Why isn't the numerical value assigned by matlab verified using == function.

I have calculated the value for R using my code which has assigned 0.0094 but if I try to verify using == function , it shows logical zero.

I had used format short but evidently isn't doing much.

what should I change here.

[Stephen23]

"I had used format short but evidently isn't doing much."

FORMAT changes how numeric data are displayed, just as the FORMAT documentation explains.

FORMAT does not change what data are stored in memory. What numeric data are stored in memeory and how they are displayed are two different things: mixing them up like you are doing will not make working with numeric data easy.

"what should I change here."

Either use ISMEMBERTOL or compare the absolute difference against some tolerance (of your choice):

abs(R-0.0094)<0.00005

Answer 1

[Steven Lord]

This behavior is a consequence of floating point arithmetic. See this Answers post and the "Avoiding Common Problems with Floating-Point Arithmetic" section of this documentation page for more information.

If you are using the == operator to attempt to locate a floating-point number in an array, instead subtract the number you're trying to find from the numbers in the array and locate those positions where the difference is smaller than some tolerance or use the ismembertol function. For example:

x = 0:0.1:1

x = 1x11

0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000

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It appears that x contains the value 0.3, but it does not contain exactly 0.3.

checkWithExactEquality = x == 0.3

checkWithExactEquality = 1x11 logical array
   0   0   0   0   0   0   0   0   0   0   0

It does contain a value that is extremely close to 0.3, however.

tolerance = 1e-15;
checkWithTolerance = abs(x-0.3) < tolerance

checkWithTolerance = 1x11 logical array
   0   0   0   1   0   0   0   0   0   0   0

whichValueTolerance = x(checkWithTolerance)

whichValueTolerance = 0.3000

How far away from 0.3 is the value we found using a tolerance?

howDifferent = whichValueTolerance - 0.3

howDifferent = 5.5511e-17

To do the same with ismembertol:

checkWithIsmembertol = ismembertol(x, 0.3, tolerance)

checkWithIsmembertol = 1x11 logical array
   0   0   0   1   0   0   0   0   0   0   0

whichValueIsmembertol = x(checkWithIsmembertol)

whichValueIsmembertol = 0.3000

The ismembertol function found the same value that the check with a tolerance did.

Don't confuse how a number is displayed with how it is stored in memory.

[Yogesh]

Thank you Steven

Answer 2

[Divyajyoti Nayak]

Hi @Yogesh, the 'format' command does not change the precision of your variables. It only changes the format in which the output is displayed. The actual value of R is unchanged.

[Yogesh]

Thank you Divya...