Integers can only be raised to positive integral powers in db2mag computation

I am converting matlab code to c using fixed point coder app

I am getting the below error

Input to function db2mag_conv

decibelTomag.u1 = 10

decibelTomag.u2 = [-90:0.1:90]

function [y] = db2mag_conv(decibelTomag)
y = power(decibelTomag.u1,double(decibelTomag.u2/20))
end

Error signature

### Begin Fixed Point Simulation using Scaled Doubles : db2mag_conv

Error using >intpower', 'C:/Program Files/MATLAB/R2020b/toolbox/eml/lib/matlab/ops/power.m', 187)">power>>intpower (line 187)

Integers can only be raised to positive integral powers

I need help in resolving the error

Answer 1

[Chidvi Modala]

The reason for the error is using negative powers as input to test the generated code. In general, power function can be used in MATLAB for negative powers by typecasting the input to "double". But power function for fixed point data ‘fi’ type has the following restriction.

Exponent must be a non-negative, real, and integer-valued scalar.

https://www.mathworks.com/help/fixedpoint/ref/embedded.fi.power.html

power(fi(1.5),fi(2.5)) 

throws error in MATLAB. Because of this restriction, if you generate code for

function [C] = power_fxpt(A,B)
C = power(double(A),double(B));

the arguments are always converted to integers in the generated code as shown below.

C = fi(power(fi_toint(double(A)),fi_toint(double(B))), 0, 16, 5, fm);

I have brought this issue to the notice of the concerned people, and it might be considered in the future releases.

As a workaround, you can keep the original types as double or single and you can use function replacement to achieve this.

Example: Write original code in power_fxpt.m as

function [C] = power_fxpt(A,B)
C = localpower(double(A),double(B));

function C = localpower(A, B)
C = power(A, B);

Write a replacement function mypower.m as

function C=mypower(A,B)
C = power(double(A),double(B));

Convert to fixed-point using:

cfg = coder.config('fixpt');
cfg.TestBenchName = 'mp_tb';
cfg.addFunctionReplacement('localpower','mypower');
codegen -config cfg power_fxpt

Another option, if the input ranges are small is to create lookup table manually for the limited set of input values.

[Life is Wonderful]

Thanks a lot for detailed write up and steps !!

Answer 2

[Nitin Kapgate]

You will need to cast both "decibelTomag.u1" and "decibelTomag.u2" as single type (Fixed Point Coder App doesn't support doubles) as follows:

y = power(single(decibelTomag.u1),single(decibelTomag.u2/20))

This should help in getting your problem resolved.

[Life is Wonderful]

Niitin,

Can you please share your settings in case you tested the working solution at your end

I see following errors - I don't see making single is helping here

y = power(single(db2mag.u1),single(db2mag.u2/20));
y = fi(power(fi_toint(single(db2mag.u1)),fi_toint(single(db2mag.u2*fi(1/20, 0, 32, 36, fm)))), 0, 32, 21, fm);

• single is not supported for fixed-point conversion
• ### Begin Fixed Point Simulation using Scaled Doubles : drc_db2mag_tb

Error using power>>intpower (line 187)

Integers can only be raised to positive integral powers.

Thanks

[Life is Wonderful]

Any feedback ?

[Steven Lord]

Raising an integer value to a non-integer power does not necessarily result in an integer value.

y = 2^(1/2)

y = 1.4142

Nor does raising that integer to a non-positive integer power always result in an integer value.

w = 2^(-1)

w = 0.5000

Here's what I receive when I replace the double precision integer value 2 with a 2 stored in an integer type.

try, x = uint32(2)^(1/2), catch ME, disp("Error was:" + newline + ME.message), end

Error was:
Integers can only be raised to positive integral powers.

try, z = uint32(2)^(-1), catch ME, disp("Error was:" + newline + ME.message), end

Error was:
Integers can only be raised to positive integral powers.

You could try:

%{
function [y] = db2mag_conv(decibelTomag)
y = power(double(decibelTomag.u1),double(decibelTomag.u2/20))
end
%}

but I'm not sure if the two casts to double will cause problems in code generation.

[Life is Wonderful]

Thanks steven, But stil the issue did not go away, can you please suggest another approach ?