bugGNU Octave - Bugs: bug #50743, dlmread producing incorrect...


bug #50743: dlmread producing incorrect results for large (64bit) integers

Submitted by:  Ben Lapid <blap>
Submitted on:  Thu Apr 6 22:02:08 2017  
Category: Octave FunctionSeverity: 3 - Normal
Priority: 5 - NormalItem Group: Inaccurate Result
Status: Need InfoAssigned to: None
Originator Name: Open/Closed: Open
Release: 4.0.0Operating System: GNU/Linux

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Fri Apr 7 17:09:13 2017, comment #3:

I was just looking into that actually. Yes, textscan appears to return values of the right class, but they are actually parsed as doubles.

See the comment here: https://hg.savannah.gnu.org/hgweb/octave/file/56c59b3f9172/libinterp/corefcn/oct-stream.cc#l3317

I still think it's valid to say that dlmread just doesn't do this feature (I agree, a Matlab test to confirm would be nice), and textscan should, but our textscan is currently broken in this regard also.

Mike Miller <mtmiller>
Project Administrator
Fri Apr 7 17:03:07 2017, comment #2:

Can the reporter, or anyone, try the the sample code in Matlab? My understanding is that dlmread always read into a double matrix and for compatibility Octave needs to do the same.

There could still be an issue, however, with extremely large integer constants. This should work, but appears to get a differently wrong answer.

Rik <rik5>
Project Administrator
Thu Apr 6 22:21:19 2017, comment #1:

I don't know for certain, but I believe that's what textscan is for, dlmread should not be that flexible.

If you want simple parsing, where "simple" means the file is delimited columns and all values can be treated as doubles, then use csvread or dlmread. If you want the full flexibility to define how each column should be interpreted, use textscan.

Mike Miller <mtmiller>
Project Administrator
Thu Apr 6 22:02:08 2017, original submission:

Steps to reproduce:
1. (shell) echo 5973459727478852968 > /tmp/1.csv
2. (octave) x = dlmread('/tmp/1.csv', ',', 0, 0)
3. (octave) uint64(x)

>> ans = 5973459727478852608

This issue comes from the fact that dlmread reads doubles (libinterp/corefcn/dlmread.cc) from the text file.
While I understand this behavior in 99% of the cases, and I agree it should be defaulted this way, I do think there should be a way to direct dlmread to a different parse method. See for example scipy's 'genfromtxt' 'dtype' argument.

Ben Lapid <blap>


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