MrArM's blog

Why Runtime Error on test 11 !!
By MrArM, 13 years ago, In English

Hi :)

Can any body help me about problem 299 sgu !?

Why my code got Runtime Error on test 11 !! ?

What's wrong with me ?!

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13 years ago, hide # |
 
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I don't know, how it can be compiled, but it is wrong:

  int n;
  cin>>n;
  string s[n+10];

Use vector instead:

  int n;
  cin>>n;
  std::vector<string> s(n);
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    13 years ago, hide # ^ |
     
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    Can you tell me why !? you say that therse is'nt array of strings !?

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      13 years ago, hide # ^ |
       
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      string s[n+10]

      is array, but according C++ standard, array's length must be constant, which is known at compile-time. In your case n is unknown at compile-time, so it is undefined behaviour.

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        13 years ago, hide # ^ |
         
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        It’s not undefined behaviour. According to the standard, the program is invalid, and a strictly conforming compiler will reject it, i. e. refuse to compile. However, in practice compilers support this as a language extension.

        Note that variable length arrays are perfectly valid in C since C99.

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13 years ago, hide # |
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Another bug:
1) Use "return a < b;" in cmp function 2) And use

if (cmp(s[i+2], PLUS(s[i], s[i+1])))

Instead

if(!cmp(PLUS(s[i],s[i+1]),s[i+2]))
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    13 years ago, hide # ^ |
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    Accepted! big thanks! :)

    But I don't understand what is difference of them !

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      13 years ago, hide # ^ |
       
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      There is no problem in 

      cmp(PLUS(a, b), c)

      here's everything ok. :)

      The potential problem is in 

      sort(s.begin(), s.end(), cmp)

      because sort and other functions from <algorithm> header expect strict predicate ( less , not lessOrEqual). In some compilers, using lessOrEqual can bring to runtime-assert-check. For example:

      bool BadPredicate(int a, int b)
{
    return a <= b;
}
...
int x[] = {1, 2, 1, 2};
std::sort(x, x + sizeof(x)/sizeof(x[0]), &BadPredicate);

      Gives debug-assert in MSVS 2010:

      if (!_Pred(_Left, _Right))
    return (false);
else if (_Pred(_Right, _Left))
    _DEBUG_ERROR2("invalid operator<", _File, _Line);

      But it's not critical for problem acceptance on contest (because problem runs not in debug config), it's right way to make reliable software. :)

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