Re: [Boost-Users] Possible interest in select_by_predicate?

From: hicks (hicks_at_[hidden])
Date: 2002-02-11 00:34:35

Hello

Dear Jonathan,

Could you answer the following 3 questions please?

(1)
For all cases where an STL predicate "pred" will suffice, e.g. your 
example using std::greater<>,
is there any case where
    std::max_element(b, e, pred)
would not also perform the job?

Apart from the issue of code bloat,
performance would seem better with std::maximum(), because there would 
be no function call in the inner loop.
(Most if not all std predicates inline cleanly.  With Borland compilers 
at least, function calls do not inline
if they contain "if" statements.  That is why select_by_predicate<> 
would result in an inner loop function call.)

(2)
For those cases where there is no suitable std predicate, you must have 
a custom predicate.
In all such cases, is there any in which the call
    std::max_element(b,e, custom_pred)
would not perform suitably?  In this case let us ignore the performance 
issue.  I only want to know
if any new functionality has been added by introducing 
select_by_predicate<>.

(3)
I suppose your functor select_by_predicate<> has "state" between calls 
which retains a pointer
to the last element which evaluated true.
I suppose that pointer is initialized to NULL, and thats how the functor 
knows which is the first call.
When the pointer is not NULL it compares the elements and resets the 
pointer if true.

Below is the code for max_element from my Borland-Rouge Wave STL library.
Note that max_element keep a copy of the best iterator, because it has 
access to the iterators,
whereas select_by_predicate<> does not.

I said I "suppose" your functor uses a pointer .  But I don't know that 
for certain.  
That means I have to worry about whether 
"ForwardIterator::value_type::operator=" might be called.
If I am not totally familiar with select_by_predicate<>, that means I 
either have to
avoid any situation where "ForwardIterator::value_type::operator=" would 
be dangerous,
or be prepared to open the doc page to double check.
Do you think this is a valid issue?

============================
  template <class ForwardIterator, class Compare>
  ForwardIterator max_element (ForwardIterator first, ForwardIterator last,
                               Compare _RWSTD_COMP)
  {
    if (first == last) return first;
    ForwardIterator result = first;
    while (++first != last)
      if (_RWSTD_COMP(*result, *first)) result = first;
    return result;
  }
=============================

Respecfully Yours

Craig Hicks
Engineer, KGK, Tokyo

Jonathan Brownell wrote:

>I work in doing programming involving quite a bit of data analysis, and
>lately I've gotten sick and tired of doing routine searches on vectors
>and other linear containers to pull out maximum/minimum values. I ended
>up sitting down and writing a unary function object that retains a value
>based on any predicate. An example is worth a thousand words:
>
>int array[10] = { 5, 4, 6, 3, 9, 0, 7, 2, 8, 1 };
>
>int maximum = std::for_each(&array[0], &array[10],
>	select_by_predicate<int, std::greater>());
>
>int minimum = std::for_each(&array[0], &array[10],
>	select_by_predicate<int, std::less>());
>
>assert(maximum == 9 && minimum == 0);
>
>
>Another example, using select_by_predicate independently:
>
>select_by_predicate<int, std::greater> max_error(0);
>
>for(loop through data samples)
>{
>	double error;
>	... analyze data sample error ...
>
>	max_error.compare(error);	// same as max_error(error)
>}
>
>std::cout << "Maximum error: " << max_error;
>
>
>Is select_by_predicate something that boost users would be interested
>in? It's been helpful to me in all kinds of contexts.
>
>-Jonathan
>
>
>
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