Decimation in C++ - c++

Question

Dear Stack Community,
I'm doing a DSP exercise to complement my C++ FIR lowpass filter with filter coefficients designed in and exported from Matlab. The DSP exercise in question is the act of decimating the output array of the FIR lowpass filter to a lower sample rate by a factor of 'M'. In C++ I made a successful but extremely simple implementation within a .cpp file and I've been trying hard to convert it to a function to which I can give the output array of the FIR filter. Here is the very basic version of the code:
int n = 0;
int length = 50;
int M = 12;
float array[length];
float array2[n];
for (int i = 0 ; i<length; i++) {
array[i] = std::rand();
}
for (int i = 0; i<length; i=i+M) {
array2[n++] = array[i];
}
for (int i = 0; i<n; i++) {
std::cout << i << " " << array2[i] << std::endl;
}
As you can see very simple. My attempt to convert this to a function using is unfortunately not working. Here is the function as is:
std::vector<float> decimated_array(int M,std::vector<float> arr){
size_t n_idx = 0;
std::vector<float> decimated(n_idx);
for (int i = 0; i<(int)arr.size(); i = i + M) {
decimated[n_idx++] = arr[i];
}
return decimated;
}
This produces a very common Xcode error of EXC_BAD_ACCESS when using this section of code in the .cpp file. The error occurs in the line 'decimated[n_idx++] = arr[i];' specifically:
int length = 50;
int M = 3;
std::vector<float> fct_array(length);
for (int i = 0 ; i<length; i++) {
fct_array[i] = std::rand();
}
FIR_LPF test;
std::vector<float> output;
output = test.decimated_array(M,fct_array);
I'm trying to understand what is incorrect with my application of or perhaps just my translation of the algorithm into a more general setting. Any help with this matter would be greatly appreciated and hopefully this is clear enough for the community to understand.
Regards, Vhaanzeit

Answer 1

The issue:
size_t n_idx = 0;
std::vector<float> decimated(n_idx);
You did not size the vector before you used it, thus you were invoking undefined behavior when assigning to element 0, 1, etc. of the decimated vector.
What you could have done is in the loop, call push_back:
std::vector<float> decimated_array(int M,std::vector<float> arr)
{
std::vector<float> decimated;
for (size_t i = 0; i < arr.size(); i = i + M) {
decimated.push_back(arr[i]);
}
return decimated;
}
The decimated vector starts out empty, but a new item is added with the push_back call.
Also, you should pass the arr vector by const reference, not by value.
std::vector<float> decimated_array(int M, const std::vector<float>& arr);
Passing by (const) reference does not invoke a copy.
Edit: Changed loop counter to correct type, thus not needing the cast.