I have a list of symbols, say
`A`B`C
. I have a table tab0; A function that takes in a table plus a string as arguments.
tab1: f[tab0;`A]
tab2: f[tab1;`B]
tab3: f[tab2;`C]
I only care about the final values. But my list of symbols can be long and can have variable length, so I don't want to hardcode above. How do I achieve it?
I think it has something to do with https://code.kx.com/q/ref/accumulators/ but I really struggle to figure out the syntax.
This is exactly the use case for the binary application of over (/) (https://code.kx.com/q/ref/accumulators/#binary-application)
So you should use:
f/[tab0;`A`B`C]
Related
I have variables with incremented numbers within, such as row0text, row1text, row2text, etc.
I've figured out how to dynamically create string versions of those variable names, but once I have those strings, how can I use them as actual variable names rather than strings in my code?
Example:
var row3text = "This is the value I need!"
var firstPart = "row"
var rowNumber = 3
var secondPart = "text"
var together = (firstPart+String(rowNumber)+secondPart)
// the below gives me the concatenated string of the three variables, but I'm looking for a way to have it return the value set at the top.
println (together)
Once I know how to do this, I'll be able to iterate through those variables using a for loop; it's just that at the moment I'm unsure of how to use that string as a variable name in my code.
Thanks!
Short Answer: There is no way to do this for good reason. Use arrays instead.
Long Answer:
Essentially you are looking for a way to define an unknown number of variables that are all linked together by their common format. You are looking to define an ordered set of elements of variable length. Why not just use an array?
Arrays are containers that allow you to store an ordered set or list of elements and access them by their ordered location, which is exactly what you're trying to do. See Apple's Swift Array Tutorial for further reading.
The advantage of arrays is that they are faster, far more convenient for larger sets of elements (and probably the same for smaller sets as well), and they come packaged with a ton of useful functionality. If you haven't worked with arrays before it is a bit of a learning curve but absolutely worth it.
I am reading a book and I am confused on what the following code does:
(defmethod execute ((o ORDER) (l SIMUL) (e MARKETUPDATE))
(values
(list (make-TRADE :timestamp (timestamp e)
:price (price e)
:quantity (orderquantity o)))
NIL))
The source to which I got this function says that it returns two values. My question is what the body does. From my understanding, the line 3-5 creates a list with :timestamp, :price, :quantity. Am I correct? What about values, the second line? Does it return this variable too? Any summary would help. Thanks
This is a method for a generic function, specializing on arguments of types order, simul, and marketupdate.
It returns 2 values:
A list of length 1 created by the eponymous function list, which contains a single object of, presumably, type trade (probably - but not necessarily - created by a defstruct), which has slots timestamp, price, and quantity.
Symbol nil.
You can probably access the slots of the trade using functions trade-timestamp &c (unless the defstruct form is non-trivial or trade is not defined by a defstruct at all).
Why the result of make-trade is wrapped in a list is hard to guess without more context, but I'd guess that an execute can be split into N trades in some scenarios.
I suspect your confusion arises almost entire because this is the first time you have encountered a use of values. Common Lisp allows functions to return multiple values. That's slightly similar to how any language allows functions to receive multiple parameters.
These multiple return values are quite efficiently implemented. Most newbies encounter multiple values for the first time on the integer division functions, which will return a remainder as their second value. Hash table look ups will return a second value to indicate if the key was actually in the table, since the value stored for the key might be nil.
In your example the second value is NIL, presumably other execute methods might return something more interesting - for example where in the update Q the order was place, or an error code if something goes wrong. Of course checking out the manual for values will be fraught with educational values(sic).
This function is a method returning two values by using the keyword values. Have a look at CLOS to better understand object orientation and at "values" for the way of returning more than one value.
Is there an idiomatic way in Matlab to bind the value of an expression to the nth return value of another expression?
For example, say I want an array of indices corresponding to the maximum value of a number of vectors stored in a cell array. I can do that by
function I = max_index(varargin)
[~,I]=max(varargin{:});
cellfun(#max_index, my_data);
But this requires one to define a function (max_index) specific for each case one wants to select a particular return value in an expression. I can of course define a generic function that does what I want:
function y = nth_return(n,fun,varargin)
[vals{1:n}] = fun(varargin{:});
y = vals{n};
And call it like:
cellfun(#(x) nth_return(2,#max,x), my_data)
Adding such functions, however, makes code snippets less portable and harder to understand. Is there an idiomatic to achieve the same result without having to rely on the custom nth_return function?
This is as far as I know not possible in another way as with the solutions you mention. So just use the syntax:
[~,I]=max(var);
Or indeed create an extra function. But I would also suggest against this. Just write the extra line of code, in case you want to use the output in another function. I found two earlier questions on stackoverflow, which adress the same topic, and seem to confirm that this is not possible.
Skipping outputs with anonymous function in MATLAB
How to elegantly ignore some return values of a MATLAB function?
The reason why the ~ operator was added to MATLAB some versions ago was to prevent you from saving variables you do not need. If there would be a syntax like the one you are searching for, this would not have been necessary.
I have a function called viewcsi(varargin) and I want to pass in three variables at most.
The first is a MBSspectrum class I made and then a string and also a number.
viewcsi is a call back, it gets called like this:
...'ButtonDownFcn','viewcsi(''pickvox_cb'', sp_viewcsi)');
sp_viewcsi is the MBSspectrum class I made and is in the workspace. I want to be able to add another argument called counter which is integer of type double.
I want to do something like this:
...'ButtonDownFcn','viewcsi(''pickvox_cb'', sp_viewcsi, counter)');
or
...'ButtonDownFcn', {#viewcsi, 'pickvox_cb', 'sp_viewcsi', counter)');
But when I do the last two thing these do not work since they do not preserve 'sp_viewcsi' as a class but treats it like a string. What can I do to fix this? I have a feeling its something easy but I havent been able to figure it out.
The ButtonDownFcn will only ever pass it two arguments. You can cheat it by saying
...'ButtonDownFcn',#(a,b)viewcsi(a,b, counter));
so that the callback will pass it a and b, while Matlab will hand it the current value of counter.
See also the doc on passing extra parameters.
i have designed a function which can return 2 different types based on the input parameters
ex: &Foo(12,"count") -> returns record count from DB for value 12
&Foo(12,"details") -> returns resultset from DB for value 12 in hash format
My question is is this a good approach? in C# i can do it with function overload.
Please think what part of your code gets easier by saying
Foo(12, "count")
instead of
Foo_count(12)
The only case I can think of is when the function name ("count") itself is input data. And even then do you probably want to perform some validation on that, maybe by means of a function table lookup.
Unless this is for an intermediate layer that just takes a command name and passes it on, I'd go with two separate functions.
Also, the implementation of the Foo function would look at the command name and then just split into a private function for every command anyway, right?
additionally you might consider the want to make foo return the details if you wanted a list.
return wantarray ? ($num, #details) : $num;