1-Visitor

Question

Bug? Error in simple sum...

Forum|Forum|14 years ago
September 2, 2011
5 replies
8004 views

Hello,

I just stumbled upon the weird fact that for Prime1 certain sums are not absolutely correct. For example, Prime1 calculates

1.4 + 0.2 = 1.6

So far, so good.

But if I check if (1.4+0.2) is the same as 1.6, Prime1 tells me it is NOT!

When I create the difference it gives out

1.4 + 0.2 - 1.6 = 2.22*10^-16

which is almost correct but just not correct enough!

In the case of

1.2 + 0.4 = 1.6

Prime1 works correctly.

Please see the attached Prime1-sheet for further examples.

Is this a common error with any calculation software or is this just mathcad?

Is there a way to avoid this error?

Best regards

Sebastian

1_Equality-test-sheet-mcdx.zip

Other

V

ValeryOchkov

24-Ruby IV

I think it is "a bug". We have same "bug" in Mathcad 15 when we have not 3-16 digits in rezult 1.4+.2-1.6= but 17 digits (maximum).

BUT why "a bug" not a bug!

The first rule by working with floating point (in Fortran, for example) - do not use the "exact equal" operator and use only the "approximately equal" (see the picture). We can use an "exact equal" operator only by working with strings, boolean, intrgers etc variables.
We must always remember it too by working with "smart" Mathcad .

V

VladimirN

24-Ruby III

Hello!

Here what result turns out in Mathcad 15 M010 (so, there is no problem with that):

And in Mathcad Prime 1.0 (It is possible to see what numerical value is accepted for "0"):

V

ValeryOchkov

24-Ruby IV

Mathcad 15

V

VladimirN

24-Ruby III

Above shows the result in Mathcad 15 with default settings (number of decimals: 3).

M

MikeArmstrong

1-Visitor

Have you formatted the results?

Increased the exponential threshold?

Mike

M

mzeftel

1-Visitor

The difference between Mathcad 15 and earlier versions and Mathcad Prime 1.0, is that Mathcad Prime displays small numbers unlike Mathcad 15.0 which displays small numbers as 0. It's an issue of floating point precision on computers. I am raising this issue with the developers since it came up in another context.

However, if this is evaluated symbolically, it shows the expected result.

Mona

M

mzeftel

1-Visitor

Mathcad Prime 1.0 has double point floating point precision. It's not possible to represent some numbers, due to rounding errors.

If you look at the Formatting tab, you will see that we chose General as the default Result Format, so that small numbers would be displayed, such as Plank's constant: