Mastering Dyalog APL: A Complete Introduction to Dyalog APL

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English is preferred, and other languages are welcome. Your software needs to be packaged as a stand-alone product for electronic distribution. Good documen- tation adds real value to your software. Copyrighted software can't be distributed without permission. Ever and C. Includes routines for multi-precision integer and float operations.

Falkoff, K. E Iverson, E. Iverson, A Programming Language, Wiley, Iverson, Concrete Mathematics Companion. An introduction to modern APL French. Stiers, M. Goovaerts, J. Thomson, Raymond P. A: User groups Note that information on officers may be somewhat out of date. Quarterly journal Quote Quad. Chapter groups around the country. Submitted papers, in Microsoft Word. Alfonseca uam. Annibali uniroma1. The tentative time limit for the new material is September 30th, Chairman:Chuck Kennedy Charles-Kennedy comcast.

Reiner Nussbaum Buchernerstr. Siegel Tel: work email: Siegel acm. Stasney Contacts: astasney earthlink. Stasney jizba gte. Skomorokhov P. Box Bedford, TX U. You can subscribe by sending email to listserve acm. You will be requested to confirm your subscription. You will receive an automatic welcome msg and updated information as it is available. This has greatly reduced sharing of functions among users, and made it difficult for users to migrate between vendors and even between machines. This has been partially implemented by various vendors.

ATF on floppy as intermediate. ATF" extension!!! ATF to file This has the advantage over 1 that the files are human-readable and editable. VisualAPL Unicode text files can also be directly compiled without Visual Studio using the command line compiler provided with the Microsoft. Net framework. All schemes suffer from the basic incompatibility between APL vendors. This includes file IO, graphics etc.

Also the mostly obsolete ibeam functions. A major fight in the community for at least the last 10 years. Each instance of the above requires someone who knows both implementations to rewrite the code. They took forms such as BOX for example. For some reason however, they all used different symbol sets and different transliterations. Hence all are incompatible.

Various users, seeing the advantage of this approach versus the WSIS have tried their hands. Approaches for APL characters : o Jim Weigang has proposed a verbose version that is probably the current standard. See his article at APL Code for various APLs is available at waterloo. See J. APL Quote Quad, 23 1 Sample code for this translator is available in this format.

The APL stack in Dyalog APL

If you're in the market for a new web hosting provider, be sure to check out our user reviews , our A-Z hosting guide and our top three popular hosting picks Your email address will not be published. Not an exclusive APL feature. History of APL In the s, a Harvard University professor created a mathematical notation system that was useful for manipulating complex arrays of information.

APL Syntax Some programming languages have been intentionally designed to be easily read by anyone — programmer or not. APL is not one of these languages. These are just a few of the dozens of symbols used in APL. Nested Multidimensional Arrays Simple one-dimenaional arrays are a common data type in computer programming.

For example, a continuation of the array listed above into a 3 by 3 multidimensional array might look like this: 0 1 2 3 4 5 6 7 8 Nested arrays are another way of enhancing the data contained in an array. Books Quite a few APL texts have been written over the past five decades. Conclusion APL is a mathematically-inclined programming language whose defining features include a highly symbolic syntax and support for nested multidimensional arrays. What Code Should You Learn?

About Jon Penland Jon has worked in many capacities in the high tech world, including engineering and development. He's written many articles for WhoIsHostingThis. He lives in Georgia with his wife and five children. Search our site Need Web Hosting? Thanks for your comment. It will show here once it has been approved. Cancel reply Your email address will not be published. Utilities to be a sibling of the assumed class MyApp. Now within the class you can do. When a namespace is included, the interpreter will execute functions from that namespace as if they had been defined in the current class.

However, the actual code is shared with the original namespace. For example, this means that if the code of means or else is changed while tracing into it from the MyApp class those changes are reflected in. Utilities immediately and any other classes that might have included it. Most of the time, this works as you expect it to, but it can lead to confusion, in particular if you were to COPY.

Utilities from another workspace. This will change the definition of the namespace, but the class has pointers to functions in the old copy of. Utilities , and will not pick up the new definitions until the class is fixed again. If you were to edit these functions while tracing into the MyApp class, the changes will not be visible in the namespace. Utilities , the class will continue to work until the class itself is edited, at which point it will complain that the namespace does not exist.

If at this stage you were to edit Goo and change 'world' to 'Universe' , and then again call Foo. Hello it would still print world to the session. If you encounter this, re-fix your classes in this case Foo. Rebuilding the WS from the source files would be even better.


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There is one major problem with this: when executing the code in the Tracer the line will be executed in one go. If you think you might want to follow the control flow and trace into the individual expressions, you should spread the control structure over 5 lines. In general: if you have a choice between a short and a long expression then go for the short one — unless the long one offers an incentive such as improved readability, better debugging or faster execution speed.

The expression above could then take its most readable form:. It works just fine in simple cases but quickly leads to confusion about which functions are called or edited, and where new names are created. Everything in that WS lives in. We have to move it into a single namespace MyApp. Execute the following steps:. This ensures we really use the same values for important system variables as the WS does by copying their values into the namespace. The last step will save the contents of the namespace.

If the folder v01 or any of its parents do not already exist the -makedir option will create them. There might be minor differences, depending on the version of the ]save user command and the version of SALT you are using. Here are some texts we can use. Many applications have functions like this at their core. By the time we give it a user interface, it will already have important capabilities, such as logging errors and recovering from crashes. Our engine will be based on the TxtToCsv function. It will take one parameter, a fully qualified filepath for a folder or file. If it is a file it will write a sibling CSV.

If it is a folder it will read all the TXT s in the folder, count the letter frequencies and write them as a CSV sibling to the folder. Simple enough. Here we go. You need a text editor that handles Unicode. Set the default font to APL Unicode. They are of course not really constants but ordinary variables. It is a common programming convention to use uppercase letters for constants. This will do nicely for now. We can do better by defining all the Dyalog constants we want to use in a single namespace in.

We have also replaced the remaining integers in the :Case statements by references to symbolic variables in Constants. Now it is simply a choice of whether you want to trace into ProcessFiles or not. In short: more often than not it is good to move a loops :For , :Repeat , :While into its own function or operator. By separating it on two lines you can open an edit window on txt , put a stop vector on line 5, and easily review the content of each file.

While you have configured your session according to your needs when you start the app with a double-click on the DYAPP on a different machine with different settings you have no idea what you are going to get. It is therefore safer — and strongly recommended — to ensure the setting is well-defined. We will come back to this later. They are part of how SALT manages meta data for scripted objects.

But we have not saved a workspace: we will always build a workspace from scripts. Examine the active session. We see. Note that MyApp contains C and U. This is nice: when you type. With version In a runtime interpreter or an EXE , there is no APL session, and output to the session which would have been visible in a development system will simply disappear. If we want to see this output, we need to write it to a log file. But how do we find out where we need to make changes? The easiest way is to associate a callback function with the SessionPrint event as in:.

You can even use this to investigate what is about to be written to the session the left argument of Catch and make the function stop when it reaches the output you are looking for. TxtToCsv has a shy result, so it won't write its result to the session. TxtToCsv needs an argument. The EXE we are about to create must fetch it from the command line.

Now MyApp is ready to be run from the Windows command line, with the name of the file to be processed following the command name. Make this a habit. It makes life easier in the long run. One example: you cannot call from a dfn a function that does not return a result. It is far easier to change a monadic function that has ignored its argument so far to one that actually requires an argument than to change a niladic function to a monadic one later on, especially when the function is called in many places, and this is something you will eventually encounter.

The reason: we want to ensure when we create a namespace with. A common source of confusion is code that relies on system variables having expected values. Your preferred values for those system variables are set in the Dyalog configuration. Whenever you execute then, say,.

But if you send your WS elsewhere then somebody with different values in their Dyalog configuration might load and run your WS. In this environment. This occasionally rarely fails for no obvious reason. If it does fail just try again and you should be fine. If it keeps failing then the by far most common reason is that the EXE is running — you cannot replace an EXE while it is running.

Although you cannot replace a running EXE what you can do is to rename it; that is possible. You can then create a new EXE with the original name. Note that you cannot really debug a console application with Ride; for details see the Debugging a stand-alone EXE chapter.

If you do not check Console application , the program is started as a separate process and you cannot catch the return code. We therefore recommend you clear the Console application checkbox unless you have a good reason to do otherwise. Use the Version button to bind to the EXE information about the application, author, version, copyright and so on. Our EXE works! Note that in the Dyalog Cookbook the words folder and directory are used interchangeably. MyApp 1. That triggered an error in the APL code. The interpreter tried to display an error message and looked for input from a developer from the session.

But a runtime task has no session, so at that point the interpreter popped the alert message and MyApp died. Prior to version However, this is true only if it is a single-threaded application, since workspaces cannot be saved when more than one thread is running. You might recreate them by re-executing the failing line — but that has other dangers, or might fail in a new way.

But Windows saves it anyway! The Logger class and its dependencies will now be included when we build MyApp :. Within MyApp , some changes. First we introduce aliases for the new modules:. We have those already in the Utilities namespace. This violates the DRY principle. We should get rid of one version and use the other everywhere. But how to choose? However, APLTreeUtils is in use for more than 10 years now, it comes with a comprehensive set of test cases and it is documented in detail. That makes the choice rather easy.

Therefore we remove the two functions from Utilities and change CountLetters :. Where is MyApp to write the logfile? We need a folder we know exists. That rules out fullfilepath. We'll write logfiles into a subfolder of the current directory, which we can be sure exists. Where will that be? When the EXE launches, the current directory is set:. If this version of MyApp were for shipping that would be a problem. That is a problem to be solved by an installer. But for this version of MyApp the logfiles are for your eyes only.

You just have to know where they are. We will put them into a subfolder Logs within the current directory. In developing and testing MyApp , we create the active workspace by running MyApp. That, too, is sure to exist. Now we set up the parameters needed to instantiate the Logger class. We shall modify them to match our needs and use the parameter namespace to create the Logger object.

When we introduce proper error handling in chapter 6, we will do away with APL characters in the log file. This is achieved by passing 'flat' as the left argument to ReadUtf8File. We ignore encoding and the newline character and allow it to default to the current operating system. Note that we now log the full command line. In an application that receives its parameters from the command line, this is important to do. This new function will take the command-line argument and return a list of files which may contain zero, one or many filenames:.

In addition we have added calls to MyLogger. Log in appropriate places:. Under Windows it will always be a backslash while under Linux it is always a slash character.

Mastering Dyalog Apl: A Complete Introduction To Dyalog Apl

Although Windows itself is quite relaxed about the separator and accepts a slash as well as a backslash, as soon as you call something else in one way or another you will find that slashes are not accepted. The foreseeable error that aborted the runtime task — an invalid filepath — has now been replaced by a message saying no files were found.

We have also changed the explicit result. So far it has returned the number of bytes written. In case something goes wrong file not found, etc. Alternatively you could set the parameter printToSession — which defaults to 0 — to 1. That would let the Logger class write all the messages not only to the log file but also to the session. That can be quite useful for test cases or during development. You can even stop the Logger class writing to the disk at all by setting fileFlag to 0. The Logger class is designed never to break your application — for obvious reasons. The drawback of this is that if something goes wrong, such as the path becoming invalid because the drive got removed, you would notice only by trying to examine the log files.

You can tell the Logger class that it should not trap all errors by setting the parameter debug to 1. Then Logger will crash if something goes wrong. Export as before and then run the new MyApp. In case you wonder what the 0 in the log file stands for: this reports the thread number that has written to the log file. So far we have used modules from the APLTree project: class and namespace scripts that might be useful when implementing an application.

APLTree also offers applications that support the programmer during her work without becoming part of the application. One of those applications is the LogDog. Its purpose is simply to watch a log file and reflect any changes immediately in the GUI. This is useful for us, as the log file is now our best view of how the application is doing. Download it into the default download location. LogDog does not come with an installer.

All you have to do is to copy it into a folder where you have the right to add, delete and change files. We recommend the Investigate folder option. The reason is: every night at a new log file with a new name is created. To display any new er log file, issue the Investigate folder menu command again. Once you have started LogDog on the MyApp log file you will see something like this:. Note that LogDog comes with an auto-scroll feature, meaning that the latest entries at the bottom of the file are always visible. If you don't want this for any reason just tick the Freeze checkbox.

From now on we will assume you have LogDog always up and running, so that you will get immediate feedback on what is going on when MyApp. We now have MyApp logging its work in a subfolder of the application folder and reporting any problems it has anticipated. Next we need to consider how to handle and report errors we have not anticipated. We should also return some kind of error code to Windows. If MyApp encounters an error, any process calling it needs to know.

But before we are doing this we will discuss how to configure MyApp.

APL Syntax

When you trace through TxtToCsv , the moment you leave the function the Tracer shows the function Cleanup of the Logger class. The function is declared as a destructor. Why that is: a destructor if any is called when the instance of a class is destroyed or shortly thereafter. MyLogger is localized in the header of TxtToCsv , meaning that when TxtToCsv ends, this instance of the Logger class is destroyed and the destructor is invoked. Since the Tracer was up and running, the destructor makes an appearance in the Tracer.

We want our logging and error handling to be configurable. In fact, we will soon have lots of state settings. Several mechanisms are available for storing configuration settings. Microsoft Windows has the Windows Registry. The Windows Registry is held in memory, so it is fast to read. It has been widely used to store configuration settings. Some would say, abused. However, for quite some time it was considered bad practice to have application-specific config files. Everything was expected to go into the Windows Registry.

The pendulum started to swing back the other way now for several years, and application-specific config files become ever more common. We follow a consensus opinion that it is best to minimise the use of the Registry. Settings needed by Windows itself have to be stored in the Registry. For example, associating a file extension with your application, so that double-clicking on its icon launches your application. We will discuss them in their own chapter. The Windows Registry is still an excellent choice for saving user-specific stuff like preferences, themes, recent files etc.

However, you have to make sure that your user has permission to write to the Windows Registry — that's by no means a certainty. INI is the oldest, simplest, and most crude. We want configuration files to be suitable for humans to read and write, so you might consider the robustness of the INI format an advantage. Or a disadvantage: a badly-formed XML document is easy to detect, and a clear indication of an error. Generally, we prefer simplicity and recommend the INI format where it will serve.

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In the chapter on Logging, we considered the question of where to keep application logs. The answer depends in part on what kind of application you are writing. Will there be single or multiple instances? For example, while a web browser might have several windows open simultaneously, it is nonetheless a single instance of the application.

Its user wants to run just one version of it, and for it to remember her latest preferences and browsing history. But a machine may have many users, and each user needs her own preferences and history remembered. Our MyApp program might well form part of other software processes, perhaps running as a service. There might be multiple instances of MyApp running at any time, quite independently of each other, each with quite different configuration settings. But an administrator should be able to revise these settings for a site. So they should be saved somewhere for all users.

Roaming means that no matter which computer a user logs on to in a Windows Domain [ 1 ], her personal settings, preferences, desktop etc. We add one line to MyApp. You can read the IniFiles documentation in a browser with ]ADoc. You can see that localhome is referred to twice in the [Folders] section, and why that is useful. Therefore it will later in the application default to 1 in a development environment and to 0 in a runtime environment. By setting this to either 1 or 0 in the INI file you can force it to be a particular value.

An optional left argument specifies a default value to be returned if the required key is not found. Note that we also changed what Initial returns: a vector of length two, the namespace Config but also an instance of the MyLogger class. Initial was called within StartFromCmdLine , and we are not going to change this but we must change the call as such because now it returns something useful:.

However, we keep it simple here. The configuration parameters, including Accents , are now collected in the namespace Config. The other option is that CountLetters just assumes the Config is around and has a variable Accents in it:. Passing everything through function arguments does not come with a performance penalty. The matter of encapsulating state — which functions have access to state information, and how it is shared between them — is very important.

Poor choices lead to tangled and obscure code. From time to time you will be offered not by us rules that attempt to make the choices simple. For example: never communicate through global or semi-global variables. There is some wisdom in these rules, but they masquerade as satisfactory substitutes for thought, which they are not.

Just as in a natural language, any rule about writing style meets occasions when it can and should be broken. One of the main reasons why globals should be used with great care is that they can easily be confused with local variables with similar or — worse — the same name. If you need to have global variables then we suggest encapsulating them in a dedicated namespace Globals. With a proper search tool like Fire [ 3 ] it is easy to get a report on all lines referring to anything in Globals.

We share these musings here so you can see what we think about when we think about encapsulating state; and also that there is often no clear right answer. We have used the most important features of the IniFiles class, but it has more to offer. We just want to mention some major topics here. In case of a name conflict the last one wins. Here this would mean that machine-specific definitions would overwrite more general ones.

And finally we create a new standalone EXE as before and run it to make sure that everything keeps working. Yes, we need test cases. So-called semi-globals are variables to be read or set by functions to which they are not localised. They are semi-globals , rather than globals, because they are local to either a function or a namespace. Fire stands for Find and Replace. It is a powerful tool for both search and replace operations in the workspace.

Fire is discussed in the chapter Useful user commands. In this situation, obviously you need to debug the EXE. In addition we will make MyApp. For debugging we are going to use Ride. See the Dyalog manuals for information about Ride. If enabled, you can use Ride to hook into a running interpreter, interrupt any running code, investigate, and even change that code. Setting Wait to 1 lets the application wait for a ride.

That simply means it enters an endless loop. If that's something you need to avoid, you have to find other ways to make the EXE communicate with Ride, perhaps by making temporary changes to the code. The approach would be the same in both cases. In MyApp we keep things simple and allow the INI file to rule whether the user may ride into the application or not. Using the default port leaves room for mistakes. Using a dedicated port rather than using the default minimises the risk of connecting to the wrong application.

If you exported the EXE with the Console application checkbox ticked there is a problem. You can connect to the EXE with Ride, but all output goes into the console window. That means you can enter statements in Ride but any response from the interpreter goes to the console window rather than Ride. For debugging we therefore recommend creating the EXE with the check box cleared.

We want to make the ride configurable. That means we cannot do it earlier than after having instantiated the INI file. But not long after either, so we change Initial :. As a result Config. Ride will be 0 if the INI rules that no Ride is permitted, otherwise the port number to be used by Ride.

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This is an important point. There is value in keeping the function independent in this way, but if you suspect that later you will need other parameters in Config , then the flexibility you gain here outweighs the value of keeping the function independent from Config. Now you can start Ride, enter both 'localhost' and the port number as parameters, connect to the interpreter or stand-alone EXE etc. From Note that 2. Prior to version 3. Since 3. This is because the OS does not bother to tell you about dependencies.

We said OS because this is not a Windows-only problem. That is not possible for all functions: for example, a function that starts a thread and must not start a second one for the same task, or a file was tied etc. But most functions can be restartable. In this example, a counter is a better way to iterate. Faster, too. MyApp already anticipates, tests for and reports certain foreseeable problems with the parameters. They might make several attempts to read or write a file before giving up and signalling an error.

We need to handle the events signalled when the utilities give up. It would be a better Windows citizen if it returned custom exit codes, letting a calling program know how it terminated. But we foresee there will be some! A mere typo in the code could break execution. We need a master trap to catch any events that would break execution, save them for analysis, and report them in an orderly way.

How do you see the exit code returned to Windows? You can access it in the command shell like this:. For calling the exported EXE we need the Execute class. For foreseen errors we check in the code and quit when something is wrong, and pass an error code to the calling environment. We define an OK value of zero for completeness; we really are trying to eliminate from our functions numerical constants that the reader has to interpret.

In Windows, an exit code of zero is a normal exit. All the exit codes are defined in this namespace. The function code can refer to them by name, so the meaning is clear. And this is the only definition of the exit-code values. We can convert the numeric value back to the symbolic name with the function GetName :. This is useful when we want to log an error code: the name is telling while the number is meaningless. We could have defined EXIT in. Constants , but we reserve that script for Dyalog constants, keeping it as a component that could be used in other Dyalog applications.

Now the result of TxtToCsv gets passed to Off to be returned to the operating system as an exit code. Note that in this particular case we set a local variable rc. Strictly speaking, this is not necessary. We learned from experience not to call several functions on a single line with the left-most being Off.


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  • If you do, you will regret it one day. This is a special case in the Dyalog parser. Note that we have replaced some constants by calls to functions in FilesAndDirs. You might find this easier to read. In general, we like functions to start at the top and exit at the bottom. Returning from the middle of a function can lead to confusion, and we have acquired a great respect for our capacity to get confused.

    Also, there is no tidying up at the end of the function that we would miss with :Return. In the line with the :Trap we call a niladic function an exception to our rule! The EventCodes class comes with a method GetName that, when fed with an integer, returns the corresponding symbolic name. We can use that to convert return codes to meaningful names:. We can convert this into something that will be useful when we change the function FileRelatedErrorCodes :.

    Now we can change FileRelatedErrorCodes by copying what we've just printed to the session into the function:. The sooner they come to light the better. For that reason we restrict the errors to be trapped to whatever might pop up when it comes to dealing with files and directories.

    Your shipped system must trap all errors. See Unforeseen errors below.

    APL language FAQ

    When you have to trap all errors, use a global flag which will allow you to switch it off in development. Back to ProcessFiles. When it fires, and control advances to its :Else fork, the trap is immediately cleared. This neatly avoids the following pitfall: a trap fires and invokes a handling expression. But the handling expression also breaks, re-invoking the trap in what now becomes an open loop.

    So with :Trap there is no need to reset the trap to avoid an open loop. But you must still consider what might happen if you call other functions in the :Else fork: if they crash the :Trap would fire again! The handling of error codes and messages can easily obscure the rest of the logic.

    Clarity is not always easy to find but is worth striving for. This is particularly true where there is no convenient test for an error, only a trap for when it is encountered. Note that here for the first time we take advantage of the [Config]Trap flag defined in the INI file, which translates to Config.

    Trap at this stage. Note that the exit code is tested against EXIT. OK being 0. The point of defining the codes in EXIT is to make the functions relate to the exit codes only by their names. Our code so far covers the errors we foresee: errors in the parameters, and errors encountered in the file system.

    There remain the unforeseen errors, chief among them, errors in our own code. But the error might not be replicable. It could, for instance, have been produced by ephemeral congestion on a network interfering with file operations. Or the parameters for your app might be so complicated that it is hard to replicate the environment and data with confidence.

    What you really want for analysing the crash is a crash workspace , a picture of the ship when it went down. For this we need a high-level — or global — trap to catch any event not trapped by any specific :Trap statements. We want it to save the workspace for analysis.

    Why not 4, the standard Windows code for a crashed application? The distinction is useful. An exit code of 4 tells you even the trap failed!