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Asked: 2026-06-07T18:40:04+00:00

Could someone tell me if c++ and matlab use the same floating point computation

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Could someone tell me if c++ and matlab use the same floating point computation implementations? Will I get the same values in C++ as I would in Matlab?

Currently I have these discrepancies from translating my Matlab code into C++:

Matlab: R = 1.0000000001623, I = -3.07178893432791e-010, C = -3.79693498864242e-011

C++:    R = 1.00000000340128 I = -3.96890964537988e-009  Z = 2.66864907949582e-009

If not what is the difference and where can I find more about floating point computation implementations?

Thanks!

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  1. Editorial Team

    Although it’s not clear what your numbers actually are, the relative difference of the first (and largest) numbers is about 1e-8, which is the relative tolerance of many double precision algorithms.

    Floating point numbers are only an approximation of the real number system, and their finite size (64 bits for double precision) limits their precision. Because of this finite precision, operations that involve floating point numbers can incur round-off error, and are thus not strictly associative. What this means is that A+(B+C) != (A+B)+C. The difference between the two is usually small, depending on their relative sizes, but it’s not always zero.

    What this means is that you should expect small differences in the relative and absolute values when you compare an algorithm coded in Matlab to one in C++. The difference may be in the libraries (i.e., there’s no guarantee that Matlab uses the system math library for routines like sqrt), or it may just be that your C++ and Matlab implementations order their operations differently.

    The section on floating point comparison tests in Boost::Test discusses this a bit, and has some good references. In particular, you should probably read What Every Computer Scientist Should Know About Floating-Point Arithmetic and consider picking up a copy of Knuth’s TAOCP Vol. II.

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