Why summing a matrix in 2 different ways can give not the same numbers ??? Does anyone know?

I had a big problem with a code because of this mistake which looks silly but maybe it's not..

suppose

f1 = .4 ; f2 =.5199 ; f4 = 0.0267 ; f3 = 0.0534 ; Total_mass = 4.799844e+22; frac_mass = [f1,f2,f3,f4]*Total_mass ;

sum(frac_mass) == sum(frac_mass(4:-1:1)) gives me '0' (FALSE) as an answer.. if you check both numbers they are the same... but setting "format long" there's a factor 1 after some zeros when you check the right-hand statement above. Where does this "1" come from? I really thought the results should've been the same.. Need some help on it please..

Answer 1

[Azzi Abdelmalek]

Because the summations are done in different order, look at this link http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

Note that the results are slightly different

[Douglas Alves]

I didn't know that!! Thank you Azzi!!