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S__MPay

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11-03-2017
02:01 AM

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I am doing an iterative calculation, but due to a progressively increasing error results start to diverge.

I was not able to find any mistake in the code especially that I am using penalty method which has a very small intrinsic error.

Since the error is small, initially iteration is converging but I noticed that the error duplicates with each iteration:

This is the outcome of the calculations, Error = strain increment 2 - strain increment 2:

strain increment 1 | strain increment 2 | Error |

0.00000200000000010109 | 0.00000200000000010121 | 1.20E-19 |

0.00000360000000018082 | 0.00000360000000018105 | 2.30E-19 |

0.00000488000000024377 | 0.00000488000000024424 | 4.70E-19 |

0.00000590400000029337 | 0.00000590400000029432 | 9.50E-19 |

0.00000672320000033230 | 0.00000672320000033416 | 1.86E-18 |

0.00000737856000036248 | 0.00000737856000036621 | 3.73E-18 |

0.00000790284800038518 | 0.00000790284800039267 | 7.49E-18 |

0.00000832227840040082 | 0.00000832227840041586 | 1.50E-17 |

0.00000865782272040866 | 0.00000865782272043871 | 3.01E-17 |

0.00000892625817640571 | 0.00000892625817646584 | 6.01E-17 |

0.00000914100654118522 | 0.00000914100654130545 | 1.20E-16 |

0.00000931280523297264 | 0.00000931280523321313 | 2.40E-16 |

0.00000945024418633035 | 0.00000945024418681130 | 4.81E-16 |

0.00000956019534887217 | 0.00000956019534983409 | 9.62E-16 |

0.00000964815627861697 | 0.00000964815628054086 | 1.92E-15 |

0.00000971852502183561 | 0.00000971852502568338 | 3.85E-15 |

0.00000977482001525616 | 0.00000977482002295169 | 7.70E-15 |

0.00000981985600768391 | 0.00000981985602307498 | 1.54E-14 |

0.00000985588479700876 | 0.00000985588482779092 | 3.08E-14 |

0.00000988470781923398 | 0.00000988470788079830 | 6.16E-14 |

At iteration 37 the error is 1.61e-8 which is very big and finally results start to diverge.

I wonder if I can get rid of this error by trimming decimals after 12th digit?

Unfortunately I don't know how to trim a decimal to 12 digits? e.g. __9.993369172794__~~37~~e-06 => 9.99336917279400e-6

I cannot simply use dnint(variable * 10d-12) / 10d12 since for the exponential of -6 it will trimmed with 6 digits.

Accepted Solutions

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Roman1

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11-03-2017
10:39 AM

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Try the following:

a = 9.99336917279437d-06 b = -floor(log10(a)) c = int(a*(10**b)*1.d12) / ((10**b)*1.d12)

9 Replies

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Roman1

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11-03-2017
10:39 AM

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Try the following:

a = 9.99336917279437d-06 b = -floor(log10(a)) c = int(a*(10**b)*1.d12) / ((10**b)*1.d12)

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jimdempseyatthecove

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11-03-2017
02:59 PM

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When performing an integration loop with fractional time quanta:

! poor coding quanta = 0.001 t = 0.0 do i=0, nIteratons ... t = t+quanta end do ==================== ! better coding quanta = 0.001d0 ! expressly specify rhs is double when lhs is double do i=0, nIteratons t = quanta * I ... end do

You may have similar issues elsewhere in your code

Jim Dempsey

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S__MPay

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11-04-2017
06:49 PM

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jimdempseyatthecove wrote:

When performing an integration loop with fractional time quanta:

... ==================== ! better coding quanta = 0.001d0 ! expressly specify rhs is double when lhs is double do i=0, nIteratons t = quanta * I ... end do...

Jim Dempsey

Thank you for your kindly replies. it seems rounding the numbers to eliminate extra decimal seems to prevent the error from increasing.

I always try to remember to use d0 to make the constants double precision. I found a couple of places that for inverse of a variable I had used A_inv = 1 / A over A_inv = 1.d0 / A. Fixing it how ever didn't prevent the error.

I am also using the old style real*8 to define double variables to obey the older code. I know it is not obeying the standard way but I could also use

use, intrinsic :: iso_fortran_env, only: REAL64 integer, parameter :: dp = REAL64

the replace real*8 with real(dp), but since we are using Intel compiler it will not make a difference; right?

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The different real definitions styles won't make a difference they are all the same thing at the end. A am perplexed that truncating numbers makes it more "accurate" logic would say it would less accurate. We don't know anything about the nature of your code, is it that the 'next guess' values for the iteration need "normalising" every few iterations to ensure consistency with the physical thing the represent?

andrew_4619

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11-05-2017
01:12 AM

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The next guess value is detected using Newton-Raphson method. The differential of the function however is a matrix (stiffness matrix). The matrix is originally singular and becomes non-singular by multiplying specific members to a very large number (penalty number; e.g. 10^30) to satisfy known boundary condition. Using this method to make matrix non-singular is not 100 percent accurate but the error becomes smaller as the chosen number is bigger. So practically we can make the error as small as we want. It is very difficult to find where is the source of increasing error but is might be those small errors that somehow find a way to add up in each iteration and put me into trouble!

S__MPay

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11-05-2017
02:07 AM

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jimdempseyatthecove

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11-05-2017
08:23 AM

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The error, determined with specific members multiplied by very large number (penalty number; e.g. 10^30), may very well flush out significant bits of the smaller numbers. Then the error determined with the combined numbers can appear smaller, yet the actual result (sans the penalty) will have unknown loss of error significance. Taken to extreme, you could set the penalty at +Infinity, get an error estimate of 0.

For arrays of elements of large variation of magnitudes, where you are summing values, it is most accurate to produce the sum in order from smallest magnitude to largest magnitude. If it is impractical to do this (sort the matrix), you can sum sections of the array. sketch

theSum = 0.0d0 from = huge(from) to = tiny(to) do i=1,ubound(array) if(abs(array(i) < from) from = abs(array(i)) if(abs(array(i) > to) to = abs(array(i)) end do do while(from < to) step = from + from do i=1,ubound(array) if((abs(array(i)) >= from) .and. (abs(array(i)) .lt. step)) theSum = theSum + array(i) end do from = from + step end do

for lesser precision you could use step = from * 4

Jim Dempsey

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jimdempseyatthecove

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11-05-2017
09:11 AM

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The above algorithm can have unnecessary passes through the while loop. The following may be better:

theSum = 0.0d0 from = huge(from) to = tiny(to) do i=1,ubound(array) if(abs(array(i) < from) from = array(i) if(abs(array(i) > to) to = array(i) end do step = from * 2 ! next bit larger do while(from < to) nextMin = to ! ** edit ** was huge(nextMin) do i=1,ubound(array) if((abs(array(i)) >= from) .and. (array(i) .lt. step)) theSum = theSum + array(i) else if(abs(array(i)) < nextMin) nextMin = abs(array(i)) end do from = from + step step = from * 2 ! new minimum step do while(step < nextMin) ! find next bit position of significance step = step * 2 end do end do

Jim Dempsey

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S__MPay

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11-05-2017
04:36 PM

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jimdempseyatthecove wrote:

The error, determined with specific members multiplied by very large number (penalty number; e.g. 10^30), may very well flush out significant bits of the smaller numbers. Then the error determined with the combined numbers can appear smaller, yet the actual result (sans the penalty) will have unknown loss of error significance. Taken to extreme, you could set the penalty at +Infinity, get an error estimate of 0.

For arrays of elements of large variation of magnitudes, where you are summing values, it is most accurate to produce the sum in order from smallest magnitude to largest magnitude. If it is impractical to do this (sort the matrix), you can sum sections of the array.

...

Jim Dempsey

Dear Jim, thanks for your important note.

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jimdempseyatthecove

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11-05-2017
05:33 PM

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See edit in number 8 for correction

Jim Dempsey

For more complete information about compiler optimizations, see our Optimization Notice.