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-So let's talk more about testing, as we said a

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-little earlier in the lesson, testing means executing the program on

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-a sample of the input domain, that is of all

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-the possible input data and I really want to stress that

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-this sample is tiny sample of the input domain. There

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-are two important aspects of testing that I'm want to mention here,

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-there first one is that testing is a dynamic technique. And

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-what that means is that the program must be executed in

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-order to perform testing. The second important point is that

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-testing is an optimistic approximation. And what does it mean

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-to be optimistic? Well, it means that the program under

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-test is exercised with a very small subset of all the

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-possible inputs as we just said. And this is done

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-under the assumption that the behavior with any other input

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-is consistent with the behavior shown for the selected subset

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-of input data, that is why it is an optimistic approach.

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-Another concept that I want to mention explicitly, is the

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-concept of successful test. And I'm going to do that,

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-using another quote. This one from Goodenough and Gerhart

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-in their paper Towards a Theory of Test Data Selection,

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-and what the quote says is that a test

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-is successful if the program fails. And this might sound

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-counterintuitive, but the point here is that testing cannot

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-prove the absence of errors, but only reveal their presence.

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-If a set of tests does not produce any failure, we are either in the extremely

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-unlikely case of a correct program, or in

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-the very likely situation of a bad set of

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-tests that are not able to reveal failures of the program. And that is why we

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-say that the test is successful if you

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-can show that there are problems in the program.