This assignment writes a 'int16' value into a 'double' type.

Hi,

I use the code below in a function. My input is int16 data type and I assigned this in the code. But I still get the error: This assignment writes a 'int16' value into a 'double' type.

Best regards

function y = fcn(u)
%Input
IC=1;
bufferlength=8000;

persistent accxsignal;

if isempty(accxsignal)
    if isequal(numel(IC),bufferlength)
        accxsignal = IC;
    elseif isscalar(IC)
        accxsignal = IC*ones(1,bufferlength);
    else
        error('IC must either be scalar or the same dimensions as bufferlength')
    end
end

%Output
y = int16(accxsignal);

%Update
accxsignal = [u accxsignal(1:end-1)];

end

Answer 1

[Andy Bartlett]

My guess is you are using this code in a situation that explicitly or implicitly requires generation to C/C++ code such as

MATLAB Coder codegen

Simulink's MATLAB Function Block

Simulink's MATLAB System Block

Stateflow MATLAB action language

These situations do not allow you to change the data type of a variable from one assignment to the next.

In your code,

accxsignal = IC*ones(1,bufferlength);

will assign doubles to accxsignal.

Because u is uint8, this line

accxsignal = [u accxsignal(1:end-1)];

will try to change accxsignal to uint8. That's not allowed in explicit or implicit code generation situations.

To solve this, your friends are "colon assignment" and "cast like".

For all "assignments" to a variable after the very first one, use "colon assignment" or "index assignment" to write a value into the existing variable rather than completely redefine the variable.

foo(:) = goo; % 2nd assignment and beyond

Use cast-like to set a variable or constants type based on another variable.

var2 = cast( 13, 'like', var1 );

For example:

function y = fcn(u)
%Input

% IC same type as u
IC= cast( 1, 'like', u);  

bufferlength=8000;
persistent accxsignal;
if isempty(accxsignal)

    % First assignment to accxsignal
    % sets the type same as u
    % because IC is same type as u
    %
    if isequal(numel(IC),bufferlength)
        accxsignal = IC;
    elseif isscalar(IC)
        accxsignal = repmat( IC, 1,bufferlength );
    else
        error('IC must either be scalar or the same dimensions as bufferlength')
    end
end
%Output
y = int16(accxsignal);

%Update
% Use Colon-assignment to prevent changing type of accxsignal
%   Given the changes above, 
%   I believe this is not really necessary in this example
%   because the right hand side of the next line will be same type as u
%   so "regular assignment" would not change type in this specific case.
%   BUT, it is a good habit for code gen usage to use "colon-assignment"
%   for all "assignments" after the first one.
%
accxsignal(:) = [u accxsignal(1:end-1)];

end

Again, I'm guessing your were in an explicit or implicit code generation situation. If that's not true, then these suggestion likely do not apply.

[Walter Roberson]

It might also be the case that the output port for y has been configured as double

[Andy Bartlett]

@Walter Roberson Good point

[Matthias Schmidt]

Thank you...this helped a lot