Re: lost precision with string2float?

From: <leiting2002_at_nyahnyahspammersnyahnyah>
Date: Wed Jul 31 2013 - 06:10:09 MDT

Hi, Arne,
It is still possible it is the machine dependent issue as you suggested though two machines gave the same results.
I will ask this question to the help staff and keep you updated if I get any information.
Best,
Ting

From: leiting2002@hotmail.com
To: arne.melsom@met.no
CC: ncl-talk@ucar.edu
Subject: RE: [ncl-talk] lost precision with string2float?
Date: Wed, 31 Jul 2013 20:04:02 +0800

Hi, Arne,
Thank you so much for your suggestions.
I had suspected on the impact of the difference between stringtofloat and stringtodouble and found no differences. Just now, I just tried it again. seems the same. In the previous example, I changed it into :
^^
fxx = stringtofloat(str_get_field(str_data(2:11),10 ," "))
dxx = stringtodouble(str_get_field(str_data(2:11),10 ," "))
print("output fxx "+ fxx)
print("output dxx "+ dxx)
VV
It gave the same results. In case it is machine dependent, I tried it on another machine (Kraken).
The test ncl file is
[tlei:/lustre/scratch/tlei/hybrid-GSI/tlei-postprocessing/dr-obfits]$ cat deb.ncl
^^
load "/sw/xt-cle3.1/ncl/5.0.0/sles11.1_pgi11.6.0/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "/sw/xt-cle3.1/ncl/5.0.0/sles11.1_pgi11.6.0/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "/sw/xt-cle3.1/ncl/5.0.0/sles11.1_pgi11.6.0/lib/ncarg/nclscripts/csm/contributed.ncl"
load "/sw/xt-cle3.1/ncl/5.0.0/sles11.1_pgi11.6.0/lib/ncarg/nclscripts/csm/shea_util.ncl"
begin
xx="2.712245356"
fxx=stringtofloat(xx)
dxx=stringtofloat(xx)
print("fxx= "+fxx)
print("dxx= "+dxx)
end

VV
the result is :
^^
[tlei:/lustre/scratch/tlei/hybrid-GSI/tlei-postprocessing/dr-obfits]$ ncl deb.ncl
 Copyright (C) 1995-2011 - All Rights Reserved
 University Corporation for Atmospheric Research
 NCAR Command Language Version 6.0.0
 The use of this software is governed by a License Agreement.
 See http://www.ncl.ucar.edu/ for more details.
(0) fxx= 2.71225
(0) dxx= 2.71225

VV
So, it is still confusing.
Thanks again.
Further suggestions/comments are still appreciated deeply.
Ting



> Date: Wed, 31 Jul 2013 08:26:00 +0000
> From: arne.melsom@met.no
> To: leiting2002@hotmail.com
> CC: ncl-talk@ucar.edu
> Subject: Re: [ncl-talk] lost precision with string2float?
>
>
> Hi Ting!
>
> You lose precision since you are storing a 12-digit number represented as type 'float' (4-byte real), I think. If you wish to keep the precision, you're really asking ncl to do the impossible, since a 4-byte real has 6-9 significant digits (see http://en.wikipedia.org/wiki/Single-precision_floating-point_format for an explanation).
>
> I advise you to use stringtodouble if you need to retain full precision in your example:
> ncl 0> str_value= "2.26410202121"
> ncl 1> r4_value = stringtofloat(str_value)
> ncl 2> print((/r4_value/))
> (0) 2.264102
> ncl 3> r8_value = stringtodouble(str_value)
> ncl 4> print((/r8_value/))
> (0) 2.26410202121
>
> As you can see, using stringtofloat my implementation gives one more digit than in your example (which is still a round-off from the original value). I think this could be machine-dependent, but I'm not sure.
>
> Good luck!
> Arne M.
>
>
> ----- Original Message -----
> > Dear NCLers,
> > The lost precision in function of stringtofloat is very confusing.
> > A snapshot of my script is :
> >
> >
> > datafile(0)="/pan2/projects/hybda/tlei/hbrid-gsi/experiments/dr-processing/dr-obsfit/dr-andrew/dr-new-test/t126-ens3dvar-factq0.005-tune1_temp_24_tl3wk_date_2011072300.txtout"
> > str_data = asciiread(datafile(0),-1,"string")
> > print("------------------")
> > print(str_data)
> > print("------------------")
> >
> > print("thinkyy1 "+str_get_field(str_data(2:11),10 ," "))
> > datarmstr(0,:) = stringtofloat(str_get_field(str_data(2:11),10 ," "))
> > print("output "+ datarmstr(0,:))
> > exit
> >
> >
> > ֣
> >
> > the output was :
> > ^^
> > (0) ------------------
> >
> >
> > Variable: str_data
> > Type: string
> > Total Size: 184 bytes
> > 23 values
> > Number of Dimensions: 1
> > Dimensions and sizes: [23]
> > Coordinates:
> > Number Of Attributes: 1
> > _FillValue : missing
> > (0) t126-ens3dvar-factq0.005-tune1
> > (1) 24
> > (2) Level: 950 GL: 8.24180969913 NH: 9.03959532222 SH: 4.20871527148
> > TR: 2.26410202121
> > (3) Level: 850 GL: 3.40322092967 NH: 3.62214877696 SH: 2.20873685424
> > TR: 1.25765875807
> > (4) Level: 750 GL: 1.64514061274 NH: 1.62367548789 SH: 1.89728488578
> > TR: 1.76729637694
> > (5) Level: 650 GL: 1.36298927422 NH: 1.33075976043 SH: 2.75972659747
> > TR: 1.00703519724
> > (6) Level: 550 GL: 1.03182087585 NH: 1.00696025671 SH: 1.56631219346
> > TR: 1.1095806198
> > (7) Level: 450 GL: 1.32992905798 NH: 1.36271631568 SH: 1.18470011083
> > TR: 1.01202546831
> > (8) Level: 350 GL: 1.21252777266 NH: 1.22452270919 SH: 1.4303300868
> > TR: 0.812163178701
> > (9) Level: 250 GL: 1.70174899943 NH: 1.72542880914 SH: 1.89613201475
> > TR: 1.00688183094
> > (10) Level: 150 GL: 2.54344055051 NH: 2.64716957083 SH: 2.15570650502
> > TR: 1.92993841914
> > (11) Level: 50 GL: 3.73654217549 NH: 3.34410145973 SH: 3.07788348411
> > TR: 5.7543725482
> > (12) tmpnumLevel: 950 GL: 16381.0 NH: 14216.0 SH: 823.0 TR: 1342.0
> > (13) tmpnumLevel: 850 GL: 7427.0 NH: 6607.0 SH: 329.0 TR: 491.0
> > (14) tmpnumLevel: 750 GL: 6001.0 NH: 5334.0 SH: 254.0 TR: 413.0
> > (15) tmpnumLevel: 650 GL: 8412.0 NH: 7609.0 SH: 303.0 TR: 500.0
> > (16) tmpnumLevel: 550 GL: 7811.0 NH: 7067.0 SH: 258.0 TR: 486.0
> > (17) tmpnumLevel: 450 GL: 8011.0 NH: 7102.0 SH: 325.0 TR: 584.0
> > (18) tmpnumLevel: 350 GL: 7837.0 NH: 6834.0 SH: 517.0 TR: 486.0
> > (19) tmpnumLevel: 250 GL: 28293.0 NH: 26201.0 SH: 937.0 TR: 1155.0
> > (20) tmpnumLevel: 150 GL: 6086.0 NH: 5067.0 SH: 441.0 TR: 578.0
> > (21) tmpnumLevel: 50 GL: 8215.0 NH: 6109.0 SH: 692.0 TR: 1414.0
> > (22) totnum tot: 104474.0 NH: 92146.0 SH: 4879.0 TR: 7449.0
> > (0) ------------------
> >
> > (0) ------------------
> > (0) thinkyy1 2.26410202121
> > (1) thinkyy1 1.25765875807
> > (2) thinkyy1 1.76729637694
> > (3) thinkyy1 1.00703519724
> > (4) thinkyy1 1.1095806198
> > (5) thinkyy1 1.01202546831
> > (6) thinkyy1 0.812163178701
> > (7) thinkyy1 1.00688183094
> > (8) thinkyy1 1.92993841914
> > (9) thinkyy1 5.7543725482
> > (0) output 2.2641
> > (1) output 1.25766
> > (2) output 1.7673
> > (3) output 1.00704
> > (4) output 1.10958
> > (5) output 1.01203
> > (6) output 0.812163
> > (7) output 1.00688
> > (8) output 1.92994
> > (9) output 5.75437
> >
> > VV
> >
> > It seems,say, the string 2.26410202121 was converted to 2.2641 with
> > quite a few numbers lost.
> > I tried direct converting the string, namely use
> > "stringtofloat(2.2641020212), the converted result would be exactly
> > the same. Why would such lost numbers happen in the above example?
> > Your help is appreciated.
> > Best,
> > Ting
> >
> >
> >
> > _______________________________________________
> > ncl-talk mailing list
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Received on Wed Jul 31 06:10:24 2013

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