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What is the benefit of using paragraphs and sections for executing pieces of code, instead of using a subprogram instead? As far as I can see paragraphs and sections are dangerous because they have an non intuitive control flow, its easy to fall through and execute stuff you never meant to execute, and there is no variable (item) scoping, therefore it encourages a style of programming where everything is visible to everything else. Its a slippery soup.
I read a lot, but I could not find anything related to the comparative benefit of paragraphs/sections vs a subprogram. I also asked online some people in some COBOL forum, but their answers were along the lines of "is this a joke" or "go learn programming"(!!!).
I do not wish to engage in a discussion of stylistic preferences, everyone writes the way that their brain works, I only want to know, is there any benefit to using paragraphs/sections for flow control? As in, are there any COBOL operations that can be done only by using paragraphs/sections? Or is it just a remnant of an early way of thinking about code?
Because no other language I know of has mimicked that, so it either has some mechanical concrete essential reason to exist in COBOL, or it is a stylistic preference of the COBOL people. Can someone illuminate me on what is happening?
These are multiple questions... the two most important ones:
Are there any COBOL operations that can be done only by using paragraphs/sections?
Yes. A likely not complete list:
USE statements in DECLARATIVES can only apply to a paragraph or a section. These are used for handling file errors and exceptions. Not all compilers support this COBOL standard feature in full.
Segmentation (primary: a program that is only partially loaded in memory) is only possible with sections; but that is to be considered a "legacy feature" (at least I don't know of people actually using it this way explicitly); see the comment of Gilbert Le Blanc for more details on this
fall-through, many other languages have this feature with a kind of a switch statement (COBOL's EVALUATE, which is not the same as a common switch but can be used similar has an explicit break and no fall-through)
GO TO DEPENDING ON (could be recoded to achieve something similar with EVALUATE and then either PERFORM, if the paragraphs are expected to fall-through, which is not uncommon, then that creates a lot of extra code)
GO TO in general and especially nice - the old obsolete ALTER statement
PERFORM statement, format 1 "out-of-line"
file state is only shared between programs when you define it as EXTERNAL, and you often want to have a file state being limited to a single program
up to COBOL85: EXIT statement (plain without anything else, actually doing nothing else then a CONTINUE would)
What is the benefit of using paragraphs and sections for executing pieces of code, instead of using a subprogram instead?
shared data (I guess you know of programs with static data or otherwise (module)global data that is shared between functions/methods and also different source code files)
much less overhead than a CALL is
consistency:
you know what's in your code, you don't know what another program does (or at least: you cannot guarantee that it will do the same some years later exactly the same)
easier to extend/change: adding another variable (and also removing part of it, change its size) to a CALL USING means that you also have to adjust the called program - and all programs that call this, even when you place the complete definition in a copybook, which is very reasonable, this means you have to recompile all programs that use this
a section/paragraph is always available (it is already loaded when the program runs), a CALLed program may not be available or lead to an exception, for example because it cannot be loaded as its parameters have changed
less stuff to code
Note: While not all compilers support this you can work around nearly all of the runtime overhead and consistency issue when you use one source files with multiple program definitions in (possibly nested) and using a static call-convention. This likely gives you the "modern" view you aim for with scope-limitation of variables, within the programs either persistent (like local-static) when defined in WORKING-STORAGE or always passed when in LINKAGE or "local-temporary" when in LOCAL-STORAGE.
Should all code of an application be in one program?
[I've added this one to not lead to bad assumptions] Of course not!
Using sub-programs and also user-defined functions (possibly even nested providing the option for "scoped" and "shared" data) is a good thing where you have a "feature boundary" (for example: access to data, user-interface, ...) or with "modern" COBOL where you have a "language boundary" (for example: direct CALLs of C/Java/whatever), but it isn't "jut for limiting a counter to a section" - in this case: either define a variable which state is not guaranteed to be available after any PERFORM or define one for the section/paragraph; in both cases it would be reasonable to use a prefix telling you this.
Using that "separate by boundary" approach also takes care of the "bad habit of everything being seen by everyone" issue (which is in any case only true for "all sections/paragraphs in the same program).
Personal side note: I would only use paragraphs where it is a "shop/team rule" (it is better to stay consistent then to do things different "just because they are better" [still providing an option to possibly change the common rule]) or for GO TO, which I normally not use.
SECTIONs and EXIT SECTION + EXIT PERFORM [CYCLE] (and very rarely GOBACK/EXIT PROGRAM) make paragraphs nearly unnecessary.
very short answer. subroutines!!
Subroutines execute in the context of the calling routine. Two virtues: no parameter passing, easy to create. In some languages, subroutines are private to (and are part of) the calling (invoking) routine (see various dialects of BASIC).
direct answer: Section and Paragraph support a different way of thinking about programming. Higher performance than call subprogram. Supports overlays. The "fall thru" aspect can be quite useful, a feature rather than a vice. They may be necessary depending on what you are doing with a specific COBOL compiler.
See also PL/1, BAL/360, architecture 360/370/...
As a veteran Cobol dinosaur, I would say asking about the benefit is not the right question. I used paragraph (or section) differently than a subprogram. The right question in my opinion is when to use them logically. If I can make an analogy, if you have a Dog java class, you will write Dog-appropriate methods within it. If there's a cat involved, you may need a helper class. In this case the helper class is the subprogram. Though, you can instead code the helper class methods inside the Dog class, but that will be bad coding.
In any other language I would recommend putting self contained functions into subroutines.
However in COBOL not so much. If the code is very likely to be used in other programs then a subroutine is a good idea. Otherwise not!
The reason being the total lack of any checks on the number type or existence of passed parameters at compile time. Small errors in call statements lead to program crashes at run time. Limiting the use of sub-routines and carefully checking the calling code for errors makes for a more reliable program.
Using paragraphs any type mismatch will be flagged at compile time, or, an automatic conversion will occur.
The goto statement is taboo at my work.
So the following question is born...
Is there a situation possible where a goto is the only valid solution?
Originally GOTO was added to Pascal for error handling, including inter procedural forms that Borland(/Embarcadero) never implemented (example: GOTOing from a inner procedure to the parent), just like Borland never implemented other inner function functionality like passing inner functions to procedure-typed parameters.(*)
In that way GOTO can be considered the precursor to exceptions.
There still some practical uses: The last time I checked, jumping out of a nested IF statement with goto was still faster in Delphi then letting the code exit from a nested if naturally.
Optimizations like these are sometimes used in e.g. compression code, and other complex tree processing code with deeply nested loops or conditional statements.
Such routines often still use goto for errorhandling, because it is faster. (exceptions are not only slow, but their border conditions inhibit some optimizations).
One could see this as part of the plain Pascal level of Object Pascal, just like C++ still allows plain C nearly completely.
(of course, since the optimized compression code in Delphi is only delivered in .o form, it is hard to find examples in the Delphi codebase. The JPEG code has some, but that is a C translation)
(*) Original pascal, and IIRC even Turbo Pascal doesn't allow prematurely exiting a procedure with EXIT. Same for CONTINUE and BREAK.
Is there a situation possible where a GOTO is the only valid solution?
I suppose it depends on what you mean by valid. I suppose you are asking if there exists a program that can only be written with the use of the goto statement. In which case the answer is that there is no such program. Delphi is Turing complete with or without the goto statement.
However, if we are prepared to widen the discussion to include other languages, there are situations where goto is a good solution, even the best solution. The scenario that most commonly comes to mind is implementing tidy-up and error handling in languages without structured exception handling. If you peruse the Linux source code you will find that goto is widely used. I expect that the same is true of the Windows source code.
Goto is very old. It predates sub-routines like functions and procedures! It is also very dangerous and can make your code less readable (to others, or to yourself a few months later).
In theory it's not possible to have a situation where goto is required. I won't repeat the theory about Turing tape machines here, but using selection and iteration, you can re-order the code so in all possible input values the same output comes about.
In practice though, it's sometimes 'handy' and 'better readable' to 'jump away' from the flow of code in certain conditions, and that's where Exceptions come in. raise breaks away from the current execution, and jump to the closest finally or except section. This is safer because they work cascaded, and provide a better way to handle the context in case of one of these border conditions. (And there's also breakand abort and exit)
GOTO is never necessary. Any computable algorithm can be expressed with assignment and the combination of IF...THEN, BEGIN...END, and your choice of WHILE...DO...END or REPEAT...UNTIL. You don't even need subroutines. :)
This is known as the structured program theorem.
For a proof, see the 1966 paper, Flow Diagrams, Turing Machines and Languages with Only Two Formation Rules (PDF) by Corrado Böhm and Giuseppe Jacopini.
Something like 15 years ago I used the goto statement in Delphi to convert one of Bob Jenkins's hash functions from C to Pascal. The C function has a switch() statement without breaks after each case, and you can't do that with Pascal's case statement. So I converted it into a bunch of Pascal labels and gotos. I guess you would still have to do it the same way with the newest Delphi versions.
Edit: I guess using gotos would still be a reasonable way to do this. Gets the job done, easy to understand, limited to a short block of code, not dangerous.
I am looking to write a basic profanity filter in a Rails based application. This will use a simply search and replace mechanism whenever the appropriate attribute gets submitted by a user. My question is, for those who have written these before, is there a CSV file or some database out there where a list of profanity words can be imported into my database? We are submitting the words that we will replace the profanities with on our own. We more or less need a database of profanities, racial slurs and anything that's not exactly rated PG-13 to get triggered.
As the Tin Man suggested, this problem is difficult, but it isn't impossible. I've built a commercial profanity filter named CleanSpeak that handles everything mentioned above (leet speak, phonetics, language rules, whitelisting, etc). CleanSpeak is capable of filtering 20,000 messages per second on a low end server, so it is possible to build something that works well and performs well. I will mention that CleanSpeak is the result of about 3 years of on-going development though.
There are a few things I tell everyone that is looking to try and tackle a language filter.
Don't use regular expressions unless you have a small list and don't mind a lot of things getting through. Regular expressions are relatively slow overall and hard to manage.
Determine if you want to handle conjugations, inflections and other language rules. These often add a considerable amount of time to the project.
Decide what type of performance you need and whether or not you can make multiple passes on the String. The more passes you make the slow your filter will be.
Understand the scunthrope and clbuttic problems and determine how you will handle these. This usually requires some form of language intelligence and whitelisting.
Realize that whitespace has a different meaning now. You can't use it as a word delimiter any more (b e c a u s e of this)
Be careful with your handling of punctuation because it can be used to get around the filter (l.i.k.e th---is)
Understand how people use ascii art and unicode to replace characters (/ = v - those are slashes). There are a lot of unicode characters that look like English characters and you will want to handle those appropriately.
Understand that people make up new profanity all the time by smashing words together (likethis) and figure out if you want to handle that.
You can search around StackOverflow for my comments on other threads as I might have more information on those threads that I've forgotten here.
Here's one you could use: Offensive/Profane Word List from CMU site
Based on personal experience, you do understand that it's an exercise in futility?
If someone wants to inject profanity, there's a slew of words that are innocent in one context, and profane in another so you'll have to write a context parser to avoid black-listing clean words. A quick glance at CMU's list shows words I'd never consider rude/crude/socially unacceptable. You'll see there are many words that could be proper names or nouns, countries, terms of endearment, etc. And, there are myriads of ways to throw your algorithm off using L33T speak and such. Search Wikipedia and the internets and you can build tables of variations of letters.
Look at CMU's list and imagine how long the list would be if, in addition to the correct letter, every a could also be 4, o could be 0 or p, e could be 3, s could be 5. And, that's a very, very, short example.
I was asked to do a similar task and wrote code to generate L33T variations of the words, and generated a hit-list of words based on several profanity/offensive lists available on the internet. After running the generator, and being a little over 1/4 of the way through the file, I had over one million entries in my DB. I pulled the plug on the project at that point, because the time spent searching, even using Perl's Regex::Assemble, was going to be ridiculous, especially since it'd still be so easy to fool.
I recommend you have a long talk with whoever requested that, and ask if they understand the programming issues involved, and low-likelihood of accuracy and success, especially over the long-term, or the possible customer backlash when they realize you're censoring them.
I have one that I've added to (obfuscated a bit) but here it is: https://github.com/rdp/sensible-cinema/blob/master/lib/subtitle_profanity_finder.rb
I am trying to sort array/lists/whatever of data based upon the unicode string values in them which contain non-english characters, I want them sorted correctly alphabetically.
I have written a lot of code (D2010, win XP), which I thought was pretty solid for future internationalisation, but it is not. Its all using unicodestring (string) data type, which up until now I have just been putting english characters into the unicode strings.
It seems I have to own up to making a very serious unicode mistake. I talked to my German friend, and tried out some German ß's, (ß is 'ss' and should come after S and before T in alphabet) and and ö's etc (note the umlaut) and none of my sorting algorithms work anymore. Results are very mixed up. Garbage.
Since then I have been reading up extensively and learnt a lot of unpleasant things with regards to unicode collation. Things are looking grim, much grimmer than I ever expected, I have seriously messed this up. I hope I am missing something and things are not actually quite as grim as they appear at present. I have been tinkering around looking at windows api calls (RtlCompareUnicodeString) with no success (protection faults), I could not get it to work. Problem with API calls I learnt is that they change on various newer windows platforms, and also with delphi going cross plat soon, with linux later, my app is client server so I need to be concerned about this, but tbh with the situation being what is it (bad) I would be grateful for any forward progress, ie win api specific.
Is using win api function RtlCompareUnicodeString to obvious solution? If so I should really try again with that but tbh I have been taken aback by all of the issues involved with unicode collation and I not clear at all what I should be doing to compare these strings this way anyway.
I learnt of the IBM ICU c++ opensource project, there is a delphi wrapper for it albeit for an older version of ICU. It seems a very comprehensive solution which is platform independant. Surely I cannot be looking at creating a delphi wrapper for this (or updating the existing one) to get a good solution for unicode collation?
I would be extremely glad to hear advice at two levels :-
A) A windows specific non portable solution, I would be glad off that at the moment, forget the client server ramifications!
B) A more portable solution which is immune from the various XP/vista/win7 variations of unicode api functions, therefore putting me in good stead for XE2 mac support and future linux support, not to mention the client server complications.
Btw I dont really want to be doing 'make-do' solutions, scanning strings prior to comparison and replacing certain tricky characters etc, which I have read about. I gave the German examplle above, thats just an example, I want to get it working for all (or at least most, far east, russian) languages, I don't want to do workarounds for a specific language or two. I also do not need any advice on the sorting algorithms, they are fine, its just the string comparison bit that's wrong.
I hope I am missing/doing something stupid, this all looks to be a headache.
Thank you.
EDIT, Rudy, here is how I was trying to call RtlCompareUnicodeString. Sorry for the delay I have been having a horrible time with this.
program Project26
{$APPTYPE CONSOLE}
uses
SysUtils;
var
a,b:ansistring;
k,l:string;
x,y:widestring;
r:integer;
procedure RtlInitUnicodeString(
DestinationString:pstring;
SourceString:pwidechar) stdcall; external 'NTDLL';
function RtlCompareUnicodeString(
String1:pstring;
String2:pstring;
CaseInSensitive:boolean
):integer stdcall; external 'NTDLL';
begin
x:='wef';
y:='fsd';
RtlInitUnicodeString(#k, pwidechar(x));
RtlInitUnicodeString(#l, pwidechar(y));
r:=RtlCompareUnicodeString(#k,#l,false);
writeln(r);
readln;
end.
I realise this is most likely wrong, I am not used to calling api unctions directly, this is my best guess.
About your StringCompareEx api function. That looked really good, but is avail on Vista + only, I'm using XP. StringCompare is on XP, but that's not Unicode!
To recap, the basic task afoot, is to compare two strings, and to do so based on the character sort order specified in the current windows locale.
Can anyone say for sure if ansicomparetext should do this or not? It don't work for me, but others have said it should, and other things i have read suggest it should.
This is what I get with 31 test strings when using AnsiCompareText when in German Locale (space delimited - no strings contain spaces) :-
arß Asß asß aßs no nö ö ön oo öö oöo öoö öp pö ss SS ßaß ßbß sß Sßa
Sßb ßß ssss SSSS ßßß ssßß SSßß ßz ßzß z zzz
EDIT 2.
I am still keen to hear if I should expect AnsiCompareText to work using the locale info, as lkessler has said so, and lkessler has also posted about these subjects before and seems have been through this before.
However, following on from Rudy's advice I have also been checking out CompareStringW - which shares the same documentation with CompareString, so it is NOT non-unicode as I have stated earlier.
Even if AnsiCompareText is not going to work, although I think it should, the win32api function CompareStringW should indeed work. Now I have defined my API function, and I can call it, and I get a result, and no error... but i get the same result everytime regardless of the input strings! It returns 1 everytime - which means less than. Here's my code
var
k,l:string;
function CompareStringW(
Locale:integer;
dwCmpFlags:longword;
lpString1:pstring;
cchCount1:integer;
lpString2:pstring;
cchCount2:integer
):integer stdcall; external 'Kernel32.dll';
begin;
k:='zzz';
l:='xxx';
writeln(length(k));
r:=comparestringw(LOCALE_USER_DEFAULT,0,#k,3,#l,3);
writeln(r); // result is 1=less than, 2=equal, 3=greater than
readln;
end;
I feel I am getting somewhere now after much pain. Would be glad to know about AnsiCompareText, and what I am doing wrong with the above CompareStringW api call. Thank you.
EDIT 3
Firstly, I fixed the api call to CompareStringW myself, I was passing in #mystring when I should do PString(mystring). Now it all works correctly.
r:=comparestringw(LOCALE_USER_DEFAULT,0,pstring(k),-1,pstring(l),-1);
Now, you can imagine my dismay when I still got the same sort result as I did right at the beginning...
arß asß aßs Asß no nö ö ön oo öö oöo öoö öp pö ss SS ßaß ßbß sß Sßa
Sßb ßß ssss SSSS ßßß ssßß SSßß ßz ßzß z zzz
You may also imagine my EXTREME dismay not to mention simultaneous joy when I realised the sort order IS CORRECT, and IT WAS CORRECT RIGHT BACK IN THE BEGGINING! It make sme sick to say it, but there was never any problem in the first place - this is all down to my lack of German knowledge. I beleived the sort was wrong, since you can see above string start with S, then later they start with ß, then s again and back to ß and so on. Well I can't speak German however I could still clearly see that they was not sorted correctly - my German friend told me ß comes after S and before T... I WAS WRONG! What is happening is that string functions (both AnsiCompareText and winapi CompareTextW) are SUBSTITUTING every 'ß' with 'ss', and every 'ö' with a normal 'o'... so if i take those result above and to a search and replace as described I get...
arss asss asss Asss no no o on oo oo ooo ooo op po ss SS ssass ssbss
sss Sssa Sssb ssss ssss SSSS ssssss ssssss SSssss ssz sszss z zzz
Looks pretty correct to me! And it always was.
I am extremely grateful for all the advice given, and extremely sorry to have wasted your time like this. Those german ß's got me all confused, there was never nothing wrong with the built in delphi function or anything else. It just looked like there was. I made the mistake of combining them with normal 's' in my test data, any other letter would have not have created this illusion of un-sortedness! The squiggly ß's have made me look a fool! ßs!
Rudy and lkessler we're both especially helpful, ty both, I have to accept lkessler's answer as most correct, sorry Rudy.
You said you had problems calling Windows API calls yourself. Could you post the code, so people here can see why it failed? It is not as hard as it may seem, but it does require some care. ISTM that RtlCompareUnicodeStrings() is too low level.
I found a few solutions:
Non-portable
You could use the Windows API function CompareStringEx. This will compare using Unicode specific collation types. You can specify how you want this done (see link). It does require wide strings, i.e. PWideChar pointers to them. If you have problems calling it, give a holler and I'll try to add some demo code.
More or less portable
To make this more or less portable, you could write a function that compares two strings and use conditional defines to choose the different comparison APIs for the platform.
Try using CompareStr for case sensitive, or CompareText for case insensitive if you want your sorts exactly the same in any locale.
And use AnsiCompareStr for case sensitive, or AnsiCompareText for case insensitive if you want your sorts to be specific to the locale of the user.
See: How can I get TStringList to sort differently in Delphi for a lot more information on this.
In Unicode the numeric order of the characters is certainly not the sorting sequence. AnsiCompareText as mentioned by HeartWare does take locale specifics into consideration when comparing characters, but, as you found out, does nothing wrt the sorting order. What you are looking for is called the collation sequence of a language, which specifies the alphabetic sorting order for a language taking diacritics etc into consideration. They were sort of implied in the old Ansi Code pages, though those didn't account for sorting difference between languages using the same character set either.
I checked the D2010 docs. Apart from some TIB* components I didn't find any links. C++ builder does seem to have a compare function that takes collation into account, but that's not much use in Delphi. There you will probably have to use some Windows' API functions directly.
Docs:
Sorting collate all out: http://www.siao2.com/2008/12/06/9181413.aspx
Collation terminology: http://msdn.microsoft.com/en-us/library/ms143726(SQL.90).aspx (though that pertains to MS SQL 2005, it may be helpful)
The 'Sorting "Collate" all out' article is by Michael Kaplan, someone who has great in-depth knowledge of all things Unicode and all intricacies of various languages. His blog has been invaluable to me when porting from D2006 to D2009.
Have you tried AnsiCompareText ? Even though it is called "Ansi", I believe it calls on to an OS-specific Unicode-able comparison routine...
It should also make you safe from cross-platform dependencies (provided that Embarcadero supplies a compatible version in the various OS's they target).
I do not know how good the comparison works with the various strange Unicode ways to encode strings, but try it out and let us know the result...
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I'm building an application that receives source code as input and analyzes several aspects of the code. It can accept code from many common languages, e.g. C/C++, C#, Java, Python, PHP, Pascal, SQL, and more (however many languages are unsupported, e.g. Ada, Cobol, Fortran). Once the language is known, my application knows what to do (I have different handlers for different languages).
Currently I'm asking the user to input the programming language the code is written in, and this is error-prone: although users know the programming languages, a small percentage of them (on rare occasions) click the wrong option just due to recklessness, and that breaks the system (i.e. my analysis fails).
It seems to me like there should be a way to figure out (in most cases) what the language is, from the input text itself. Several notes:
I'm receiving pure text and not file names, so I can't use the extension as a hint.
The user is not required to input complete source codes, and can also input code snippets (i.e. the include/import part may not be included).
it's clear to me that any algorithm I choose will not be 100% proof, certainly for very short input codes (e.g. that could be accepted by both Python and Ruby), in which cases I will still need the user's assistance, however I would like to minimize user involvement in the process to minimize mistakes.
Examples:
If the text contains "x->y()", I may know for sure it's C++ (?)
If the text contains "public static void main", I may know for sure it's Java (?)
If the text contains "for x := y to z do begin", I may know for sure it's Pascal (?)
My question:
Are you familiar with any standard library/method for figuring out automatically what the language of an input source code is?
What are the unique code "tokens" with which I could certainly differentiate one language from another?
I'm writing my code in Python but I believe the question to be language agnostic.
Thanks
Vim has a autodetect filetype feature. If you download vim sourcecode you will find a /vim/runtime/filetype.vim file.
For each language it checks the extension of the file and also, for some of them (most common), it has a function that can get the filetype from the source code. You can check that out. The code is pretty easy to understand and there are some very useful comments there.
build a generic tokenizer and then use a Bayesian filter on them. Use the existing "user checks a box" system to train it.
Here is a simple way to do it. Just run the parser on every language. Whatever language gets the farthest without encountering any errors (or has the fewest errors) wins.
This technique has the following advantages:
You already have most of the code necessary to do this.
The analysis can be done in parallel on multi-core machines.
Most languages can be eliminated very quickly.
This technique is very robust. Languages that might appear very similar when using a fuzzy analysis (baysian for example), would likely have many errors when the actual parser is run.
If a program is parsed correctly in two different languages, then there was never any hope of distinguishing them in the first place.
I think the problem is impossible. The best you can do is to come up with some probability that a program is in a particular language, and even then I would guess producing a solid probability is very hard. Problems that come to mind at once:
use of features like the C pre-processor can effectively mask the underlyuing language altogether
looking for keywords is not sufficient as the keywords can be used in other languages as identifiers
looking for actual language constructs requires you to parse the code, but to do that you need to know the language
what do you do about malformed code?
Those seem enough problems to solve to be going on with.
One program I know which even can distinguish several different languages within the same file is ohcount. You might get some ideas there, although I don't really know how they do it.
In general you can look for distinctive patterns:
Operators might be an indicator, such as := for Pascal/Modula/Oberon, => or the whole of LINQ in C#
Keywords would be another one as probably no two languages have the same set of keywords
Casing rules for identifiers, assuming the piece of code was writting conforming to best practices. Probably a very weak rule
Standard library functions or types. Especially for languages that usually rely heavily on them, such as PHP you might just use a long list of standard library functions.
You may create a set of rules, each of which indicates a possible set of languages if it matches. Intersecting the resulting lists will hopefully get you only one language.
The problem with this approach however, is that you need to do tokenizing and compare tokens (otherwise you can't really know what operators are or whether something you found was inside a comment or string). Tokenizing rules are different for each language as well, though; just splitting everything at whitespace and punctuation will probably not yield a very useful sequence of tokens. You can try several different tokenizing rules (each of which would indicate a certain set of languages as well) and have your rules match to a specified tokenization. For example, trying to find a single-quoted string (for trying out Pascal) in a VB snippet with one comment will probably fail, but another tokenizer might have more luck.
But since you want to perform analysis anyway you probably have parsers for the languages you support, so you can just try running the snippet through each parser and take that as indicator which language it would be (as suggested by OregonGhost as well).
Some thoughts:
$x->y() would be valid in PHP, so ensure that there's no $ symbol if you think C++ (though I think you can store function pointers in a C struct, so this could also be C).
public static void main is Java if it is cased properly - write Main and it's C#. This gets complicated if you take case-insensitive languages like many scripting languages or Pascal into account. The [] attribute syntax in C# on the other hand seems to be rather unique.
You can also try to use the keywords of a language - for example, Option Strict or End Sub are typical for VB and the like, while yield is likely C# and initialization/implementation are Object Pascal / Delphi.
If your application is analyzing the source code anyway, you code try to throw your analysis code at it for every language and if it fails really bad, it was the wrong language :)
My approach would be:
Create a list of strings or regexes (with and without case sensitivity), where each element has assigned a list of languages that the element is an indicator for:
class => C++, C#, Java
interface => C#, Java
implements => Java
[attribute] => C#
procedure => Pascal, Modula
create table / insert / ... => SQL
etc. Then parse the file line-by-line, match each element of the list, and count the hits.
The language with the most hits wins ;)
How about word frequency analysis (with a twist)? Parse the source code and categorise it much like a spam filter does. This way when a code snippet is entered into your app which cannot be 100% identified you can have it show the closest matches which the user can pick from - this can then be fed into your database.
Here's an idea for you. For each of your N languages, find some files in the language, something like 10-20 per language would be enough, each one not too short. Concatenate all files in one language together. Call this lang1.txt. GZip it to lang1.txt.gz. You will have a set of N langX.txt and langX.txt.gz files.
Now, take the file in question and append to each of he langX.txt files, producing langXapp.txt, and corresponding gzipped langXapp.txt.gz. For each X, find the difference between the size of langXapp.gz and langX.gz. The smallest difference will correspond to the language of your file.
Disclaimer: this will work reasonably well only for longer files. Also, it's not very efficient. But on the plus side you don't need to know anything about the language, it's completely automatic. And it can detect natural languages and tell between French or Chinese as well. Just in case you need it :) But the main reason, I just think it's interesting thing to try :)
The most bulletproof but also most work intensive way is to write a parser for each language and just run them in sequence to see which one would accept the code. This won't work well if code has syntax errors though and you most probably would have to deal with code like that, people do make mistakes. One of the fast ways to implement this is to get common compilers for every language you support and just run them and check how many errors they produce.
Heuristics works up to a certain point and the more languages you will support the less help you would get from them. But for first few versions it's a good start, mostly because it's fast to implement and works good enough in most cases. You could check for specific keywords, function/class names in API that is used often, some language constructions etc. Best way is to check how many of these specific stuff a file have for each possible language, this will help with some syntax errors, user defined functions with names like this() in languages that doesn't have such keywords, stuff written in comments and string literals.
Anyhow you most likely would fail sometimes so some mechanism for user to override language choice is still necessary.
I think you never should rely on one single feature, since the absence in a fragment (e.g. somebody systematically using WHILE instead of for) might confuse you.
Also try to stay away from global identifiers like "IMPORT" or "MODULE" or "UNIT" or INITIALIZATION/FINALIZATION, since they might not always exist, be optional in complete sources, and totally absent in fragments.
Dialects and similar languages (e.g. Modula2 and Pascal) are dangerous too.
I would create simple lexers for a bunch of languages that keep track of key tokens, and then simply calculate a key tokens to "other" identifiers ratio. Give each token a weight, since some might be a key indicator to disambiguate between dialects or versions.
Note that this is also a convenient way to allow users to plugin "known" keywords to increase the detection ratio, by e.g. providing identifiers of runtime library routines or types.
Very interesting question, I don't know if it is possible to be able to distinguish languages by code snippets, but here are some ideas:
One simple way is to watch out for single-quotes: In some languages, it is used as character wrapper, whereas in the others it can contain a whole string
A unary asterisk or a unary ampersand operator is a certain indication that it's either of C/C++/C#.
Pascal is the only language (of the ones given) to use two characters for assignments :=. Pascal has many unique keywords, too (begin, sub, end, ...)
The class initialization with a function could be a nice hint for Java.
Functions that do not belong to a class eliminates java (there is no max(), for example)
Naming of basic types (bool vs boolean)
Which reminds me: C++ can look very differently across projects (#define boolean int) So you can never guarantee, that you found the correct language.
If you run the source code through a hashing algorithm and it looks the same, you're most likely analyzing Perl
Indentation is a good hint for Python
You could use functions provided by the languages themselves - like token_get_all() for PHP - or third-party tools - like pychecker for python - to check the syntax
Summing it up: This project would make an interesting research paper (IMHO) and if you want it to work well, be prepared to put a lot of effort into it.
There is no way of making this foolproof, but I would personally start with operators, since they are in most cases "set in stone" (I can't say this holds true to every language since I know only a limited set). This would narrow it down quite considerably, but not nearly enough. For instance "->" is used in many languages (at least C, C++ and Perl).
I would go for something like this:
Create a list of features for each language, these could be operators, commenting style (since most use some sort of easily detectable character or character combination).
For instance:
Some languages have lines that start with the character "#", these include C, C++ and Perl. Do others than the first two use #include and #define in their vocabulary? If you detect this character at the beginning of line, the language is probably one of those. If the character is in the middle of the line, the language is most likely Perl.
Also, if you find the pattern := this would narrow it down to some likely languages.
Etc.
I would have a two-dimensional table with languages and patterns found and after analysis I would simply count which language had most "hits". If I wanted it to be really clever I would give each feature a weight which would signify how likely or unlikely it is that this feature is included in a snippet of this language. For instance if you can find a snippet that starts with /* and ends with */ it is more than likely that this is either C or C++.
The problem with keywords is someone might use it as a normal variable or even inside comments. They can be used as a decider (e.g. the word "class" is much more likely in C++ than C if everything else is equal), but you can't rely on them.
After the analysis I would offer the most likely language as the choice for the user with the rest ordered which would also be selectable. So the user would accept your guess by simply clicking a button, or he can switch it easily.
In answer to 2: if there's a "#!" and the name of an interpreter at the very beginning, then you definitely know which language it is. (Can't believe this wasn't mentioned by anyone else.)