I am trying to make equivalence tests on an algorithm written in C++ and in Matlab.
The algorithm contains some kind of a loop in time and runs more than 1000 times. It has arithmetic operations and some math functions.
I feed the initial inputs to both platforms by hand (like a=1.767, b=6.65, …) and when i check the hexadecimal representations of those inputs they are the same. So no problem for inputs. And get the outputs of c++ to matlab by a text file with 16 decimal digits. (i use “setprecision(32)” statement)
But here comes the problem; although after the 614’th step of both code, all the results are exactly the same, on the step of 615 I get a difference about 2.xxx..xxe-19? And after this step the error becomes larger and larger, and at the end of the runs it is about 5.xx..xxe-14.
0x3ff1 3e42 a211 6cca—>[C++ function]—>0x3ff4 7619 7005 5a42
0x3ff1 3e42 a211 6cca—>[MATLAB function]—>ans
ans – 0x3ff4 7619 7005 5a42
= 2.xxx..xxe-19
I searched how matlab behaves the numbers and found really interesting things like “denormalized mantissa”. While realmin is about e-308, by denormalizing the mantissa matlab has the smallest real number about e-324. Further matlab holds many more digits for “pi” or “exp(1)” than that of c++.
On the other hand, matlab help says that whatever the format it displays, matlab uses the double precision internally.
So,I’d really appreciate if someone explains what the exact reason is for these differences? How can we make equivalence tests on matlab and c++?
A similar discussion occurred before, the conclusion was that IEEE 754 tolerates error in the last bit for transcendental functions (cos, sin, exp, etc..). So you can’t expect exactly same results between MATLAB and C (not even same C code compiled in different compilers).