Reverse channel values from 1D array - c++

Question

I have a 1D array float64 arr[60] being used to store voltage values from a DAQ. The array is populated as such:
10 SAMPLES_PER_CHANNEL, 6 CHANNELS, grouped by channel
transducer_0 transducer_1 ... transducer_6
[0, 1 ..., 9, 10, 11 ..., 19 ... 50, 51 ..., 59]
I would like to store this data into a 1D array of pointers and to pass to a function whose definition is: bool func2(int samples, double* const* arr2)
I have asked a similar question in this post to which the accepted solution given by #Yakk was:
template<size_t stride, class T, size_t N, size_t count = N/stride>
std::array<T*, count> make_2d( T(&raw)[N] ) {
std::array<T*, count> retval;
for (size_t i = 0; i < count; ++i)
retval[i] = raw + i*stride;
return retval;
}
I am looking for help to generate the argument double* const* arr2 for func2() which is the reverse order of the values stored in the channels. E.g.
[channel 0] [channel 1] [channel 5]
{val 9, val 8 ...val 0}, {val 19, val 18 ...val 10}, ...
The input can either be the initial 1D array, or the 1D array of pointers as constructed through the templated make_2d() function. I'm wondering what the most efficient means to do this is as the array is being updated at a rate of 5000Hz. I'm hoping there is a way which I won't have to copy values from the input array, though I am more interested in time efficiency then space efficiency.
Currently I am creating a second array of pointers and copying the reverse channel values in.
// DOF == 6, SAMPLES_PER_CHANNEL = 10
double ch0[SAMPLES_PER_CHANNEL];
double ch1[SAMPLES_PER_CHANNEL];
double ch2[SAMPLES_PER_CHANNEL];
double ch3[SAMPLES_PER_CHANNEL];
double ch4[SAMPLES_PER_CHANNEL];
double ch5[SAMPLES_PER_CHANNEL];
double* data_rev[DOF] = {
ch0, ch1, ch2, ch3, ch4, ch5};
// generate reversed order array
for (int i = DOF - 1; i > 0; --i) {
for (int j = SAMPLES_PER_CHANNEL - 1; j > 0; --j) {
data_rev[DOF - 1 - i][SAMPLES_PER_CHANNEL - 1 - j] = static_cast<double>(data[(i*10)+j]);
}
}

Answer 1

If I understand you correctly, what you want is not possible:
You cannot pass an array of pointers to func2 that makes it appear as if the samples would be in reversed order (I assume, you can't modify func2), but as long as you are not working on a very low power machine, something like this is dirt cheap (both in time and space overhead):
double arr[60];
std::array<double,60> tmp;
int main() {
auto t = make_2d<10>(tmp);
std::reverse(t.begin(), t.end());
while (true) {
readData(arr, 60);
reverse_copy(begin(arr),end(arr), begin(tmp));
func2(60, t.data());
}
}