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' \ O ' is stored in s[O] Similar checking for inputs of one and two characters followed by a newline give us confidence that the loop works near that boundary There are other boundary conditions to check, though If the input contains a long line or no newlines, that is protected by the check that i stays less than MAX-1 But what if the input is empty, so the first call to getchar returns EOF We must check for that:
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f o r ( i = O ; i < M A X - 1 ; i++) if ( ( s [ i ] = getchar()) == ' \ n ' break; s [ i ] = '\O';
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== EOF)
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Boundary condition testing can catch lots of bugs, but not all of them We will return to this example in 8, where we will show that it still has a portability bug The next step is to check input at the other boundary, where the array is nearly full, exactly full, and over-full, particularly if the newline arrives at the same time We won't write out the details here, but it's a good exercise Thinking about the boundaries raises the question of what to d o when the buffer fills before a '\n' occurs; this gap in the specification should be resolved early, and testing boundaries helps to identify it Boundary condition checking is effective for finding off-by-one errors With practice, it becomes second nature, and many trivial bugs are eliminated before they ever happen
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Test pre- and post-conditions Another way to head off problems is to verify that expected or necessary properties hold before (pre-condition) and after (post-condition) some piece of code executes Making sure that input values are within range is a common example of testing a pre-condition This function for computing the average of n elements in an array has a problem if n is less than or equal to zero:
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double avg(doub1e a [ ] , i n t n)
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i n t i; double sum; sum = 0 0 ; f o r (i = 0 ; i < n; i++) sum += a [ i l ; r e t u r n sum / n;
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What should avg do if n is zero An array with no elements is a meaningful concept although its average value is not Should avg let the system catch the division by zero Abort Complain' Quietly return some innocuous value What if n is negative, which is nonsensical but not impossible As suggested in 4, our preference would probably be to return 0 as the average if n is less than or equal to zero:
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r e t u r n n <= 0 0 0 : sum/n;
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but there's no single right answer The one guaranteed wrong answer is to ignore the problem An article in the November, 1998 Scientific Americcin describes an incident aboard the USS Yorktown, a guided-missile cruiser A crew member mistakenly entered a zero for a data value, which resulted in a division by zero, an error that cascaded and eventually shut down the ship's propulsion system The Yorktown was dead in the water for a couple of hours because a program didn't check for valid input
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Use assertions C and C++ provide an assertion facility in < a s s e r t h> that encourages adding pre- and post-condition tests Since a failed assertion aborts the program, these are usually reserved for situations where a failure is really unexpected and there's no way to recover We might augment the code above with an assertion before the loop:
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If the assertion is violated, it will cause the program to abort with a standard message: Assertion f a i l e d : n > 0 , f i l e a v g t e s t - c , l i n e 7 Abort(crash) Assertions are particularly helpful for validating properties of interfaces because they draw attention to inconsistencies between caller and callee and may even indicate who's at fault If the assertion that n is greater than zero fails when the function is called, it points the finger at the caller rather than at avg itself as the source of trouble If an interface changes but we forget to fix some routine that depends on it, an assertion may catch the mistake before it causes real trouble
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Program defensively A useful technique is to add code to handle "can't happen" cases, situations where it is not logically possible for something to happen but (because of some failure elsewhere) it might anyway Adding a test for zero or negative array lengths to avg was one example As another example, a program processing grades might expect that there would be no negative or huge values but should check anyway:
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i f (grade < 0 1 I grade > 100) l e t t e r = ' ' ; e l s e i f (grade >= 90) l e t t e r = 'A' ; else
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/* c a n ' t happen */
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This is an example of defensive progrtrmming: making sure that a program protects itself against incorrect use or illegal data Null pointers, out of range subscripts, division by zero, and other errors can be detected early and warned about or deflected Defensive programming (no pun intended) might well have caught the zero-divide problem on the Yorktown
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