MATLAB: Colon operator numerical error

colonnumerical errror

When I create a vector using the colon operator I noticed for certain vectors there is a small numerical error in the values returned. For example: >> a = 0:.1:1

a =

            0          0.1          0.2          0.3          0.4          0.5          0.6          0.7          0.8          0.9            1

>> a(4)

ans =

0.3

>> a(4) == .3

ans =

>> a(4) – .3

ans =

   5.5511e-17

I find this very strange. Do other people have this same problem?

Best Answer

Yes, everyone has this problem. Please read this FAQ

Related Solutions

MATLAB: Different result when adding variables

You are doing two different operation sequences, so why the surprise that you get two different results?

First Case

>> 1:3 + [0 0 5]
ans =
 1   2   3

because of the operator precedence (plus is higher than colon) is equivalent to

>> 1: (3 + [0 0 5])
ans =
 1   2   3

which is equivalent to

>> 1:[3,3,8]
ans =
 1   2   3

and thus is just

>> 1:3
ans =
 1   2   3

because the colon operator only uses the first element of the vector and ignore the rest: "If you specify nonscalar arrays, MATLAB interprets j:i:k as j(1):i(1):k(1)."

Second Case

 >> x = 1:3;
 >> y = [0 0 5];
 >> x+y
 ans =
   1   2   8

performs an element-wise addition on two vectors.

Summary

Beginners often forget about operator precedence and to read the documentation...

MATLAB: Colon operator : problem

FAQ

Best Answer

Related Solutions

MATLAB: Different result when adding variables

MATLAB: Colon operator : problem

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