Very simple 'if' statement with == operator not working

I have some very basic logic that is not working. It's a simple 'if' statement with an '==' operator. Please help.

Program:

%simple_script.m

clear all

A = 0.40;
B = 0.20;
C = 0.60;
D = A + B;
        if C == D
            disp('It worked.')
        else
            disp('It did not work.')
        end

Command Window:

    >> simple_script
    It did not work.
    >>

Answer 1

[Star Strider]

Welcome to the apparent inconsistencies of floating-point calculations. See the discussion in Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? (link) for more informaiton.

Floating point approximations are similar in this regard to decimal 1/3 being approximated as 0.3333... Multiplying by 3 then gives 0.9999..., not 1.

[Graham]

Thanks!

[Star Strider]

My pleasure!

A further illustration:

Test = C - D

Test =
    -1.110223024625157e-16

Answer 2

[Geoff Hayes]

Graham - you are incorrectly trying to use equality with floating point numbers. See the Compare Floating-Point Numbers section of determine equality for an example of what to do.

Also, please see https://www.mathworks.com/matlabcentral/answers/267964-while-comparing-the-equality-of-two-floating-numbers-a-b-we-use-abs-a-b-tolerance-could-anyo and https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero.

[Graham]

Thank you!