How to minupulate the elements (counting by 3) of an array usingSTL?

[Tony Young]

Hi,

I have an array of integer. I need to find the max of the 0th, 3th, 6th
, 9th, ... elements in the array. I wonder if STL (standard template
library) provides a way to manipulate only part of an array (rather than
whole), e.g. 0th, 3th, 6th , 9th, ... in this case. Please advise. Thanks.

Tony

[Victor Bazarov]

Tony Young wrote:

I have an array of integer. I need to find the max of the 0th, 3th,
6th , 9th, ... elements in the array. I wonder if STL (standard
template library) provides a way to manipulate only part of an array
(rather than whole), e.g. 0th, 3th, 6th , 9th, ... in this case.

No, it doesn't. Roll your own.

V

[Marcus Kwok]

Victor Bazarov <v.********@comacast.net> wrote:

Tony Young wrote:

I have an array of integer. I need to find the max of the 0th, 3th,
6th , 9th, ... elements in the array. I wonder if STL (standard
template library) provides a way to manipulate only part of an array
(rather than whole), e.g. 0th, 3th, 6th , 9th, ... in this case.

No, it doesn't. Roll your own.

What about std::valarray and std::slice? In fact, there may be a better
way, but I do not use valarrays very often (does anybody?).
#include <iostream>
#include <valarray>

int main()
{
const int N = 300;
int arr[N];

for (int i = 0; i < N; ++i) {
arr[i] = i;
}

std::valarray<int> varr(arr, N);
std::valarray<int> arr3 = varr[std::slice(0, varr.size() / 3, 3)];

std::cout << "max = " << arr3.max() << '\n';
}

--
Marcus Kwok

[Neil Cerutti]

On 2005-12-19, Marcus Kwok <ri******@gehennom.net> wrote:

Victor Bazarov <v.********@comacast.net> wrote:

Tony Young wrote:

I have an array of integer. I need to find the max of the
0th, 3th, 6th , 9th, ... elements in the array. I wonder if
STL (standard template library) provides a way to manipulate
only part of an array (rather than whole), e.g. 0th, 3th, 6th
, 9th, ... in this case.

No, it doesn't. Roll your own.

What about std::valarray and std::slice? In fact, there may be
a better way, but I do not use valarrays very often (does
anybody?).

It's one of the things included in the standard library that's
hard to understand. B.Stroustrup puts it something like this in
_TCPL_, but it was unenlightening to me: A vector optimized for
computations. Huh?

My decision was that if I needed it I would know what he meant,
so I ignored valarrays.

--
Neil Cerutti

[Victor Bazarov]

Marcus Kwok wrote:

Victor Bazarov <v.********@comacast.net> wrote:

Tony Young wrote:

I have an array of integer. I need to find the max of the 0th, 3th,
6th , 9th, ... elements in the array. I wonder if STL (standard
template library) provides a way to manipulate only part of an array
(rather than whole), e.g. 0th, 3th, 6th , 9th, ... in this case.

No, it doesn't. Roll your own.

What about std::valarray and std::slice? In fact, there may be a better
way, but I do not use valarrays very often (does anybody?).
[...]

Neither do I, that's why it didn't occur to me to suggest it.

V

[Marcus Kwok]

Neil Cerutti <le*******@email.com> wrote:

On 2005-12-19, Marcus Kwok <ri******@gehennom.net> wrote:

What about std::valarray and std::slice? In fact, there may be
a better way, but I do not use valarrays very often (does
anybody?).

It's one of the things included in the standard library that's
hard to understand. B.Stroustrup puts it something like this in
_TCPL_, but it was unenlightening to me: A vector optimized for
computations. Huh?

My decision was that if I needed it I would know what he meant,
so I ignored valarrays.

Yeah, I found this (TC++PL:SE section 22.4):

In particular, the primary design criterion [of valarray] wasn't ease
of use, but rather effective use of high-performance computers when
relying on aggressive optimization techniques. If your aim is
flexibility and generality rather than efficiency, you are probably
better off building on the standard containers ... than trying to fit
into the simple, efficient, and deliberately traditional framework of
valarray.

...

A valarray is a vector optimized for numeric computation, a vector is
a flexible container designed for holding and manipulating objects of
a wide variety of types, and an array is a low-level, built-in type.

I do agree that they seem pretty esoteric though.

--
Marcus Kwok

[Neil Cerutti]

On 2005-12-19, Victor Bazarov <v.********@comAcast.net> wrote:

Marcus Kwok wrote:

Victor Bazarov <v.********@comacast.net> wrote:

Tony Young wrote:

I have an array of integer. I need to find the max of the 0th, 3th,
6th , 9th, ... elements in the array. I wonder if STL (standard
template library) provides a way to manipulate only part of an array
(rather than whole), e.g. 0th, 3th, 6th , 9th, ... in this case.

No, it doesn't. Roll your own.

What about std::valarray and std::slice? In fact, there may
be a better way, but I do not use valarrays very often (does
anybody?).
[...]

Neither do I, that's why it didn't occur to me to suggest it.

Looking at the SGI library docs, slice_array does not provide
iterators, so valarray doesn't provide what he needs. He'll need
to roll his own, unless he doesn't mind making copies of the
items over which he iterates.

--
Neil Cerutti

[pookiebearbottom]

Just curious, if 0th, 3rd, 6th, ... are significant, are you sure that
array[int] is the correct data structure?
Tony Young wrote:

Hi,

I have an array of integer. I need to find the max of the 0th, 3th, 6th
, 9th, ... elements in the array. I wonder if STL (standard template
library) provides a way to manipulate only part of an array (rather than
whole), e.g. 0th, 3th, 6th , 9th, ... in this case. Please advise. Thanks.

Tony

[Marcus Kwok]

Neil Cerutti <le*******@email.com> wrote:

Looking at the SGI library docs, slice_array does not provide
iterators, so valarray doesn't provide what he needs. He'll need
to roll his own, unless he doesn't mind making copies of the
items over which he iterates.

Also, Stroustrup provides an implementation for a Slice_iter class on
p.670 of _TC++PL:SE_ :
template <class T>
class Slice_iter {
valarray<T>* v;
slice s;
size_t curr; // index of current element

T& ref(size_t i) const { return (*v)[s.start() + i*s.stride()]; }
public:
Slice_iter(valarray<T>* vv, slice ss) : v(vv), s(ss), curr(0) { }

Slice_iter end() const
{
Slice_iter t = *this;
t.curr = s.size(); // index of last-plus-one element
return t;
}

Slice_iter& operator++() { curr++; return *this; }
Slice_iter operator++(int) { Slice_iter t = *this; curr++; return t; }

T& operator[](size_t i) { return ref(i); } // C style subscript
T& operator()(size_t i) { return ref(i); } // Fortran-style subscript
T& operator*() { return ref(curr); } // current element

friend bool operator==<>(const Slice_iter& p, const Slice_iter& q);
friend bool operator!=<>(const Slice_iter& p, const Slice_iter& q);
friend bool operator< <>(const Slice_iter& p, const Slice_iter& q);
};
template <class T>
bool operator==(const Slice_iter<T>& p, const Slice_iter<T>& q)
{
return p.curr == q.curr &&
p.s.stride() == q.s.stride() &&
p.s.start() == q.s.start();
}

template <class T>
bool operator!=(const Slice_iter<T>& p, const Slice_iter<T>& q)
{
return !(p == q);
}

template <class T>
bool operator<(const Slice_iter<T>& p, const Slice_iter<T>& q)
{
return p.curr < q.curr &&
p.s.stride() == q.s.stride() &&
p.s.start() == q.s.start();
}

--
Marcus Kwok

[Robbie Hatley]

"Tony Young" <jd*******@yahoo.com> wrote:

I have an array of integer. I need to find the max of the 0th, 3th, 6th
, 9th, ... elements in the array. I wonder if STL (standard template
library) provides a way to manipulate only part of an array (rather than
whole), e.g. 0th, 3th, 6th , 9th, ... in this case. Please advise.

Thanks.

This is one of those rare cases where I'd advice you
to use built-in arrays and for loops intstead of the
STL. If you want to process just every third element,
simply increment through the array by threes instead
of by ones.

Something like this sould do the trick (untested;
debug it yourself):

int DivThreeMax(int Array[], int Size)
{
int MaxBuffer = Array[0];
for (int i = 0; i < Size; ++++++i) // note triple increment!
{
if (Array[i] > MaxBuffer) MaxBuffer = Array[i]
}
return MaxBuffer;
}

If you really need to apply STL functionality to this
kind of problem, use the for-loop technique above to
syphon every third element into an STL container.
Then you can use std. containers, iterators, algorithms,
etc. on the problem.

--
Cheers,
Robbie Hatley
Tustin, CA, USA
lone wolf intj at pac bell dot net
home dot pac bell dot net slant earnur slant

[Victor Bazarov]

Robbie Hatley wrote:

"Tony Young" <jd*******@yahoo.com> wrote:

I have an array of integer. I need to find the max of the 0th, 3th, 6th
, 9th, ... elements in the array. I wonder if STL (standard template
library) provides a way to manipulate only part of an array (rather than
whole), e.g. 0th, 3th, 6th , 9th, ... in this case. Please advise.
Thanks.

This is one of those rare cases where I'd advice you
to use built-in arrays and for loops intstead of the
STL. If you want to process just every third element,
simply increment through the array by threes instead
of by ones.

Something like this sould do the trick (untested;
debug it yourself):

int DivThreeMax(int Array[], int Size)

I'd probably generalize it to be either

template<int howmany> int DivXXXMax(int Array[], int Size) // var 1

or

int DivXXXMax(int Array[], int Size, int howmany) // var 2

(and see below), or even further

template<class T... (T Array[], ...
{
int MaxBuffer = Array[0];
for (int i = 0; i < Size; ++++++i) // note triple increment!
Instead write

for (int i = howmany; i < Size; i += howmany) // var 1 or 2
{
if (Array[i] > MaxBuffer) MaxBuffer = Array[i] ^^^
A semicolon might be required there...
}
return MaxBuffer;
}

[..]

V