if (Q%i == 0) return (false); i++; } return (true); } Input x –73

4 Proof that H is not solvable

5 Definition of list L The set of all programs is countably infinite –For simplicity, assume these programs have one input the type of this input is an unsigned int This means there exists a list L where –each program is on this list –the programs will appear on this list by order of length length 0 programs first length 1 programs next … Simpler compared to last time –We built an algorithm D to work with any countably infinite list –Now, we get to work with this specific list

6 Definition of P H If H is solvable, some program must solve H Let P H be a procedure which solves H –We declare it as a procedure because we will use P H as a subroutine Declaration of P H –bool P H (program P, unsigned int x) In general, the type of x should be the type of the input to P Comments –We do not know how P H works –However, if H is solvable, we can build programs which call P H as a subroutine

7 Argument Overview List L contains all programs (with one input of type unsigned int) –We use list L to specify a pattern of program behavior B which is distinct from all real program behaviors We construct a program D (with one input of type unsigned int) with the following properties –D satisfies the specification of program behavior B and is thus not on list L –D uses P H as a subroutine which means D exists if P H exists These two facts imply H is not solvable –Since L contains all programs but not D, D must not exist –Since D exists if P H exists, P H does not exist

8 Visualizing List L of Programs P0P0 P1P1 P2P2 P3P3 P4P4...  1234 HHHHH NHHHH HH H H H HH #Rows is countably infinite  p * is countably infinite #Cols is countably infinite Set of n  nnegative integers is countably infinite Consider each number to be a feature –A program halts or doesn’t halt on each integer –Several programs may be identical now (e.g. several always halt) –We have a fixed L this time

9 Specifying a non-existent program behavior B P0P0 P1P1 P2P2 P3P3 P4P4...  1234 HHHHH NHHHH HH H H H HH We specify a non-existent program behavior B by using a unique feature (number) to differentiate B from P i NH H H H B

10 Specification Insufficient Task not complete –The previous slide only provides a specification B for program D That is, B describes how program D should behave in order to not be on the list of all programs –We still need to construct an actual program D that behaves this way –Previous number of languages proof The specification of language D(L) was sufficient We were done at this point

11 Overview of D Program D with input y –(type for y: unsigned int) Given input y, generate the program (string) P y Run P H on P y and y Guaranteed to halt since P H decides H IF (P H (P y,y)) while (1>0); else return (yes); P0P0 P1P1 P2P2 PyPy...  12 HH HH NH H D...y HNH

12 Generating P y from y We won’t go into this in detail here This was the basis of the question at the bottom of slide 21 of lecture 5. This is the main place where our assumption about the input type for program P is important –for other input types, how to do this would vary

13 Observation For some y, the computed P y of  P * will not be a legal program In particular, the computed P y will not be the y th legal program a b P i-1...  12 HH HH NH H D...i Not a program First Legal Program... i th Legal Program P0P0... y

14 bool main(unsigned int y) /* main for program D */ { program P = generate(y); if (P H (P,y)) while (1>0); else return (yes); } bool P H (program P, unsigned int x) /* how P H solves H is unknown */ Claims –Program D exhibits behavior B –Program D exists if P H exists Code for D and P H

15 Alternate Proof For every program P y, there is a number y which we associate with it The number we use to distinguish program P y from D is this number y Using this idea, we can arrive at a contradiction without explicitly using the table L –The diagonalization is hidden

16 H is not solvable, proof II Assume H is solvable –Let P H be the program which solves H –Use P H to construct program D which cannot exist Contradiction –This means program P H cannot exist. –This implies H is not solvable D is the same as before

17 Arguing D cannot exist If D is a program, it must have an associated number y What does D do on this number y? 2 cases –D halts on y This means P H (D,y) = NO –Definition of D This means D does not halt on y –P H solves H Contradiction This case is not possible

18 Continued –D does not halt on this number y This means P H (D,y) = YES –Definition of D This means D halts on y –P H solves H Contradiction This case is not possible –Both cases are not possible, but one must be for D to exist –Thus D cannot exist

19 Implications The Halting Problem is one of the simplest problems we can formulate about program behavior We can use the fact that it is unsolvable to show that other problems about program behavior are also unsolvable This has important implications restricting what we can do in the field of software engineering –In particular, “perfect” debuggers/testers do not exist –We are forced to “test” programs for correctness even though this approach has many flaws

20 Summary Halting Problem definition –Basic problem about program behavior Halting Problem is unsolvable –We have identified a specific unsolvable problem –Diagonalization technique Proof more complicated because we actually need to construct D, not just give a specification