Replacing For loop with memcopy, memmove, or std:copy? - c++

Question

I've got shift function where i an continuously sending it new data points and it will shift my points by an offset of 1. This is to achieve a "graphical shifting" where the points represent points on a graph.
The shifting function is the following:
void Chart_Buffer::ShiftData()
{
for(int index = 0; index < (_channel_Samples - 1); ++index)
{
_sample_Points[index].y = _sample_Points[index + 1].y;
}
return;
}
The problem with this is that it is running through a huge array of up to 800 data points and it does this every time for every new data point added, so i wanted to see if i can optimize this process by shifting all values out by an offset of 1 without running through a for loop. I looked at implementations of memcopy, memmove, and std::copy, but i cant figure out how to use them for my purpose.
Basically, if i have elements 0-799 in the array, i want to shift elements 1-799 by 1 so that i have 0-798 and then just add the new element to the array.
Edit: _sample_Points is type tagPOINT with the following structure:
typedef struct tagPOINT
{
LONG x;
LONG y;
} POINT, *PPOINT, NEAR *NPPOINT, FAR *LPPOINT;

Answer 1

It's hard to give a firm answer to this without knowing what you are doing with _sample_Points. But I believe that I can firmly say that copying every element in the array down one is an expensive approach.
In the best case: You just need to access the front of the array and add to the back of the array. If that's the case you're describing a queue.
To add a new element to the back of a queue use: push
To inspect the front element use: front
To "copy everything down one" (just delete the front element) use: pop.
Otherwise you'd be in the case where: You need random access to the array. If that's the case you can still get potentially better performance from a deqeu.
To add a new element to the back of a deque use: push_back
To inspect the front element use: front
To "copy everything down one" (just delete the front element) use: pop_front
So if you use a queue for your _sample_Points Chart_Buffer::ShiftData could be replaced by _sample_Points.pop().
If you use a deque for your _sample_Points Chart_Buffer::ShiftData could be replaced by _sample_Points.pop_front().

Answer 2

It looks like that you are looking for a std::deque. It is a double ended queue, which means you can pop an element from the back and push on the front.
If what you are looking for is to keep the elements of your array in a certain order, this will help you do just that.
Now if you also want to have them contiguously on memory, then you could do it like this:
memmove(array+1, array, sizeof(element)*(array_size-1));
array[0] = new_element;

Answer 3

You cannot do this without less operations than you are already doing, whether you spell all of them or you call an algorithm. The problem is that the operation is not what you described initially, it is not shifting the data, but shifting part of the data (only the y coordinate) but leaving the other half as it is.
If you don't want to spell out the operation, you can play with the transform algorithm in a way similar to the answer by id256, but I am not sure whether that is an improvement really, the loop in the question is easier and cleaner than the transform...

Answer 4

If it is an acceptable amount of refactoring of your code, you could also let go of tagPOINT and instead of having one _sample_Points, have two arrays, one for the x and one for the y. Then you can memmove() the array of ys. Like:
LONG _sample_Points_x[DIMENSION];
LONG _sample_Points_y[DIMENSION];
void Chart_Buffer::ShiftData() {
memmove(_sample_Points_y, _sample_Points_y + 1, (DIMENSION-1) * sizeof _sample_Points_y[0]);
}