I've been developing a MathLink application with a function that accepts two lists, e.g.
:Pattern: g[zi_List, fi_List]
which I intended to pull in to the function manually. Both lists can be real or complex with the result being complex if either parameter is complex. Additionally, fi could be a list of square matrices, but zi is to remain a one dimensional list.
Within the MathLink C API, the most straightforward seeming function to use is MLGetReal64Array which can handle both real and complex data types as Complex shows up as the innermost Head of the array. And, once complexity is determined, the array can be cast to std::complex<double> or the C99 complex type, if appropriate. Now, MLGetReal64Array doesn't handle non-rectangular Lists, so each List element must have the dimensionality of the others and be of the same type: real of complex. Oddly, though, with a function that accepts a single List parameter, MLGetReal64Array returns a data structure that has a one element List as its outermost element, i.e. inputing h[ {1, 3, 5} ] returns List[List[1,3,5]] on the c-side of things.
It turns out that for a two list function, like g, a single call to MLGetReal64Array will return both parameters at once, i.e. g receives List[ zi, fi ]. Since I plan on preprocessing each list for uniformity of structure and element type, ensuring that both had the same element type wouldn't be a problem. But, I'd like for fi to be a list of matrices, and MLGetReal64Array causes a MLEGSQ: MLGet() called out of sequence error.
So, my questions are: can I use MLGetReal64Array to get both lists? how would I go about it? And, if I can't use MLGetReal64Array, what are my alternatives?
I'm thinking that if MLGetReal64Array is correct about the structure, I can pop the outer List off the link by using MLGetFunction which would then allow me to use MLGetReal64Array for each parameter. As of yet, I haven't tried it. But, in the meantime, I would appreciate any suggestions.
I'd create separate functions for the different cases you have. It's much easier to handle this logic on the Mathematica side than figure out what you have coming over the link in C.
Related
I know that one can act a function on every member of a list using the map function, e.g.,
f(x):=block([x:x], x^2)$
foo:[1,2,3];
map('f,foo);
However, I haven't been able to work out how one would do the equivalent with an array of lists, and a function that acts on lists (vs. list members).
E.g., given bar:[[1,2,3],[a,b,c]]$ I'd like to find a general way to obtain a list of each member of bar concatenated into a string, i.e., the output would be the same as:
[apply('sconcat, bar[1]), apply('sconcat,bar[2])];
I'd actually like to then concatenate the resulting list; however, that seems pretty straightforward, once at this point.
XY problem
How do I convert an array to a list in PureScript?
arrayToList :: forall a. Array a -> List a
arrayToList = ???
Actual problem
Must I necessarily write this function?
Neither purescript-arrays nor purescript-lists define such a function, which leads me wonder if there is an idiomatic way to deal with arrays in the contexts of functions taking a list.
For example Matrix.getRow returns an array which needs to be transformed into a list of Pux Html elements (in the process of rendering the matrix as HTML). What is the best way to do this?
With compiler version 0.10.2, you can simply write
arrayToList :: forall a. Array a -> List a
arrayToList = ?whatGoesHere
and the compiler will give you a list of things to fill in, based on the type information. ?whatGoesHere is called a typed hole.
In this case, you probably want Data.Array.toUnfoldable or Data.List.fromFoldable.
While reading about Julia on http://learnxinyminutes.com/docs/julia/ I came across this:
# You can define functions that take a variable number of
# positional arguments
function varargs(args...)
return args
# use the keyword return to return anywhere in the function
end
# => varargs (generic function with 1 method)
varargs(1,2,3) # => (1,2,3)
# The ... is called a splat.
# We just used it in a function definition.
# It can also be used in a fuction call,
# where it will splat an Array or Tuple's contents into the argument list.
Set([1,2,3]) # => Set{Array{Int64,1}}([1,2,3]) # produces a Set of Arrays
Set([1,2,3]...) # => Set{Int64}(1,2,3) # this is equivalent to Set(1,2,3)
x = (1,2,3) # => (1,2,3)
Set(x) # => Set{(Int64,Int64,Int64)}((1,2,3)) # a Set of Tuples
Set(x...) # => Set{Int64}(2,3,1)
Which I'm sure is a perfectly good explanation, however I fail to grasp the main idea/benefits.
From what I understand so far:
Using a splat in a function definition allows us to specify that we have no clue how many input arguments the function will be given, could be 1, could be 1000. Don't really see the benefit of this, but at least I understand (I hope) the concept of this.
Using a splat as an input argument to a function does... What exactly? And why would I use it? If I had to input an array's contents into the argument list, I would use this syntax instead: some_array(:,:) (for 3D arrays i would use some_array(:,:,:) etc.).
I think part of the reason why I don't understand this is that I'm struggling with the definition of tuples and arrays, are tuples and arrays data types (like Int64 is a data type) in Julia? Or are they data structures, and what is a data structure? When I hear array I typically think about a 2D matrix, perhaps not the best way to imagine arrays in a programming context?
I realize that you could probably write entire books about what a data structure is, and I could certainly Google it, however I find that people with a profound understanding of a subject are able to explain it in a much more succinct (and perhaps simplified) way then let's say the wikipedia article could, which is why I'm asking you guys (and girls).
You seem like you get the mechanism and how/what they do but are struggling with what you would use it for. I get that.
I find them useful for things where I need to pass an unknown number of arguments and don't want to have to bother constructing an array first before passing it in when working with the function interactively.
for instance:
func geturls(urls::Vector)
# some code to retrieve URL's from the network
end
geturls(urls...) = geturls([urls...])
# slightly nicer to type than building up an array first then passing it in.
geturls("http://google.com", "http://facebook.com")
# when we already have a vector we can pass that in as well since julia has method dispatch
geturls(urlvector)
So a few things to note. Splat's allow you to turn an iterable into an array and vice versa. See the [urls...] bit above? Julia turns that into a Vector with the urls tuple expanded which turns out to be much more useful than the argument splatting itself in my experience.
This is just 1 example of where they've proved useful to me. As you use julia you'll run across more.
It's mostly there to aid in designing api's that feel natural to use.
My project has classes which, unavoidably, contain hundreds upon hundreds of variables that I'm always having to keep straight. For example, I'm always having to keep track of specific kinds of variables for a recurring set of "items" that occur inside of a class, where placing those variables between multiple classes would cause a lot of confusion.
How do I better sort my variables to keep from going crazy, especially when it comes time to save my data?
Am I missing something? Actionscript is an Object Oriented language, so you might have hundreds of variables, but unless you've somehow treated it like a grab bag and dumped it all in one place, everything should be to hand. Without knowing what all you're keeping track of, it's hard to give concrete advice, but here's an example from a current project I'm working on, which is a platform for building pre-employment assessments.
The basic unit is a Question. A Question has a stem, text that can go in the status bar, a collection of answers, and a collection of measures of things we're tracking about what the user does in that particular type of questions.
The measures are, again, their own type of object, and come in two "flavors": one that is used to track a time limit and one that isn't. The measure has a name (so we know where to write back to the database) and a value (which tells us what). Timed ones also have a property for the time limit.
When we need to time the question, we hand that measure to yet another object that counts the time down and a separate object that displays the time (if appropriate for the situation). The answers, known as distractors, have a label and a value that they can impart to the appropriate measure based on the user selection. For example, if a user selects "d", its value, "4" is transferred to the measure that stores the user's selection.
Once the user submits his answer, we loop through all the measures for the question and send those to the database. If those were not treated as a collection (in this case, a Vector), we'd have to know exactly what specific measures are being stored for each question and each question would have a very different structure that we'd have to dig through. So if looping through collections is your issue, I think you should revisit that idea. It saves a lot of code and is FAR more efficient than "var1", "var2", "var3."
If the part you think is unweildy is the type checking you have to do because literally anything could be in there, then Vector could be a good solution for you as long as you're using at least Flash Player 10.
So, in summary:
When you have a lot of related properties, write a Class that keeps all of those related bits and pieces together (like my Question).
When objects have 0-n "things" that are all of the same or very similar, use a collection of some sort, such as an Array or Vector, to allow you to iterate through them as a group and perform the same operation on each (for example, each Question is part of a larger grouping that allows each question to be presented in turn, and each question has a collection of distractors and another of measures.
These two concepts, used together, should help keep your information tidy and organized.
While I'm certain there are numerous ways of keeping arrays straight, I have found a method that works well for me. Best of all, it collapses large amounts of information into a handful of arrays that I can parse to an XML file or other storage method. I call this method my "indexed array system".
There are actually multiple ways to do this: creating a handful of 1-dimensional arrays, or creating 2-dimensional (or higher) array(s). Both work equally well, so choose the one that works best for your code. I'm only going to show the 1-dimensional method here. Those of you who are familiar with arrays can probably figure out how to rewrite this to use higher dimensional arrays.
I use Actionscript 3, but this approach should work with almost any programming or scripting language.
In this example, I'm trying to keep various "properties" of different "activities" straight. In this case, we'll say these properties are Level, High Score, and Play Count. We'll call the activities Pinball, Word Search, Maze, and Memory.
This method involves creating multiple arrays, one for each property, and creating constants that hold the integer "key" used for each activity.
We'll start by creating the constants, as integers. Constants work for this, because we never change them after compile. The value we put into each constant is the index the corresponding data will always be stored at in the arrays.
const pinball:int = 0;
const wordsearch:int = 1;
const maze:int = 2;
const memory:int = 3;
Now, we create the arrays. Remember, arrays start counting from zero. Since we want to be able to modify the values, this should be a regular variable.
Note, I am constructing the array to be the specific length we need, with the default value for the desired data type in each slot. I've used all integers here, but you can use just about any data type you need.
var highscore:Array = [0, 0, 0, 0];
var level:Array = [0, 0, 0, 0];
var playcount:Array = [0, 0, 0, 0];
So, we have a consistent "address" for each property, and we only had to create four constants, and three arrays, instead of 12 variables.
Now we need to create the functions to read and write to the arrays using this system. This is where the real beauty of the system comes in. Be sure this function is written in public scope if you want to read/write the arrays from outside this class.
To create the function that gets data from the arrays, we need two arguments: the name of the activity and the name of the property. We also want to set up this function to return a value of any type.
GOTCHA WARNING: In Actionscript 3, this won't work in static classes or functions, as it relies on the "this" keyword.
public function fetchData(act:String, prop:String):*
{
var r:*;
r = this[prop][this[act]];
return r;
}
That queer bit of code, r = this[prop][this[act]], simply uses the provided strings "act" and "prop" as the names of the constant and array, and sets the resulting value to r. Thus, if you feed the function the parameters ("maze", "highscore"), that code will essentially act like r = highscore[2] (remember, this[act] returns the integer value assigned to it.)
The writing method works essentially the same way, except we need one additional argument, the data to be written. This argument needs to be able to accept any
GOTCHA WARNING: One significant drawback to this system with strict typing languages is that you must remember the data type for the array you're writing to. The compiler cannot catch these type errors, so your program will simply throw a fatal error if it tries to write the wrong value type.
One clever way around this is to create different functions for different data types, so passing the wrong data type in an argument will trigger a compile-time error.
public function writeData(act:String, prop:String, val:*):void
{
this[prop][this[act]] = val;
}
Now, we just have one additional problem. What happens if we pass an activity or property name that doesn't exist? To protect against this, we just need one more function.
This function will validate a provided constant or variable key by attempting to access it, and catching the resulting fatal error, returning false instead. If the key is valid, it will return true.
function validateName(ID:String):Boolean
{
var checkthis:*
var r:Boolean = true;
try
{
checkthis = this[ID];
}
catch (error:ReferenceError)
{
r = false;
}
return r;
}
Now, we just need to adjust our other two functions to take advantage of this. We'll wrap the function's code inside an if statement.
If one of the keys is invalid, the function will do nothing - it will fail silently. To get around this, just put a trace (a.k.a. print) statement or a non-fatal error in the else construct.
public function fetchData(act:String, prop:String):*
{
var r:*;
if(validateName(act) && validateName(prop))
{
r = this[prop][this[act]];
return r;
}
}
public function writeData(act:String, prop:String, val:*):void
{
if(validateName(act) && validateName(prop))
{
this[prop][this[act]] = val;
}
}
Now, to use these functions, you simply need to use one line of code each. For the example, we'll say we have a text object in the GUI that shows the high score, called txtHighScore. I've omitted the necessary typecasting for the sake of the example.
//Get the high score.
txtHighScore.text = fetchData("maze", "highscore");
//Write the new high score.
writeData("maze", "highscore", txtHighScore.text);
I hope ya'll will find this tutorial useful in sorting and managing your variables.
(Afternote: You can probably do something similar with dictionaries or databases, but I prefer the flexibility with this method.)
I have a need for an efficient sort that doesn't have a callback, but is as customizable as using qsort(). What I want is for it to work like an iterator, where it continuously calls into the sort API in a loop until it is done, doing the comparison in the loop rather than off in a callback function. This way the custom comparison is local to the calling function (and therefore has access to local variables, is potentially more efficient, etc). I have implemented this for an inefficient selection sort, but need it to be efficient, so prefer a quick sort derivative.
Has anyone done anything like this? I tried to do it for quick sort, but trying to turn the algorithm inside out hurt my brain too much.
Below is how it might look in use.
// the array of data we are sorting
MyData array[5000], *firstP, *secondP;
// (assume data is filled in)
Sorter sorter;
// initialize sorter
int result = sortInit (&sorter, array, 5000,
(void **)&firstP, (void **)&secondP, sizeof(MyData));
// loop until complete
while (sortIteration (&sorter, result) == 0) {
// here's where we do the custom comparison...here we
// just sort by member "value" but we could do anything
result = firstP->value - secondP->value;
}
Turning the sort function inside out as you propose isn't likely to make it faster. You're trading indirection on the comparison function for indirection on the item pointers.
It appears you want your comparison function to have access to state information. The quick-n-dirty way to create global variables or a global structure, assuming you don't have more than one thread going at once. The qsort function won't return until all the data is sorted, so in a single threaded environment this should be safe.
The only other thing I would suggest is to locate a source to qsort and modify it to take an extra parameter, a pointer to your state structure. You can then pass this pointer into your comparison function.
Take an existing implementation of qsort and update it to reference the Sorter object for its local variables. Instead of calling a compare function passed in, it would update its state and return to the caller.
Because of recursion in qsort, you'll need to keep some sort of a state stack in your Sorter object. You could accomplish that with an array or a linked-list using dynamic allocation (less efficient). Since most qsort implementations use tail recursion for the larger half and make a recursive call to qsort for the smaller half of the pivot point, you can sort at least 2n elements if your array can hold n states.
A simple solution is to use a inlineble sort function and a inlineble compare callback. When compiled with optimisation, both call get flatten into each other exactly like you want. The only downside is that your choice of sort algorithm is limited because if you recurse or alloc more memory you potentially lose any benefit from doing this. Method with small overhead, like this, work best with small data set.
You can use generic sort function with compare method, size, offset and stride.This way custom comparison can be done by parameter rather then callback. With this way you can use any algorithm. Just use some macro to fill in the most common case because you will have a lot of function argument.
Also, check out the STB library (https://github.com/nothings/stb).
It has sorting function similar to this among many other useful C tools.
What you're asking for has already been done -- it's called std::sort, and it's already in the C++ standard library. Better support for this (among many other things) is part of why well-written C++ is generally faster than C.
You could write a preprocessor macro to output a sort routine, and have the macro take a comparison expression as an argument.
#define GENERATE_SORT(name, type, comparison_expression) \
void name(type* begin, type* end) \
{ /* ... when needed, fill a and b and use comparison_expression */ }
GENERATE_SORT(sort_ints, (*a<*b))
void foo()
{
int array[10];
sort_ints(array, array+10);
}
Two points. I).
_asm
II). basic design limits of compilers.
Compilers have, as a basic purpose, the design goal of avoiding assembler or machine code. They achieve this by imposing certain limits. In this case, we give up a flexibility that we can easily do in assembly code. i.e. split the generated code of the sort into two pieces at the call to the compare function. copy the first half to somewhere. next copy the generated code of the compare function to there, just after the previous copied code of the first part. then copy the last half of the sort code. Finally, we have to deal with a whole series of minor details. See also the concept of "hot patching" running programs.