In the Fortran program, is it possible to read an expression including the variables ?
For example, the input file is （if necessary, we can change the input form of the expression,e.g.,the binary form）,
2(a-4b)
It should be noted that the input expression has a very simple form and it only contains integer or fraction or some variables,like the following in the list,
{0,232,-2/5a,3a-b,b/9}
Here 2a means 2*a
The Fortran program is
Program test
implicit none
real(kind=8)::a,b,exp
a=10.
b=3.
! open file and read the input expression
! that is, exp=2*(a-4*b)
write(*,*) exp ! we can get exp=-4.0
end program
For the complicated expressions, it is obviously not a good idea for Fortran. I just want to know, in this simple input expression case, is it possible to find a better way ?
A site with tests of three expression evaluators in Fortran is http://www.angelfire.com/ab5/extensao/report.htm . Only the link to the "Brazilian" one works.
There is also a Sourceforge project fparser http://fparser.sourceforge.net/ .
Fortran cannot do this unless you write code that can parse the arbitrary expression, substitute and solve, which is a bit of work (see the comment below for details). Fortran is compiled and there is no way to load source on the fly, compile and run it, which is essentially what you are asking. You might look into a language such as Lisp where doing this is should be somewhat trivial. Likewise any scripted language will have facilities to evaluate code, which can do what you are asking.
Related
I am implementing an interpreter that codegen to another language using Racket. As a novice I'm trying to avoid macros to the extent that I can ;) Hence I came up with the following "interpreter":
(define op (open-output-bytes))
(define (interpret arg)
(define r
(syntax-case arg (if)
[(if a b) #'(fprintf op "if (~a) {~a}" a b)]))
; other cases here
(eval r))
This looks a bit clumsy to me. Is there a "best practice" for doing this? Am I doing a totally crazy thing here?
Short answer: yes, this is a reasonable thing to do. The way in which you do it is going to depend a lot on the specifics of your situation, though.
You're absolutely right to observe that generating programs as strings is an error-prone and fragile way to do it. Avoiding this, though, requires being able to express the target language at a higher level, in order to circumvent that language's parser.
Again, it really has a lot to do with the language that you're targeting, and how good a job you want to do. I've hacked together things like this for generating Python myself, in a situation where I knew I didn't have time to do things right.
EDIT: oh, you're doing Python too? Bleah! :)
You have a number of different choices. Your cleanest choice is to generate a representation of Python AST nodes, so you can either inject them directly or use existing serialization. You're going to ask me whether there are libraries for this, and ... I fergits. I do believe that the current Python architecture includes ... okay, yes, I went and looked, and you're in good shape. Python's "Parser" module generates ASTs, and it looks like the AST module can be constructed directly.
https://docs.python.org/3/library/ast.html#module-ast
I'm guessing your cleanest path would be to generate JSON that represents these AST modules, then write a Python stub that translates these to Python ASTs.
All of this assumes that you want to take the high road; there's a broad spectrum of in-between approaches involving simple generalizations of python syntax (e.g.: oh, it looks like this kind of statement has a colon followed by an indented block of code, etc.).
If your source language shares syntax with Racket, then use read-syntax to produce a syntax-object representing the input program. Then use recursive descent using syntax-case or syntax-parse to discern between the various constructs.
Instead of printing directly to an output port, I recommend building a tree of elements (strings, numbers, symbols etc). The last step is then to print all the elements of the tree. Representing the output using a tree is very flexible and allows you to handle sub expressions out of order. It also allows you to efficiently concatenate output from different sources.
Macros are not needed.
In MATLAB, you can declare a function dependency with:
%#function myExtraFunctionName
Doing so tells MATLAB that myExtraFunctionName is required by the script or function to operate, even if it's called by an eval statement or some other method that the various dependency checkers or compilers can't figure out.
I have several files that load in .mat or other data files that are required for the script to run, and I would like to include them in a similar manner so that when I run a dependency check with, say fList = matlab.codetools.requiredFilesAndProducts, it will find these data files as well. Ultimately what I would like to be able to do is generate the list of files and pass it to zip to archive every file required to run a given script or function, including data files.
Trying to find any documentation on this feature is challenging because the MATLAB help won't let you just type in %# and searching for %#function just searches for function. Google does the same thing: "hash percent function" returns lots of information on hash tables, "%#function matlab" strips out the important characters, and "declare matlab function" "declare matlab function dependency" turns up nothing useful. I don't remember where I encountered this syntax, so I don't even know if this is a documented feature or not.
I have two questions:
Can someone point me to documentation on this syntax along with some clues as to what keywords I should be using to search?
Can this be used to declare dependencies other than m-files and, if not, how can I go about doing that?
%#function is a pragma directive that informs MATLAB Compiler that the specified function will be called indirectly using feval, eval, or the like.
This is important because the static code analyzer will not be able to detect such dependencies on its own. For instance the name of the function could be stored in a string as in:
fcn = 'myFunction';
feval(fcn)
As far as I know, this is only used by the MATLAB Compiler, nothing else.
There are other similar pragmas. For example MATLAB Coder has %#codegen compiler directive.
I don't have any answer, but maybe you can use this website:
http://www.symbolhound.com/
It let you do search using symbols.
I am totally a newbie in Matlab
I want to ask that when we write a program in Matlab software or IDE and save it with a
.m (dot m) file and then compile and execute it, then that .m (dot m) file is converted into which file? I want to know this because i heard that matlab is platform independent and i did google this but i got converting matlab file to C, C++ etc
Sorry for the silly question and thanks in advance.
Matlab is an interpreted language. So in most cases there is no persistent intermediate form. However, there is an encrypted intermediate form called pcode and there are also the MATLAB compiler and MATLAB coder which delivers code in other high level languages such as C.
edit:
pcode is not generated automatically and should be platform/version independent. But it's major purpose is to encrypt the code, not to compile it (although, it does some partial compilation). To use pcode, you still need the MATLAB environment installed, so in many ways it acts like interpreted code.
But from your follow-up question I guess you don't quite understand how MATLAB works. The code gets interpreted (although with a bit of Just-In-Time Compilation), so there is no need for a persistent intermediate code file: the actual data structures representing your code are maintained by MATLAB. In contrast to compiled languages, where your development cycle is something like "write code, compile & link, execute", the compilation (actually: interpretation) step is part of the execution, so you end up with "write code, execute" in most of the cases.
Just to give you some intuitive understanding of the difference between a compiler and an interpreter. A compiler translates a high level language to a lower level language (let's say machine code that can be executed by your computer). Afterwards that compiled code (most likely stored in a file) is executed by your computer. An interpreter on the other hand, interprets your high level code piece by piece, determining what machine code corresponds to your high level code during the runtime of the program and immediately executes that machine code. So there is no real need to have a machine code equivalent of your entire program available (so in many cases an interpreter will not store the complete machine code, as that is just wasted effort and space).
You could look at interpretation more or less as a human would interpret code: when you try to manually determine the output of some code, you follow the calculations line by line and keep track of your results. You don't generally translate that entire code into some different form and afterwards execute that code. And since you don't translate the code entirely, there is no need to persistently store the intermediate form.
As I said above: you can use other tools such as MATLAB coder to convert your MATLAB code to other high languages such as C/C++, or you can use the MATLAB compiler to compile your code to executable form that depends on some runtime libraries. But those are only used in very specific cases (e.g. when you have to deploy a MATLAB application on computers/embedded devices without MATLAB, when you need to improve performance of your code, ...)
note: My explanation about compilers and interpreters is a quick comparison of the archetypal interpreter and compiler. Many real-life cases are somewhere in between, e.g. Java generally compiles to (JVM) bytecode which is then interpreted by the JVM and something similar can be said about the .NET languages and its CLR.
Since MATLAB is an interpreter, you can write code and just execute it from the IDE, without compilation.
If you want to deploy your program, you can use the MATLAB compiler to create an stand-alone executable or a shared library that you can use in a C++ project. On Windows, MATLAB code would compile to an .EXE file or a .DLL file, respectively.
I have an Xtext/Antlr grammar that parses a subset of coffeescript. I have some test cases, but I thought of doing another sort of test:
Generate random, syntactically correct snippets from my Antlr grammar
Feed these snippets to the original coffeescript parser (calling coffee -ne "the sentence")
Check if each sentence is parsed by coffeescript
Thus I could ensure that my parser accepts a proper subset, and it's not too permissive in some cases. Now, I am stuck with the first step. How can I generate sentences from my Antlr grammar (which also makes heavy use of syntactic predicates)? So I'm interested in the opposite of parsing a sentence.
I found some related attempts, but the answers are not using Antlr at all, but a custom grammar in python, or in clojure, or in ruby. I'd prefer a working solution rather than a hint about how it could be implemented.
No, you can't do this. If you look at the code that ANTLR compiles into, you can see that it's only a recognizer, not a generator.
The links you provided are your best bet -- take your ANTLR grammar, strip out all the rules to make it into a formal grammar, and then try to run it through one of those programs.
Or if your coffeescript subset is very small, you could take the approach of generating strings of random tokens and throwing away all the strings that don't parse.
This is C macro weirdness question.
Is it possible to write a macro that takes string constant X ("...") as argument and evaluates to sting Y of same length such that each character of Y is [constant] arithmetic expression of corresponding character of X.
This is not possible, right ?
No, the C preprocessor considers string literals to be a single token and therefore it cannot perform any such manipulation.
What you are asking for should be done in actual C code. If you are worried about runtime performance and wish to delegate this fixed task at compile time, modern optimising compilers should successfully deal with code like this - they can unroll any loops and pre-compute any fixed expressions, while taking code size and CPU cache use patterns into account, which the preprocessor has no idea about.
On the other hand, you may want your code to include such a modified string literal, but do not want or need the original - e.g. you want to have obfuscated text that your program will decode and you do not want to have the original strings in your executable. In that case, you can use some build-system scripting to do that by, for example, using another C program to produce the modified strings and defining them as macros in the C compiler command line for your actual program.
As already said by others, the preprocessor sees entire strings as tokens. There is only one exception the _Pragma operator, that takes a string as argument and tokenizes its contents to pass it to a #pragma directive.
So unless your targeting a _Pragma the only way to do things in the preprocessing phases is to have them written as token sequences, manipulate them and to stringify them at the end.