Testing floating point numbers for equality

[ulfaazmi]

Anybody can help me?? The problem is when the value of (r1o = r, and r2o=r), the result will be infinite,,,what the the probably test for this case?? I can't test floating point numbers for equality ( if r == 0.5 and r == -0.5). But, when running this program without test floating point, it is success. however, the result is not true because it should be infine value or missing.
This is my code :

Program EfieldEpotential
 implicit none
 integer,parameter :: ikind = selected_real_kind(p=15)
 real (kind=ikind)::E1,E2,E,V,V1,V2
 real :: k,r1o,r2o,q1,q2,r,r1,r2
 integer :: i,n=100

! initial position
 r1o = -0.5
 r2o = 0.5
 q1 = -1  ! the charge at r1o
 q2 = 1   ! the charge at r2o
 k = 9E+9

 do i = 1,n
 r  = -2.1 + real(i)*0.1
 r1 = abs(r1o-r)
 r2 = abs(r2o-r)
 E1 = k*q1/r1**2
 E2 = k*q2/r2**2
 E  = E1+E2
 V1 = k*q1/r1
 V2 = k*q2/r2
 V  = V1+V2

if ( r>2.1) then
     stop 'Program End'
     else
        write(*,30) r,E,V
    30 format(7x,f8.2,8x,E15.7,8x,E15.7)
    
 end if
 end do
 End

<Moderator's note: code tags added>

Thank you.

 

[DrClaude]

You have to test for r1 or r2 being smaller than a very small number, to see if it is near 0.

Some comments on your program. First, please use code tags when posting code at PF.

Second, your loop is quite strange. Either calculate the step size (instead of using a fixed value of 0.1) so that n = 100 points take you to r = 2.1, or calculate the value of n needed to get just the right number of points. Alternatively, you can use a do without a counter

do
   ! some code here
   if (r > 2.1) exit
   ! some code here
end do

 

[Nik_2213]

"You have to test for r1 or r2 being smaller than a very small number, to see if it is near 0."

Or their absolute difference being less than 'baseline noise'...

Met this using HP Technical BASIC (Run under Unix on a plasma-screen 'luggable' with integral printer !), which, IIRC, had a totally-huge dynamic range for its FP variables, and a real-neat 'close enough' function to prevent near misses and infinite iterations ...

 

[ulfaazmi]

You have to test for r1 or r2 being smaller than a very small number, to see if it is near 0.

Some comments on your program. First, please use code tags when posting code at PF.

Second, your loop is quite strange. Either calculate the step size (instead of using a fixed value of 0.1) so that n = 100 points take you to r = 2.1, or calculate the value of n needed to get just the right number of points. Alternatively, you can use a do without a counter

do
   ! some code here
   if (r > 2.1) exit
   ! some code here
end do

You have to test for r1 or r2 being smaller than a very small number, to see if it is near 0.

Some comments on your program. First, please use code tags when posting code at PF.

Second, your loop is quite strange. Either calculate the step size (instead of using a fixed value of 0.1) so that n = 100 points take you to r = 2.1, or calculate the value of n needed to get just the right number of points. Alternatively, you can use a do without a counter

do
   ! some code here
   if (r > 2.1) exit
   ! some code here
end do

thank you for your advices and help. I will do it.

 

[Baluncore]

It will depend on the application. I define something like; TAD = 1e-12.
Then I can use; If Abs( f1 – f2 ) < TAD Then equal.
Or ratiometric; If Abs( f1 / f2 – 1 ) < TAD Then equal.

 

[newjerseyrunner]

Try this function

template<int DECIMALS> inline bool equals(float f1, float f2){
    return (fabs(f1 - f2) < 1.0f / (10.0f * static_cast<float>(DECIMALS)));
}

...

if (equals<3>(f1, f2)) dostuff();

 

[ulfaazmi]

It will depend on the application. I define something like; TAD = 1e-12.
Then I can use; If Abs( f1 – f2 ) < TAD Then equal.
Or ratiometric; If Abs( f1 / f2 – 1 ) < TAD Then equal.

I have tried it and success. thank you very much.

 

[scottdave]

It will depend on the application. I define something like; TAD = 1e-12.
Then I can use; If Abs( f1 – f2 ) < TAD Then equal.
Or ratiometric; If Abs( f1 / f2 – 1 ) < TAD Then equal.

I have used something along the same lines.