Problem Detail: How to prove that there exist two different programs A and B such that A printing code of B and B printing code of A without giving actual examples of such programs?
Asked By : KirillSk
Answered By : A.Schulz
You maybe know the recursion theorem. It implies, that you can assume that you have a procedure called get_your_own_codeif your programming language is at least as powerful as a Turing machine. Now do the following
Progr. A w=get_your_own_code print "print "+w
Note that some code is hidden in the subroutine of get_you_own_code. So we assume the complete source code is a string called source_code_of_A.
Progr. B print source_code_of_A
Program A prints:
print source_code_of_A
Program B prints
source_code_of_A
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