# (Matlab Coder)Domain error. To compute complex results from real x, use 'acosd(complex(x))'

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cui,xingxing on 28 Sep 2023
Commented: cui,xingxing on 28 Sep 2023
When I try to evaluate an acosd value, the generated mex execution is not consistent with the original function and often errors out?
function out = triFun(in)%#codegen
out = real(acosd(in));
end
I use the following statement to generate successfully：
codegen -config:mex triFun.m -args {0.5}
Then test it with the following statement:
for i = 1:10
a = rand(1,2);
b = a;
distA = sqrt(a(1).^2+a(2).^2);
distB = sqrt(b(1).^2+b(2).^2);
val = dot(a,b)./(distA.*distB);
out1 = triFun(val) % always OK
out2 = triFun_mex(val) % sometimes failed, Domain error. To compute complex results from real x, use 'acosd(complex(x))' ???
end
output error:
test_codegen_mex
out1 =
0
Domain error. To compute complex results from real x, use 'acosd(complex(x))'.
Error in acosd (line 13)
coder.internal.error('Coder:toolbox:ElFunDomainError',mfilename);
Error in triFun (line 2)
out = real(acosd(in));
Error in test_codegen_mex (line 12)
out2 = triFun_mex(val) % sometimes failed, Domain error. To compute complex results from real x, use 'acosd(complex(x))' ???
Question:
Obviously, the a,b I gave are equal and the val value should be equal to 1. There will be no complexity, but out2 will mistake the result for a complex number and out1 will not. How can I modify this to achieve the goal of consistent results?
Run In Matlab R2023a

Bruno Luong on 28 Sep 2023
Edited: Bruno Luong on 28 Sep 2023
"Obviously, the a,b I gave are equal and the val value should be equal to 1"
wrong. Before claming anything, you should first check.
format long
while true
a = rand(1,2);
b = a;
distA = sqrt(a(1).^2+a(2).^2);
distB = sqrt(b(1).^2+b(2).^2);
val = dot(a,b)./(distA.*distB);
if abs(val) > 1
a
b
val
exceedvalto1 = val-1
fprintf('val can not be == 1 with numerical error\n')
break
end
end
a = 1×2
0.894442198246962 0.268453115545912
b = 1×2
0.894442198246962 0.268453115545912
val =
1.000000000000000
exceedvalto1 =
2.220446049250313e-16
val can not be == 1 with numerical error
Fix: you need to truncate val by safety
val = max(min(val,1),-1);
before calling acosd
cui,xingxing on 28 Sep 2023
Is this a precision issue with Floating-Point Numbers（Unexpected Results with Floating-Point Arithmetic）?

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