Efficient way to reorganize data in a buffer in C++

Question

Let's say I have a unsigned char buffer that looks like this:

unsigned char buffer = {'A', 'B', 'C', 'A', 'B', 'C', 'A', 'B', 'C', 'A', 'B', 'C'}

Basically I just want to grab that A's and B's, and pack them into a new buffer like this:

unsigned char buffer2 = {'A', 'B', 'A', 'B', 'A', 'B'  ... etc

is there an efficient way to do this besides looping through every single element? Is there some trick you can do with memcpy or memset?

Thanks!

Can you think of a way you can examine each element to see if it is 'A' or 'B' *without* looping at least *once* ?? [copy_if](http://en.cppreference.com/w/cpp/algorithm/copy) will do it if you configure the comparator correctly, but it is still, none-the-less, a loop. (and *neither* of those are valid C++; they're both initializer lists sent to a single scaler var of type `unsigned char`). — WhozCraig, Apr 27 '13 at 00:35
You want some trick with memcpy that skips every 3rd byte? heh. You'll have to write that yourself. — David, Apr 27 '13 at 00:38
Yes I meant buffer[] and buffer2[], and I figured it was a hard problem, but I just wanted to make sure incase I was missing something. — Bobby Pardridge, May 03 '13 at 16:29

score 0 · Answer 1 · answered Apr 27 '13 at 01:06

0

I've never used memcpy, but I would do it like this:

#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

class Modulo3
{
public:
    Modulo3();
    bool operator()(const char c);
private:
    int num_;
};

Modulo3::Modulo3()
{
    num_ = 0;
}

bool Modulo3::operator()(const char c)
{
    return num_++ % 3 == 0;
}

int main()
{
    string test = "ABCABCABC";

    Modulo3 func;
    remove_if(test.begin(),test.end(),func);

    return 0;
}

The code is the same for vector, list, etc.

answered Apr 27 '13 at 01:06

lucas92

464
2
11

Use `Modulo3() : num_1(0) {}` instead. – David G Apr 27 '13 at 01:07
I think he wants to make a new array, not remove from the first one – David Apr 27 '13 at 01:09
You could copy the string into an another one, then use the code. – lucas92 Apr 27 '13 at 01:18

Adnan Akbar · Answer 2 · 2013-04-27T11:53:21.750

0

If your data is sorted the way in example you could just loop with incrementing 3 and copying first two of the bytes. Otherwise there is no way you will have to loop on each and every value.

edited Apr 27 '13 at 11:53

answered Apr 27 '13 at 06:16

Adnan Akbar

718
6
16

Efficient way to reorganize data in a buffer in C++

2 Answers2