Problem with variable size

Hello, I am trying to do a customize buffer but it says that my output has a variable size and I don't wont it.

Can someone help me to resolve it ? I put my code below ( u is a vector of 8 double and FFTLength is a size of the buffer)

function data = custom_buffer( FFTLength, u)
%#codegen

persistent buffer next_index


% Define initial values
if isempty(buffer)
    buffer = zeros(FFTLength,1);
    next_index = 1;
end

% Populate the buffer
for k = 1 :8
    buffer(next_index) = u(k);
end

% Increment the location to write to at the next time
if next_index < FFTLength
    next_index = next_index + 9;
else
    next_index = 1;
end

    data = buffer;

end

[Walter Roberson]

    buffer(next_index) = u(k);

Why are you copying 8 u(k) values to the same buffer(next_index) location ?

[Walter Roberson]

for k = 1 :8
    buffer(next_index) = u(k);
    next_index = mod(next_index, FFTLength) + 1;
end

[Catteau Ophélie]

OK, thank you I forgot this, I change it but it doesn't solve my problem about the variable size of data.

function data = custom_buffer( FFTLength, u)
%#codegen

persistent buffer next_index


% Define initial values
if isempty(buffer)
    buffer = zeros(FFTLength,1);
    next_index = 1;
end

% Populate the buffer
for k = 1 :8
    buffer(next_index) = u(k);
    next_index = next_index +1;
end

% Increment the location to write to at the next time
if next_index < FFTLength
   next_index = next_index + 1;
else
    next_index = 1;
end

    data = buffer;

end

[Walter Roberson]

No, that way of coding can expand the array.

Suppose that we have reached next_index = FFTLength - 1. Then buffer(next_index)=u(1) puts u(1) into second last location, increment next_index to FFTLength - 0. u(2) is put there, increment next_index to FFTLength + 1. u(3) is put there... Ooops, we expanded the array!

Answer 1

[Walter Roberson]

function data = custom_buffer( FFTLength, u)
%#codegen

persistent buffer next_index


% Define initial values
if isempty(buffer)
    buffer = zeros(FFTLength,1);
    next_index = 1;
end

% Populate the buffer
for k = 1 : 8
    buffer(next_index) = u(k);
    next_index = mod(next_index, FFTLength) + 1;
end

data = buffer;

end

[Catteau Ophélie]

I corrected like to write but there is always the variable size 's issue...

[Walter Roberson]

Is your FFTLength input defined as a Constant block? If there is any chance that the value could change between function calls, then the block is variable length.

The typical work-around for this is to define a maximum length using coder.varsize() or to

buffer = zeros(maximum_FFTLength,1);

and

assert(FFTLength <= maximum_FFTLength)