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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.