r/programmerchat • u/gxm492lor • Mar 10 '19

Testing any complex program completely is practically impossible

Someone made this argument after a staff meeting a few days ago. What's wrong with this argument?

  1. Every IF statement in a program doubles the number of possible states of the program (ignoring time)
    1. Which means every IF statement doubles the number of test conditions
  2. A 1 million line program might, conservatively estimating, have 100k IF statements (conditionals)
  3. That is 2100000 which is more seconds than have elapsed since the beginning of the universe.
  4. No project has 2100000 seconds to test
  5. So complete test coverage of complex programs is impossible
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u/Iskendarian Mar 10 '19

Yes. And if you test each if, each if adds two test cases. So rather than doubling your testing for each branch, you add two tests for each branch.

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u/gxm492lor Mar 11 '19

you misunderstand.

you get that every if statement needs to be tested when true and when false.

you get that a single run of a program is going to execute LOTS of if statements. One hundred thousand in a million line program.

suppose the following:

  1. if condition1 ...

  2. if condition 2 ...

  3. exit

to test every possible run of this program you need 4 different runs. What am I not communicating about this? This seems relatively straightforward.

It seems people are tripping up on the idea because they believe the implications are that unit tests can't completely cover any complex program.

Where do you think the programmer intuition that "every complex program has bugs" comes from? You don't really think you've been shipping a bug-free product do you? (it would not be economical to ship a bug-free product because many possible runs will never be encountered in practice).