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• Wrapping a point-to-point link

  - by user3712955

  I'm using a pair of IP radios (non-WiFi) to bridge my office engineering LAN (172.0.0.0/8) to a lab in another building. The radios work fine, but they expose a web management interface I'd like to hide, and they also generate traffic (ARP, STP, and more) that I need to keep off my (very, very clean) LAN segments. I have some ARM-Linux boards (similar to Beagle/Panda/RasPi) running Ubuntu, and I've put one at each end of the link, between the radio and the LAN. Each of the boards has 2 wired Ethernet interfaces, eth0 and eth1. The LAN segments are connected to eth0, and the radios are connected to eth1. I'd like to accomplish the following: Keep radio-originated traffic off my LAN segments! Hide all services provided by the radio (web, ssh, etc.) Transparently pass all traffic between the LAN segments (including things like ARP). The above also applies to the ARM-Linux boards: No stray traffic my LAN from them either! I'd like the system to look like a switch: LAN packets arriving at one eth0 appear at the other. And neither eth0 should have an IP address: The working system should behave like a CAT6 cable with some latency (instead of ARM boards and radios). Unfortunately, I'm confused about how to properly configure the ARM Ubuntu systems. What I'm guessing I need is a bridge on each board (br0?) and a VLAN (vlan0 or eth0.0?) to isolate the LAN traffic from everything else as it passes through the ARM boards and the radios. Then I need some kind of a firewall to block sending anything out eth0 that isn't from the other eth0 (via the VLAN). I've looked at the ip and ebtables commands (especially -t broute). While the concepts sorta-kinda make sense, I'm completely lost in the details. Edit: In the perverse case that a system on one of my LAN segments has the same IP address as one of the radios, or as eth1 on the ARM-Ubuntu boards, a VLAN won't work. Which I believe means I need to tunnel all traffic between the two eth0 interfaces to get that "like a wire" behavior. Help? Finally, I'd like to have a way to temporarily expose services on the ARM boards (ssh) and the radios (web) for maintenance purposes. Ideally, it would expose an IP address with ssh available on port 22. Once connected, I figure I'd start an X11 session and run a browser on the ARM board to access the radios. Or something. I could login via the console to enable/disable this, or perhaps could use a GPIO to trigger a script. I feel I've identified most of the pieces needed to make all this happen, but I have no idea how to combine them to make a working system. Thanks!

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• floating point hex octal binary

  - by workinprogress

  Hi, I am working on a calculator that allows you to perform calculations past the decimal point in octal, hexadecimal, binary, and of course decimal. I am having trouble though finding a way to convert floating point decimal numbers to floating point hexadecimal, octal, binary and vice versa. The plan is to do all the math in decimal and then convert the result into the appropriate number system. Any help, ideas or examples would be appreciated. Thanks!

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• SQL Queries for Creating a rollback point and to rollback to that specific point

  - by Santhosha

  Hi, As per my project requirement i want to perform two operation Password Change Unlock Account(Only unlocking account, no password change!) I want return success only if both the transactions succeeds. Say if password change succeeds and unlock fails i cannot send success or failure. So i want to create a rollback point before password change, if both queries executes successfully i will commit the transaction. If one of the query fails i will discard the changes by rolling back to the rollback point. I am doing this in C++ using ADO. Is there any SQL Queries,using i can create the rollback point and reverting to rollback point and commiting the transaction I am using below commands for Password change ALTER LOGIN [username] WITH PASSWORD = N'password' for Unlock account ALTER LOGIN [%s] WITH CHECK_POLICY = OFF ALTER LOGIN [%s] WITH CHECK_POLICY = ON Thanks in advance!! Santhosh

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• Floating point precision in Visual C++

  - by Luigi Giaccari

  HI, I am trying to use the robust predicates for computational geometry from Jonathan Richard Shewchuk. I am not a programmer, so I am not even sure of what I am saying, I may be doing some basic mistake. The point is the predicates should allow for precise aritmthetic with adaptive floating point precision. On my computer: Asus pro31/S (Core Due Centrino Processor) they do not work. The problem may stay in the fact the my computer may use some improvements in the floating point precision taht conflicts with the one used by Shewchuk. The author says: /* On some machines, the exact arithmetic routines might be defeated by the / / use of internal extended precision floating-point registers. Sometimes / / this problem can be fixed by defining certain values to be volatile, / / thus forcing them to be stored to memory and rounded off. This isn't / / a great solution, though, as it slows the arithmetic down. */ Now what I would like to know is that there is a way, maybe some compiler option, to turn off the internal extended precision floating-point registers. I really appriaciate your help

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• Drag UILabel from point A to point B

  - by Matt S.

  How can I go about dragging a UILabel from point A to point B on my iPhone application?

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• How do you handle floating point rounding problems?

  - by Doc Brown

  Seeing this post about floating point errors on slashdot, I got curious what other kind of solutions exist to deal with such kind of floating point rounding errors. So what was the floating point bug you learned most from, and what did you learn from it?

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• calculate space in mount point

  - by user175084

  i have a folder tempAgents within a mount point drive. This mount point drive is within my E drive. so the address is like this "E:\mountpoint\tempAgents" now i need to calculate the free space and total space of the mount point. is there a way to do so.. thanks

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• What are some of the best answer file settings for a WDS Deployment?

  - by drpcken

  I've had my head buried in answer files for days now and have gotten quite comfortable setting them up, test, etc... I use a handful of Components to help my migrations, for my unattend.xml I like: Windows-International-Core-WinPE -- this is good for setting Locales the preboot environment (en-us for us english US speakers). Keeps me from having to set these on the initial image boot. Windows-Setup_neutral -- I like the WindowsDeploymentServices -> ImageSelection, especially if I'm only pushing a single image. This keeps me from having to select it each time. My OOBE_Unattend.xml is really useful and I barely have to touch anything during this part of the installation: Windows-Shell-Setup_neutral -- This lets me put a ProductKey in for my MAK volume license (very useful and time saving). I can also set the TimeZone for the installation. Windows UnattendedJoin_neutral -- I couldn't live without this component. It joins the machine on my domain before logging in as a domain administrator. I would hate to not have this ability. Windows-International-Core -- Again this component really speeds up the OOBE process. I configure my locals and time zone so I don't have to do it by hand when the machine enteres OOBE. Windows-Shell-Setup -- Allows you to configure an autologon when the new machine is finished. I like to logon as a domain admin automatically for customizing and troubleshooting the new machine immediately after it is imaged. Also the OOBE component under here lets me skip the EULA, Hide Wireless Setup, and set my default NetworkLocation. All of this makes the entire OOBE totally automated. What are some other good components I am missing as far as helping me get these images pushed and configured as quickly as possible?

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• Floating point arithmetic is too reliable.

  - by mcoolbeth

  I understand that floating point arithmetic as performed in modern computer systems is not always consistent with real arithmetic. I am trying to contrive a small C# program to demonstrate this. eg: static void Main(string[] args) { double x = 0, y = 0; x += 20013.8; x += 20012.7; y += 10016.4; y += 30010.1; Console.WriteLine("Result: "+ x + " " + y + " " + (x==y)); Console.Write("Press any key to continue . . . "); Console.ReadKey(true); } However, in this case, x and y are equal in the end. Is it possible for me to demonstrate the inconsistency of floating point arithmetic using a program of similar complexity, and without using any really crazy numbers? I would like, if possible, to avoid mathematically correct values that go more than a few places beyond the decimal point.

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• C - floating point rounding

  - by hatorade

  I'm trying to understand how floating point numbers work. I think I'd like to test out what I know / need to learn by evaluating the following: I would like to find the smallest x such that x + 1 = x, where x is a floating point number. As I understand it, this would happen in the case where x is large enough so that x + 1 is closer to x than the next number higher than x representable by floating point. So intuitively it seems it would be the case where I don't have enough digits in the significand. Would this number x then be the number where the significand is all 1's. But then I can't seem to figure out what the exponent would have to be. Obviously it would have to be big (relative to 10^0, anyway).

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• MySQL floating point comparison issues

  - by Sharief

  I ran into an issue by introducing floating point columns in the MySQL database schema that the comparisons on floating point values don't return the correct results always. 1 - 50.12 2 - 34.57 3 - 12.75 4 - ...(rest all less than 12.00) SELECT COUNT(*) FROM `users` WHERE `points` > "12.75" This returns me "3". I have read that the comparisons of floating point values in MySQL is a bad idea and decimal type is the better option. Do I have any hope of moving ahead with the float type and get the comparisons to work correctly?

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• Converting byte[] of binary fixed point to floating point value?

  - by Sean Donohue

  I'm reading some data over a socket. The integral data types are no trouble, the System.BitConverter methods are correctly handling the conversion. (So there are no Endian issues to worry about, I think?) However, BitConverter.ToDouble isn't working for the floating point parts of the data...the source specification is a bit low level for me, but talks about a binary fixed point representation with a positive byte offset in the more significant direction and negative byte offset in the less significant direction. Most of the research I've done has been aimed at C++ or a full fixed-point library handling sines and cosines, which sounds like overkill for this problem. Could someone please help me with a C# function to produce a float from 8 bytes of a byte array with, say, a -3 byte offset?

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• Point in polygon OR point on polygon using LINQ

  - by wageoghe

  As noted in an earlier question, How to Zip enumerable with itself, I am working on some math algorithms based on lists of points. I am currently working on point in polygon. I have the code for how to do that and have found several good references here on SO, such as this link Hit test. So, I can figure out whether or not a point is in a polygon. As part of determining that, I want to determine if the point is actually on the polygon. This I can also do. If I can do all of that, what is my question you might ask? Can I do it efficiently using LINQ? I can already do something like the following (assuming a Pairwise extension method as described in my earlier question as well as in links to which my question/answers links, and assuming a Position type that has X and Y members). I have not tested much, so the lambda might not be 100% correct. Also, it does not take very small differences into account. public static PointInPolygonLocation PointInPolygon(IEnumerable<Position> pts, Position pt) { int numIntersections = pts.Pairwise( (p1, p2) => { if (p1.Y != p2.Y) { if ((p1.Y >= pt.Y && p2.Y < pt.Y) || (p1.Y < pt.Y && p2.Y >= pt.Y)) { if (p1.X < p1.X && p2.X < pt.X) { return 1; } if (p1.X < pt.X || p2.X < pt.X) { if (((pt.Y - p1.Y) * ((p1.X - p2.X) / (p1.Y - p2.Y)) * p1.X) < pt.X) { return 1; } } } } return 0; }).Sum(); if (numIntersections % 2 == 0) { return PointInPolygonLocation.Outside; } else { return PointInPolygonLocation.Inside; } } This function, PointInPolygon, takes the input Position, pt, iterates over the input sequence of position values, and uses the Jordan Curve method to determine how many times a ray extended from pt to the left intersects the polygon. The lambda expression will yield, into the "zipped" list, 1 for every segment that is crossed, and 0 for the rest. The sum of these values determines if pt is inside or outside of the polygon (odd == inside, even == outside). So far, so good. Now, for any consecutive pairs of position values in the sequence (i.e. in any execution of the lambda), we can also determine if pt is ON the segment p1, p2. If that is the case, we can stop the calculation because we have our answer. Ultimately, my question is this: Can I perform this calculation (maybe using Aggregate?) such that we will only iterate over the sequence no more than 1 time AND can we stop the iteration if we encounter a segment that pt is ON? In other words, if pt is ON the very first segment, there is no need to examine the rest of the segments because we have the answer. It might very well be that this operation (particularly the requirement/desire to possibly stop the iteration early) does not really lend itself well to the LINQ approach. It just occurred to me that maybe the lambda expression could yield a tuple, the intersection value (1 or 0 or maybe true or false) and the "on" value (true or false). Maybe then I could use TakeWhile(anontype.PointOnPolygon == false). If I Sum the tuples and if ON == 1, then the point is ON the polygon. Otherwise, the oddness or evenness of the sum of the other part of the tuple tells if the point is inside or outside.

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• Grails validation problems with sets of data: only getting one error message for all errors in a set

  - by Matt

  Hi, I'm trying to validate a domain class that has a number of subsets. class IebeUser { ... static hasMany = [openUserAnswers:OpenUserAnswer, closedUserAnswers:ClosedUserAnswer] } class OpenUserAnswer { OpenQuestion openQuestion String text static belongsTo = [user:IebeUser] static constraints = { openQuestion(nullable:false) text(blank:false) } } class ClosedUserAnswer { ClosedQuestion closedQuestion ClosedAnswer answer static belongsTo = [user:IebeUser] static constraints = { closedQuestion(nullable:false) answer(nullable:false) } } A closed question has a set of predefined answers and an open question lets the user enter a freeform answer. All is well until I come to validate the object after entry in a form: params: [closedUserAnswers[0].answer.id:, closedUserAnswers[0]:[answer:[id:], answer.id:], password:dfgdfgdf, openUserAnswers[0].text:gdfgdfgdfg, openUserAnswers[0]:[text:gdfgdfgdfg], _isOptedOut:, create:Continue, username:gdfgdfggdf, email:[email protected], closedUserAnswers[1].answer.id:, closedUserAnswers[1]:[answer:[id:], answer.id:], openUserAnswers[1].text:, openUserAnswers[1]:[text:], firstName:dfgdf, lastName:gdfgdfgd, action:save, controller:main] The key bits being: closedUserAnswers[0].answer.id:, closedUserAnswers[0]:[answer:[id:] closedUserAnswers[1].answer.id:, closedUserAnswers[1]:[answer:[id:] openUserAnswers[1].text:, openUserAnswers[1]:[text:] In my tests I have two objects of type closedUserAnswers and two of openUserAnswers. But when I call validation on IebeUser I only get validation errors for the closedUserAnswers or the openUserAnswers as a whole. I don't get validation errors for each object with a problem which is what I need. I really need an error per instance. Does anyone know what I'm doing wrong? Even when I call the validate method against each closedUserAnswer/openUserAnswer I still only get one per type. Here are my errors. Sorry for all the code, but thought I'd include as much of the code as possible so that it makes sense. Field error in object 'uk.co.cascaid.iebe.IebeUser' on field 'openUserAnswers.text': rejected value []; codes [uk.co.cascaid.iebe.OpenUserAnswer.text.blank.error.uk.co.cascaid.iebe.IebeUser.openUserAnswers.text,uk.co.cascaid.iebe.OpenUserAnswer.text.blank.error.openUserAnswers.text,uk.co.cascaid.iebe.OpenUserAnswer.text.blank.error.text,uk.co.cascaid.iebe.OpenUserAnswer.text.blank.error,openUserAnswer.text.blank.error.uk.co.cascaid.iebe.IebeUser.openUserAnswers.text,openUserAnswer.text.blank.error.openUserAnswers.text,openUserAnswer.text.blank.error.text,openUserAnswer.text.blank.error,uk.co.cascaid.iebe.OpenUserAnswer.text.blank.uk.co.cascaid.iebe.IebeUser.openUserAnswers.text,uk.co.cascaid.iebe.OpenUserAnswer.text.blank.openUserAnswers.text,uk.co.cascaid.iebe.OpenUserAnswer.text.blank.text,uk.co.cascaid.iebe.OpenUserAnswer.text.blank,openUserAnswer.text.blank.uk.co.cascaid.iebe.IebeUser.openUserAnswers.text,openUserAnswer.text.blank.openUserAnswers.text,openUserAnswer.text.blank.text,openUserAnswer.text.blank,blank.uk.co.cascaid.iebe.IebeUser.openUserAnswers.text,blank.openUserAnswers.text,blank.text,blank]; arguments [text,class uk.co.cascaid.iebe.OpenUserAnswer]; default message [Property [{0}] of class [{1}] cannot be blank] Field error in object 'uk.co.cascaid.iebe.IebeUser' on field 'closedUserAnswers.answer': rejected value [null]; codes [uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.error.uk.co.cascaid.iebe.IebeUser.closedUserAnswers.answer,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.error.closedUserAnswers.answer,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.error.answer,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.error,closedUserAnswer.answer.nullable.error.uk.co.cascaid.iebe.IebeUser.closedUserAnswers.answer,closedUserAnswer.answer.nullable.error.closedUserAnswers.answer,closedUserAnswer.answer.nullable.error.answer,closedUserAnswer.answer.nullable.error,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.uk.co.cascaid.iebe.IebeUser.closedUserAnswers.answer,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.closedUserAnswers.answer,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable.answer,uk.co.cascaid.iebe.ClosedUserAnswer.answer.nullable,closedUserAnswer.answer.nullable.uk.co.cascaid.iebe.IebeUser.closedUserAnswers.answer,closedUserAnswer.answer.nullable.closedUserAnswers.answer,closedUserAnswer.answer.nullable.answer,closedUserAnswer.answer.nullable,nullable.uk.co.cascaid.iebe.IebeUser.closedUserAnswers.answer,nullable.closedUserAnswers.answer,nullable.answer,nullable]; arguments [answer,class uk.co.cascaid.iebe.ClosedUserAnswer]; default message [Property [{0}] of class [{1}] cannot be null]

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• C99 and floating point environment

  - by yCalleecharan

  Hi, I was looking at new features of C99 and saw the floating point environment: #include <fenv.h> My question is simple. If I'm performing floating point number computations, do I have to include the above preprocessor directive in my code? If no, then what does this directive do and when does it become important to include? Thanks a lot...

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• Ruby BigDecimal sanity check (floating point newb)

  - by Andy

  Hello, Hoping to get some feedback from someone more experienced here. I haven't dealt with the dreaded floating-point calculation before... Is my understanding correct that with Ruby BigDecimal types (even with varying precision and scale lengths) should calculate accurately or should I anticipate floating point shenanigans? All my values within a Rails application are BigDecimal type and I'm seeing some errors (they do have different decimal lengths), hoping it's just my methods and not my object types... Thanks!

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• How does GDI+ support floating point values?

  - by Vulcan Eager

  Given that GDI+ is a wrapper around GDI, how does it handle floating point values? I don't see any support for floating point co-ordinates in the GDI documentation.

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• Meshing of Point Clouds from a 3Dlaser scanner

  - by iceman

  Is there any package/software which can do Meshing of Point Clouds in real time? What is the data structure used to represent 3D Point Clouds ?

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• Keeping sync in multiplayer RTS game that uses floating point arithmetic

  - by Calmarius

  I'm writing a 2D space RTS game in C#. Single player works. Now I want to add some multiplayer functionality. I googled for it and it seems there is only one way to have thousands of units continuously moving without a powerful net connection: send only the commands through the network while running the same simulation at every player. And now there is a problem the entire engine uses doubles everywhere. And floating point calculations are depends heavily on compiler optimalizations and cpu architecture so it is very hard to keep things syncronized. And it is not grid based at all, and have a simple phisics engine to move the space-ships (space ships have impulse and angular-momentum...). So recoding the entire stuff to use fixed point would be quite cumbersome (but probably the only solution). So I have 2 options so far: Say bye to the current code and restart from scratch using integers Make the game LAN only where there is enough bandwidth to have 8 players with thousands of units and sending the positions and orientation etc in (almost) every frame... So I looking for better opinions, (or even tips on migrating the code to fixed-point without messing everything up...)

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• Finding furthermost point in game world

  - by user13414

  I am attempting to find the furthermost point in my game world given the player's current location and a normalized direction vector in screen space. My current algorithm is: convert player world location to screen space multiply the direction vector by a large number (2000) and add it to the player's screen location to get the distant screen location convert the distant screen location to world space create a line running from the player's world location to the distant world location loop over the bounding "walls" (of which there are always 4) of my game world check whether the wall and the line intersect if so, where they intersect is the furthermost point of my game world in the direction of the vector Here it is, more or less, in code: public Vector2 GetFurthermostWorldPoint(Vector2 directionVector) { var screenLocation = entity.WorldPointToScreen(entity.Location); var distantScreenLocation = screenLocation + (directionVector * 2000); var distantWorldLocation = entity.ScreenPointToWorld(distantScreenLocation); var line = new Line(entity.Center, distantWorldLocation); float intersectionDistance; Vector2 intersectionPoint; foreach (var boundingWall in entity.Level.BoundingWalls) { if (boundingWall.Intersects(line, out intersectionDistance, out intersectionPoint)) { return intersectionPoint; } } Debug.Assert(false, "No intersection found!"); return Vector2.Zero; } Now this works, for some definition of "works". I've found that the further out my distant screen location is, the less chance it has of working. When digging into the reasons why, I noticed that calls to Viewport.Unproject could result in wildly varying return values for points that are "far away". I wrote this stupid little "test" to try and understand what was going on: [Fact] public void wtf() { var screenPositions = new Vector2[] { new Vector2(400, 240), new Vector2(400, -2000), }; var viewport = new Viewport(0, 0, 800, 480); var projectionMatrix = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, viewport.Width / viewport.Height, 1, 200000); var viewMatrix = Matrix.CreateLookAt(new Vector3(400, 630, 600), new Vector3(400, 345, 0), new Vector3(0, 0, 1)); var worldMatrix = Matrix.Identity; foreach (var screenPosition in screenPositions) { var nearPoint = viewport.Unproject(new Vector3(screenPosition, 0), projectionMatrix, viewMatrix, worldMatrix); var farPoint = viewport.Unproject(new Vector3(screenPosition, 1), projectionMatrix, viewMatrix, worldMatrix); Console.WriteLine("For screen position {0}:", screenPosition); Console.WriteLine(" Projected Near Point = {0}", nearPoint.TruncateZ()); Console.WriteLine(" Projected Far Point = {0}", farPoint.TruncateZ()); Console.WriteLine(); } } The output I get on the console is: For screen position {X:400 Y:240}: Projected Near Point = {X:400 Y:629.571 Z:599.0967} Projected Far Point = {X:392.9302 Y:-83074.98 Z:-175627.9} For screen position {X:400 Y:-2000}: Projected Near Point = {X:400 Y:626.079 Z:600.7554} Projected Far Point = {X:390.2068 Y:-767438.6 Z:148564.2} My question is really twofold: what am I doing wrong with the unprojection such that it varies so wildly and, thus, does not allow me to determine the corresponding world point for my distant screen point? is there a better way altogether to determine the furthermost point in world space given a current world space location, and a directional vector in screen space?

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• SQL SERVER – Three Puzzling Questions – Need Your Answer

  - by pinaldave

  Last week I had asked three questions on my blog. I got very good response to the questions. I am planning to write summary post for each of three questions next week. Before I write summary post and give credit to all the valid answers. I was wondering if I can bring to notice of all of you this week. Why SELECT * throws an error but SELECT COUNT(*) does not This is indeed very interesting question as not quite many realize that this kind of behavior SQL Server demonstrates out of the box. Once you run both the code and read the explanation it totally makes sense why SQL Server is behaving how it is behaving. Also there is connect item is associated with it. Also read the very first comment by Rob Farley it also shares very interesting detail. Statistics are not Updated but are Created Once This puzzle has multiple right answer. I am glad to see many of the correct answer as a comment to this blog post. Statistics are very important and it really helps SQL Server Engine to come up with optimal execution plan. DBA quite often ignore statistics thinking it does not need to be updated, as they are automatically maintained if proper database setting is configured (auto update and auto create). Well, in this question, we have scenario even though auto create and auto update statistics are ON, statistics is not updated. There are multiple solutions but what will be your solution in this case? When to use Function and When to use Stored Procedure This question is rather open ended question – there is no right or wrong answer. Everybody developer has always used functions and stored procedures. Here is the chance to justify when to use Stored Procedure and when to use Functions. I want to acknowledge that they can be used interchangeably but there are few reasons when one should not do that. There are few reasons when one is better than other. Let us discuss this here. Your opinion matters. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, Readers Contribution, Readers Question, SQL, SQL Authority, SQL Performance, SQL Puzzle, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQLAuthority News, SQLServer, T SQL, Technology

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• When the obvious answer is obviously wrong

  - by John Paul Cook

  This post is about how simple math in T-SQL can produce undesirable results, but first we begin with a math quiz. Answer the following as quickly as possible: You just read pages 100-300 of a book. How many pages did you read? QUICKLY NOW! For those of you who answered 200 pages, I have a new question: Which page did you not read? There were 201 pages to read. If you read 200 pages, you skipped a page! What your answer be if I asked you how many pages did you read if you read pages 1-3? Three pages!...(read more)

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• Floating point vs integer calculations on modern hardware

  - by maxpenguin

  I am doing some performance critical work in C++, and we are currently using integer calculations for problems that are inherently floating point because "its faster". This causes a whole lot of annoying problems and adds a lot of annoying code. Now, I remember reading about how floating point calculations were so slow approximately circa the 386 days, where I believe (IIRC) that there was an optional co-proccessor. But surely nowadays with exponentially more complex and powerful CPUs it makes no difference in "speed" if doing floating point or integer calculation? Especially since the actual calculation time is tiny compared to something like causing a pipeline stall or fetching something from main memory? I know the correct answer is to benchmark on the target hardware, what would be a good way to test this? I wrote two tiny C++ programs and compared their run time with "time" on Linux, but the actual run time is too variable (doesn't help I am running on a virtual server). Short of spending my entire day running hundreds of benchmarks, making graphs etc. is there something I can do to get a reasonable test of the relative speed? Any ideas or thoughts? Am I completely wrong? The programs I used as follows, they are not identical by any means: #include <iostream> #include <cmath> #include <cstdlib> #include <time.h> int main( int argc, char** argv ) { int accum = 0; srand( time( NULL ) ); for( unsigned int i = 0; i < 100000000; ++i ) { accum += rand( ) % 365; } std::cout << accum << std::endl; return 0; } Program 2: #include <iostream> #include <cmath> #include <cstdlib> #include <time.h> int main( int argc, char** argv ) { float accum = 0; srand( time( NULL ) ); for( unsigned int i = 0; i < 100000000; ++i ) { accum += (float)( rand( ) % 365 ); } std::cout << accum << std::endl; return 0; } Thanks in advance!

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• Floating point equality and tolerances

  - by doron

  Comparing two floating point number by something like a_float == b_float is looking for trouble since a_float / 3.0 * 3.0 might not be equal to a_float due to round off error. What one normally does is something like fabs(a_float - b_float) < tol. How does one calculate tol? Ideally tolerance should be just larger than the value of one or two of the least significant figures. So if the single precision floating point number is use tol = 10E-6 should be about right. However this does not work well for the general case where a_float might be very small or might be very large. How does one calculate tol correctly for all general cases? I am interested in C or C++ cases specifically.

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• Double precision floating point in CUDA

  - by cuda-dev

  Does CUDA support double precision floating point numbers ? Also need reasons for the same.

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