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From: | Tim Daly |
Subject: | [Axiom-developer] Re: [sage-devel] Re: doctest failures due to rounding errors on Solaris. |
Date: | Fri, 01 Jan 2010 01:09:06 -0500 |
User-agent: | Thunderbird 2.0.0.21 (Windows/20090302) |
William Stein wrote:
And therein lies the problem. We use a regression that does a comparison of the printed representation of the output of the run with a stored copy of the output.On Thu, Dec 31, 2009 at 9:13 PM, Tim Daly <address@hidden> wrote:Dr. David Kirkby wrote:rjf wrote:On Dec 31, 11:15 am, "Dr. David Kirkby" <address@hidden> wrote:RJFThe point you are missing is that we want to compare the output what Sage prints to a human.The point you are missing is that the following item, which presumably could be printed by Sage, is perfectly readable to a human: 6121026514868073 * 2^(-51). It exactly dictates the bits in an IEEE double-float, and does not require any conversion from binary to decimal. It does not need rounding. This kind of representation does not have any hidden unprinted digits. It does not ever need to be longer because of delicate edge conditions of certain numbers. It happens to evaluate to APPROXIMATELY 2.718281828459045Sure, Sage could print that. It would also be worth printing the sign bit, so we could verify the values of 1) Sign bit 2) Significand 3) Exponent. All of those could be correct. But there is still the software which does the non-trivial task of converting that into the base 10 representation used by humans. Then in additon to that, there is the software which takes a base 10 number, shows it with the Sage prompt, adding carriage returns etc where necessary. All of these can go wrong. I would think in an almost ideal world, the test would be done at a higher level, using hardware/software which checked what the monitor actually displayed. That's not quite as easy to do though. Even better would be some way to scan the brain of the user to see what he/she believes Sage is showing. Perhaps we use a font that is not very good, so despite being displayed properly, it misunderstood. Given most of time people want to see a base 10 representation of a number, and not a base 2, base 16 or IEE 754 representation, I believe most testing should be done at the base 10 level. If there is a reason for testing the IEEE 754 representation as first choice, then you have yet to convince me of it. DaveDave, Axiom has the same issues. My take on this is that what you check depends on the reason you are checking. If you are generating the output for human use (e.g. a table) then you want decimal. If you are generating the output for regression testing (e.g. checking the answers on multiple hardware) then you probably want Fateman's solution. TimThe output is used both for human use and for regression testing. Its primary use is human -- it's an example in the Sage reference manual: sage: float(e) 2.7182818284590451 This is something a user will look at when reading the documentation for some function. It illustrates what happens when they convert the symbolic constant e to float. William
All of our regression tests were passing until I installed another, unrelated program. Suddenly about 30 regression tests started failing. It turns out that the unrelated program upgraded one of the system libraries. The net effect of that change was to cause the last digit in the output to "wobble" so that some of the table values differ in the nth place (20th, 30th, or thereabouts digit). This caused the regression
comparisons to fail. Common lisp will give you the exact bit pattern of the float and this value does not wobble so the text comparison succeeds with both the old and the new libraries against the bit pattern. So I can tell you from experience that what you would like to do is not going to succeed. Our solution to the human vs regression problem is to include the stable bit values in the actual compare and keep the human values in a latex table. This is easy to do with literate input. Tim
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