dear sir,
sir can i learn from you the basic programming c???????????
----- Original Message ----
From: Thomas Hruska <[EMAIL PROTECTED]>
To: [email protected]
Sent: Wednesday, August 29, 2007 7:50:21 AM
Subject: Re: [c-prog] Re: What's wrong with qsort here
Thomas Hruska wrote:
> Nico Heinze wrote:
>> --- In [EMAIL PROTECTED] com, "Indika Bandara" <indikabandara19@ ...>
>> wrote:
>>> man qsort
>>> ============ ========= ========= ========= ========= ==
>>>
>>> void qsort(void *base, size_t nmemb, size_t size,
>>> int(*compar) (const void *, const void *));
>>>
>>> DESCRIPTION
>>> The qsort() function sorts an array with nmemb elements of size
>>> size. The base argument points to the start of the array.
>>>
>>> The contents of the array are sorted in ascending order
>>> according to a comparison function pointed to by compar, which is
>>> called with two arguments
>>> that point to the objects being compared.
>>> ============ ========= ========= ========= ========= ==
>>>
>>> as man says, "sorts an array with nmemb elements of size size."
>>> my array contains MAX_NUM of (P*)
>>>>> P** p = new P*[MAX_NUM];
>>> but if it were
>>> int *i = new int[MAX_NUM]
>>> and
>>> qsort(i, MAX_NUM, sizeof(i), CompInt);
>>> works
>>>
>>> am i doing something silly? can u pls try urself?
>> <snip>
>>
>> I won't; I don't have the time. But even if I did have the time:
>> frankly I don't understand C++ well enough to even judge whether
>> qsort() can be used with objects this way. I have to leave the
>> response to this question to other people like Victor, Paul, and many
>> other ones. Sorry. I'm out of the game here.
>>
>> Regards,
>> Nico
>
> qsort() is C...
> (sort() is C++ - the OP is mixing C and C++ though)
>
> Anyway to answer the OP's question:
>
> qsort(p, MAX_NUM, sizeof(P*), Comp);
>
> Should be:
>
> qsort(p, MAX_NUM, sizeof(P**), Comp);
>
> (Although it probably won't affect anything).
> Then this:
>
> int Comp(const void* p1, const void* p2)
> {
> P* i1 = (P*)p1;
> P* i2 = (P*)p2;
> if(i1->_i>i2- >_i)
> return 1;
> if(i1->_i<i2- >_i)
> return -1;
>
> return 0;
> }
>
> Should be:
>
> int Comp(const void *p1, const void *p2)
> {
> P *i1 = *((P **)p1);
> P *i2 = *((P **)p2);
> if(i1->_i>i2- >_i)
> return 1;
> if(i1->_i<i2- >_i)
> return -1;
>
> return 0;
> }
>
> However, since you are using C++, you should be using std::sort(),
> FastSort() (from Safe C++ Design Principles - free e-book for group
> members), or equivalent with an appropriate sort class in P that
> overloads the () operator.
Oops - forgot to mention why. See, qsort() can't inline the comparison
function because, well, it is C. It is a function. This means that you
incur the overhead of a function call for each and every comparison. On
the other hand, C++ can use templates, which means it CAN inline the
comparison function. This means that there is no function call
overhead. As a result, you see anywhere from a 25 to 50% increase in
performance. Sorting is one area where C++ blows C out of the water in
terms of performance (although equivalent performance can be achieved by
rolling custom sorting functions... why bother? That's a huge waste of
time).
--
Thomas Hruska
CubicleSoft President
Ph: 517-803-4197
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