* Newbie question: How does one do bit manipulation in Ada? @ 2003-12-23 1:58 Peter C. Chapin 2003-12-23 3:01 ` Georg Bauhaus ` (2 more replies) 0 siblings, 3 replies; 9+ messages in thread From: Peter C. Chapin @ 2003-12-23 1:58 UTC (permalink / raw) Hello! I'm a C/C++ programmer in the process of teaching myself Ada. I've been writing toy programs to get a feeling for the language. There is obviously a lot to learn, but I'm enjoying myself. Anyway, I have a need to do some bit manipulations in one of my programs. In particular I want to invert certain bits in a number (integer). I'm used to doing this with a bitwise XOR operator (C/C++) so I looked around for some kind of equivalent in Ada but I didn't find anything in package Standard or in any of the "usual" library packages. Am I missing something or am I just thinking about this wrong? Thanks! Peter ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-23 1:58 Newbie question: How does one do bit manipulation in Ada? Peter C. Chapin @ 2003-12-23 3:01 ` Georg Bauhaus 2003-12-23 3:23 ` Peter C. Chapin 2003-12-23 3:33 ` Steve 2003-12-23 9:09 ` Stephen Leake 2 siblings, 1 reply; 9+ messages in thread From: Georg Bauhaus @ 2003-12-23 3:01 UTC (permalink / raw) Peter C. Chapin <pchapin@sover.net> wrote: : Anyway, I have a need to do some bit manipulations in one of my : programs. In particular I want to invert certain bits in a number : (integer). I'm used to doing this with a bitwise XOR operator (C/C++) so : I looked around for some kind of equivalent in Ada but I didn't find : anything in package Standard or in any of the "usual" library packages. : Am I missing something or am I just thinking about this wrong? xor is available for Boolean types, modular types, and one-dimensional arrays of Booleans, as well as for (Wide_)Character_Sets. Some signed and modular integer types are predefined in package Interfaces. If you cannot use the modular variant of integers, but you know that some integers values in question are bitwise represented like modular values, you could do an unchecked_conversion. In addition, package Interfaces has Shift_Left/Right, Shift_Right_Arithmentic, and Rotate_Left/Right. It is also possible to use representation clauses and assign names to bits or groups of bits that are components of a record (and possibly again do unchecked_conversions if needed). -- Georg ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-23 3:01 ` Georg Bauhaus @ 2003-12-23 3:23 ` Peter C. Chapin 2003-12-23 3:36 ` tmoran 0 siblings, 1 reply; 9+ messages in thread From: Peter C. Chapin @ 2003-12-23 3:23 UTC (permalink / raw) In article <bs8b5o$3h1$1@a1-hrz.uni-duisburg.de>, sb463ba@l1-hrz.uni- duisburg.de says... > xor is available for Boolean types, modular types, and one-dimensional > arrays of Booleans, as well as for (Wide_)Character_Sets. > Some signed and modular integer types are predefined in package > Interfaces. Okay, thanks. I didn't look at package Interfaces, but obviously I should. I think that will get me going. Peter ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-23 3:23 ` Peter C. Chapin @ 2003-12-23 3:36 ` tmoran 0 siblings, 0 replies; 9+ messages in thread From: tmoran @ 2003-12-23 3:36 UTC (permalink / raw) >Okay, thanks. I didn't look at package Interfaces, but obviously I You can define your own modular types, or arrays of booleans, or packed bit fields in a representation clause. Interfaces does conveniently predefine the sort of thing you are used to in C, though. ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-23 1:58 Newbie question: How does one do bit manipulation in Ada? Peter C. Chapin 2003-12-23 3:01 ` Georg Bauhaus @ 2003-12-23 3:33 ` Steve 2003-12-23 9:09 ` Stephen Leake 2 siblings, 0 replies; 9+ messages in thread From: Steve @ 2003-12-23 3:33 UTC (permalink / raw) Ada only permits bit masking on arrays of booleans or modular types. To do an XOR on integers convert the integers to modular types using unchecked conversion, xor the resulting modular values, and convert the result back to integer. This question came up recently, here's the sample program I posted earlier. with Ada.Integer_Text_IO; with Ada.Text_IO; with Ada.Unchecked_Conversion; procedure DoIntXOR is function "xor"(Left,Right : in Integer) return Integer is pragma Inline("xor"); type Int_As_Mod is mod 2 ** Integer'Size; function To_Mod is new Ada.Unchecked_Conversion( Integer, Int_As_Mod ); function To_Int is new Ada.Unchecked_Conversion( Int_As_Mod, Integer ); begin return To_Int( To_Mod(Left) xor To_Mod(Right) ); end "xor"; a_value : Integer; b_value : Integer; begin Ada.Text_IO.Put( "Enter first value > " ); Ada.Integer_Text_IO.Get( a_value ); Ada.Text_IO.Put( "Enter second value > " ); Ada.Integer_Text_IO.Get( b_value ); Ada.Text_IO.Put( "XOR value is " ); Ada.Integer_Text_IO.Put( a_value xor b_value ); Ada.Text_IO.New_Line; end DoIntXOR; BTW: Unless you're doing interface work, you rarely need to do bit fiddling in Ada. Steve (The Duck) "Peter C. Chapin" <pchapin@sover.net> wrote in message news:MPG.1a516ba5b76b5bee989684@news.sover.net... > > Hello! > > I'm a C/C++ programmer in the process of teaching myself Ada. I've been > writing toy programs to get a feeling for the language. There is > obviously a lot to learn, but I'm enjoying myself. > > Anyway, I have a need to do some bit manipulations in one of my > programs. In particular I want to invert certain bits in a number > (integer). I'm used to doing this with a bitwise XOR operator (C/C++) so > I looked around for some kind of equivalent in Ada but I didn't find > anything in package Standard or in any of the "usual" library packages. > Am I missing something or am I just thinking about this wrong? > > Thanks! > > Peter ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-23 1:58 Newbie question: How does one do bit manipulation in Ada? Peter C. Chapin 2003-12-23 3:01 ` Georg Bauhaus 2003-12-23 3:33 ` Steve @ 2003-12-23 9:09 ` Stephen Leake 2003-12-28 13:20 ` Peter C. Chapin 2 siblings, 1 reply; 9+ messages in thread From: Stephen Leake @ 2003-12-23 9:09 UTC (permalink / raw) To: comp.lang.ada Peter C.Chapin <pchapin@sover.net> writes: > I'm a C/C++ programmer in the process of teaching myself Ada. I've been > writing toy programs to get a feeling for the language. There is > obviously a lot to learn, but I'm enjoying myself. Good to hear! > Anyway, I have a need to do some bit manipulations in one of my > programs. In particular I want to invert certain bits in a number > (integer). Others have told you how to do this using XOR. I'd like to point out that Ada often has better ways to accomplish things that are done with bit-fiddling in other languages. So if you can explain why you need to invert those bits, we might be able to give you a better way to accomplish the same thing. For example, specifying the exact position of bits in registers is much simpler in Ada than in C. -- -- Stephe ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-23 9:09 ` Stephen Leake @ 2003-12-28 13:20 ` Peter C. Chapin 2003-12-28 18:40 ` Robert I. Eachus 0 siblings, 1 reply; 9+ messages in thread From: Peter C. Chapin @ 2003-12-28 13:20 UTC (permalink / raw) In article <mailman.170.1072170574.31149.comp.lang.ada@ada-france.org>, stephen_leake@acm.org says... > Others have told you how to do this using XOR. > > I'd like to point out that Ada often has better ways to accomplish > things that are done with bit-fiddling in other languages. So if you > can explain why you need to invert those bits, we might be able to > give you a better way to accomplish the same thing. Sorry about the long delay for this follow-up; I've been away for the Christmas holiday. Anyway, I can appreciate that there might be a better way to accomplish what I'm trying to do. My little program is trying to experiment with the effectiveness of different checksumming techniques. I made a package that simulates a noisy channel (random noise). The package has a procedure to set the bit error rate and another procedure, Transceive, that is intended to accept an 8 bit octet and then randomly change some bits in that octet according to the selected bit error rate. It then hands back the result. The main program generates random data, computes checksums on this data in various ways, and then passes the data, together with the checksums, through the simulated noisy channel. By comparing the result with the original data it can count the number of erronous bits. It can also verify the checksum and thus find out if the checksum used was able to detect the error. After using a large number of data blocks one can accumulate statistically significant results. The program shows, for example, that your typical CRC checksum does a much better job at detecting errors than, for example, a simple checksum using the same space overhead. There is nothing revolutionary in this, of course. As I said, this is just a toy program that I'm using right now to help me learn Ada. Anyway, I have yet to deal with writing CRC checksumming code in Ada (that's for later). At the moment I'm getting my noisy channel simulation to work. My Transceive procedure works by looping over the eight bits in an octet and, if a particular bit is to be corrupted, XORing a suitable mask into the number to invert the bit. The C program that I wrote originally uses the type int for the simulated octets. My first pass at an Ada translation thus used Integer. However, I can see that Integer is not entirely the most appropriate type here so I have no problem using one of the modular types (or a subtype thereof). In fact, I could even use a record type of some kind if there was some advantage to doing so. I don't actually use the data for anything aside from this simulation; an array of 8 boolean values would work for me. Hmmmm. Peter ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-28 13:20 ` Peter C. Chapin @ 2003-12-28 18:40 ` Robert I. Eachus 2003-12-29 2:57 ` Peter C. Chapin 0 siblings, 1 reply; 9+ messages in thread From: Robert I. Eachus @ 2003-12-28 18:40 UTC (permalink / raw) Peter C. Chapin wrote: > Anyway, I have yet to deal with writing CRC checksumming code in Ada > (that's for later). At the moment I'm getting my noisy channel > simulation to work. My Transceive procedure works by looping over the > eight bits in an octet and, if a particular bit is to be corrupted, > XORing a suitable mask into the number to invert the bit. You could certainly do this in Ada with say: type Octet is mod 256; for Octet'Size use 8; -- optional, doesn't affect semantics. But first you should think about where you want to go with this. Right now you are doing unbiased errors. You could also OR 1 and AND 0 error bits in to create a bias toward one or zero. You might also want to implement the error generator as a finite state machine to create burst errors: type State is (Clear, Burst); Current: State := Clear; function Error return Boolean is begin case Current is when Clear => if Random < 0.01 then Current := Burst; return True; else Return False; end if; when Burst => if Random < 0.5 then Current := Clear; return False; else return True; end if; end case; end Error; (You really should start the error generator in a state chosen based on the calculated probability from the generator. I'd just throw the first few dozen bytes away...) As for the choice between a boolean array and a modular type for Octet, it is a close call. Or you could even convert back and forth. Personally, I would use the modular type, and generate error bytes by: function Error_Octet is Result: Octet := 0; begin for I in 1..8 loop Result := Result + Result; -- or 2 * Result; if Error then Result := Result + 1; end loop; return Result; end Error_Octet; Seems simple enough, and allows Error to be as complex a function as you want. In any case, I think you probably want to count an octet as an error if any bits were flipped, and are not really planning on counting the number of errors in a > > The C program that I wrote originally uses the type int for the > simulated octets. My first pass at an Ada translation thus used Integer. > However, I can see that Integer is not entirely the most appropriate > type here so I have no problem using one of the modular types (or a > subtype thereof). In fact, I could even use a record type of some kind > if there was some advantage to doing so. I don't actually use the data > for anything aside from this simulation; an array of 8 boolean values > would work for me. > > Hmmmm. > > Peter -- Robert I. Eachus "The war on terror is a different kind of war, waged capture by capture, cell by cell, and victory by victory. Our security is assured by our perseverance and by our sure belief in the success of liberty." -- George W. Bush ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Newbie question: How does one do bit manipulation in Ada? 2003-12-28 18:40 ` Robert I. Eachus @ 2003-12-29 2:57 ` Peter C. Chapin 0 siblings, 0 replies; 9+ messages in thread From: Peter C. Chapin @ 2003-12-29 2:57 UTC (permalink / raw) In article <bN6dnYXsR6wQvnKiRVn-ug@comcast.com>, rieachus@comcast.net says... > Seems simple enough, and allows Error to be as complex a function as you > want. Thanks for your comments. I will probably experiment with a few of your ideas. I like the thought of making Error a separate function, for example. I hadn't planned on playing around with burst errors, but I might do so. At least it makes sense to lay the groundwork for doing so in a future version. Peter ^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2003-12-29 2:57 UTC | newest] Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2003-12-23 1:58 Newbie question: How does one do bit manipulation in Ada? Peter C. Chapin 2003-12-23 3:01 ` Georg Bauhaus 2003-12-23 3:23 ` Peter C. Chapin 2003-12-23 3:36 ` tmoran 2003-12-23 3:33 ` Steve 2003-12-23 9:09 ` Stephen Leake 2003-12-28 13:20 ` Peter C. Chapin 2003-12-28 18:40 ` Robert I. Eachus 2003-12-29 2:57 ` Peter C. Chapin
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