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std::rotate substitute
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bitrex
Oct 18, 2018, 7:23:57 PM10/18/18
to
I need an implementation of rotate for a microprocessor platform where I
have C++11 compiler but really nothing else available to work with. I'd
like to be able to rotate the elements of a short array of characters of
arbitrary length e.g. ABCD -> BCDA -> CDAB etc.
Something like the following should be good enough, but what's good to
use for "swap"? "algorithm" is a non-starter here, too.
#include <algorithm>
string rotate(string s) {
for (int i = 1; i < s.size(); i++)
swap(s[i-1], s[i]);
return s;
}
have C++11 compiler but really nothing else available to work with. I'd
like to be able to rotate the elements of a short array of characters of
arbitrary length e.g. ABCD -> BCDA -> CDAB etc.
Something like the following should be good enough, but what's good to
use for "swap"? "algorithm" is a non-starter here, too.
#include <algorithm>
string rotate(string s) {
for (int i = 1; i < s.size(); i++)
swap(s[i-1], s[i]);
return s;
}
bitrex
Oct 18, 2018, 7:27:08 PM10/18/18
to
#define SWAP(a, b) (((a) ^= (b)), ((b) ^= (a)), ((a) ^= (b)))
?
Alf P. Steinbach
Oct 18, 2018, 9:16:06 PM10/18/18
to
On 19.10.2018 01:23, bitrex wrote:
> I need an implementation of rotate for a microprocessor platform where I
> have C++11 compiler but really nothing else available to work with. I'd
> like to be able to rotate the elements of a short array of characters of
> arbitrary length e.g. ABCD -> BCDA -> CDAB etc.
If the microprocessor is 32-bit or better, just use its rotate instruction.
> I need an implementation of rotate for a microprocessor platform where I
> have C++11 compiler but really nothing else available to work with. I'd
> like to be able to rotate the elements of a short array of characters of
> arbitrary length e.g. ABCD -> BCDA -> CDAB etc.
> Something like the following should be good enough, but what's good to
> use for "swap"? "algorithm" is a non-starter here, too.
> #include <algorithm>
>
> string rotate(string s) {
> for (int i = 1; i < s.size(); i++)
> swap(s[i-1], s[i]);
> return s;
> }
Like,
void rotate_buffer( char* buf, int size )
{
// Use memmov for the moving.
}
void rotate( string& s )
{
rotate_buffer( s.data(), s.size() );
}
Cheers & hth.,
- Alf
David Brown
Oct 19, 2018, 2:43:06 AM10/19/18
to
(Alf has already posted the ideas I had.)
Juha Nieminen
Oct 19, 2018, 10:27:30 AM10/19/18
to
bitrex <us...@example.net> wrote:
> Perhaps the old "hack":
>
> #define SWAP(a, b) (((a) ^= (b)), ((b) ^= (a)), ((a) ^= (b)))
Which at least in modern architectures is slower than the "naive"
> Perhaps the old "hack":
>
> #define SWAP(a, b) (((a) ^= (b)), ((b) ^= (a)), ((a) ^= (b)))
swap.
Juha Nieminen
Oct 19, 2018, 10:30:16 AM10/19/18
to
bitrex <us...@example.net> wrote:
> string rotate(string s) {
> for (int i = 1; i < s.size(); i++)
> swap(s[i-1], s[i]);
> return s;
> }
Why swapping? You are performing tons of unnecessary assignments.
> string rotate(string s) {
> for (int i = 1; i < s.size(); i++)
> swap(s[i-1], s[i]);
> return s;
> }
In fact, you are performing twice as many as would be needed.
(For a string of length n, you only need n+1 assignments,
not (n-1)*3 assignments as you are doing there.)
bitrex
Oct 20, 2018, 1:24:59 AM10/20/18
to
since I've had to resort to dusty ol' C...
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