MATLAB: Strange approximation during vector creation!

Question

Hello,

I am using Matlab for a few years now and I surprisely discovered a strange Matlab Behavior:

0:0.2:10 provides an array of numbers sometime approximated which is rather bad for testing…

Here is the command result:

>> 0:0.2:10

ans = Columns 1 through 4

                   0   0.200000000000000   0.400000000000000   0.600000000000000
Columns 5 through 8
   0.800000000000000   1.000000000000000   1.200000000000000   1.400000000000000
Columns 9 through 12
   1.600000000000000   1.800000000000000   2.000000000000000   2.200000000000000
Columns 13 through 16
   2.400000000000000   2.600000000000000   2.800000000000000   3.000000000000000
Columns 17 through 20
   3.200000000000000   3.400000000000000   3.600000000000000   3.800000000000000
Columns 21 through 24
   4.000000000000000   4.200000000000000   4.400000000000000   4.600000000000001
Columns 25 through 28
   4.800000000000001   5.000000000000000   5.199999999999999   5.399999999999999
Columns 29 through 32
   5.600000000000000   5.800000000000000   6.000000000000000   6.199999999999999
Columns 33 through 36
   6.400000000000000   6.600000000000000   6.800000000000000   7.000000000000000
Columns 37 through 40
   7.199999999999999   7.400000000000000   7.600000000000000   7.800000000000000
Columns 41 through 44
   8.000000000000000   8.199999999999999   8.400000000000000   8.600000000000000
Columns 45 through 48
   8.800000000000001   9.000000000000000   9.199999999999999   9.400000000000000
Columns 49 through 51
   9.600000000000000   9.800000000000001  10.000000000000000

As you can see 4.6, 5.2, 5.4, 6.2, 7.2, 8.2, 8.8 and 9.1 are approximated!

Is it normal? Does a workaround exist???

Thanks for your time,

regards,

Robin

Answer 1

That’s normal. It has to do with floating-point calculations and approximations. See: Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? (link) for details.

The only work-around I can imagine is to round them:

V1 = 0:0.2:10;
V2 = round(V1,1);
Test = V1-V2;

The more recent versions of round allow rounding to a specific number of decimal places. If you don’t have it, this anonymous funciton will do the same thing (with the same accuracy):

roundn = @(x,n) round(x .* 10.^n)./10.^n;   % Round ‘x’ To ‘n’ Digits, Emulates Latest ‘round’ Function