My professor likes work to be turned in in Flowgorithm.
I can't figure out how to take the variable X=0 from the main method to the displayLoop method and then allow the displayLoop method to modify the stored value so it can be used as a countdown (or countup in this case).
Flowgorithm, like Java, uses pass-by-value for types like Internet, Real, etc...
To return one less, you can create a function and return the decremented value. So, maybe try something like this:
count = displayLoop(count)
The assignment sounds a tad strange. Is this a recursion assignment?
Related
In Matlab, it is easy to generate "help" for a function, as follows.
function out = foo()
% helpful information about foo
end
When we execute help foo, we get "helpful information about foo".
However, suppose we would like to define help for a variable, probably as a definition. How could we do such a thing? It would be nice if we could do something like
x = 3; % m ... position
help x
and get "m ... position". However, I don't believe such functionality exists.
The only reasonable way I see around this is to define every variable as a struct with keys value and description.
x.value = 3;
x.description = 'm/s ... position';
This requires we define every variable as a struct, which is kind of annoying and, I worry (should I?), unperformant (it's simulation code and these variables are accessed repeatedly).
Is there another solution I'm not considering? Should I be worried about making every variable a struct?
Your code should be self-documenting. Instead of variable name x, use position.
Furthermore, all variables should be local, so you can easily look for its definition (with comment) within the function you are editing.
Variables declared further away (with larger scope within the function) should have longer, more self-explanatory names than variables with a smaller scope (e.g. use within a short loop.
There are only two three cases where variables are declared outside the function’s scope:
Class properties. You can actually document these.
In a script, you have access to variables that already existed before the script started. A good reason not to use scripts or depend on the base namespace in larger projects.
Global variables. You should never use global variables for many reasons. Just don’t.
I'm working with some code which calls ADJUST-ARRAY. I am getting a warning message from the Lisp interpreter (CMUCL) that the return value of ADJUST-ARRAY should not be ignored.
In the code I am working on, ADJUST-ARRAY modifies its argument in place, if I am not mistaken. So it's not necessary to do anything with the return value. Is there a designated way to ignore a return value in Common Lisp? Of course, I could assign the return value to some variable, and then ignore the variable. But that feels clumsy.
I could also assign the return value to the ADJUST-ARRAY argument, something like:
(setq my-array (adjust-array my-array ...))
but that seems to suggest that I'm not sure if ADJUST-ARRAY will modify MY-ARRAY in place.
Any advice is welcome, thanks in advance.
You are correct. As the documentation states:
The result is an array of the same type and rank as array, that is
either the modified array, or a newly created array to which array
can be displaced, and that has the given new-dimensions.
If the result is a newly created array then of course the function would have had no effect on the argument.
Common Lisp almost always require you to use the returned value rather than old bindings in order to have portable code.
The specification of adjust-array is that the adjusted array is the one returned.
What you can expect of the argument array afterwards to be is a bit complicated and may differ between implementations in some cases.
Just use the one returned. You might use setf to modify or let to create a binding.
I am attempting to use LuaJ with Scala. Most things work (actually all things work if you do them correctly!) but the simple task of setting object values has become incredibly complicated thanks to Scala's setter implementation.
Scala:
class TestObject {
var x: Int = 0
}
Lua:
function myTestFunction(testObject)
testObject.x = 3
end
If I execute the script or line containing this Lua function and pass a coerced instance of TestObject to myTestFunction this causes an error in LuaJ. LuaJ is trying to direct-write the value, and Scala requires you to go through the implicitly-defined setter (with the horrible name x_=, which is not valid Lua so even attempting to call that as a function makes your Lua not parse).
As I said, there are workarounds for this, such as defining your own setter or using the #BeanProperty markup. They just make code that should be easy to write much more complicated:
Lua:
function myTestFunction(testObject)
testObject.setX(testObject, 3)
end
Does anybody know of a way to get luaj to implicitly call the setter for such assignments? Or where I might look in the luaj source code to perhaps implement such a thing?
Thanks!
I must admit that I'm not too familiar with LuaJ, but the first thing that comes to my mind regarding your issue is to wrap the objects within proxy tables to ease interaction with the API. Depending upon what sort of needs you have, this solution may or may not be the best, but it could be a good temporary fix.
local mt = {}
function mt:__index(k)
return self.o[k] -- Define how your getters work here.
end
function mt:__newindex(k, v)
return self.o[k .. '_='](v) -- "object.k_=(v)"
end
local function proxy(o)
return setmetatable({o = o}, mt)
end
-- ...
function myTestFunction(testObject)
testObject = proxy(testObject)
testObject.x = 3
end
I believe this may be the least invasive way to solve your problem. As for modifying LuaJ's source code to better suit your needs, I had a quick look through the documentation and source code and found this, this, and this. My best guess says that line 71 of JavaInstance.java is where you'll find what you need to change, if Scala requires a different way of setting values.
f.set(m_instance, CoerceLuaToJava.coerce(value, f.getType()));
Perhaps you should use the method syntax:
testObject:setX(3)
Note the colon ':' instead of the dot '.' which can be hard to distinguish in some editors.
This has the same effect as the function call:
testObject.setX(testObject, 3)
but is more readable.
It can also be used to call static methods on classes:
luajava.bindClass("java.net.InetAddress"):getLocalHost():getHostName()
The part to the left of the ':' is evaluated once, so a statement such as
x = abc[d+e+f]:foo()
will be evaluated as if it were
local tmp = abc[d+e+f]
x = tmp.foo(tmp)
I have a function that returns two values, like so:
[a b] = myfunc(x)
Is there a way to get the second return value without using a temporary variable, and without altering the function?
What I'm looking for is something like this:
abs(secondreturnvalue(myfunc(x)))
not that i know of. subsref doesn't seem to work in this case, possibly because the second variable isn't even returned from the function.
since matlab 2009b it is possible to use the notation
[~, b] = function(x)
if you don't need the first argument, but this still uses a temporary variable for b.
Unless there is some pressing need to do this, I would probably advise against it. The clarity of your code will suffer. Storing the outputs in temporary variables and then passing these variables to another function will make your code cleaner, and the different ways you could do this are outlined here: How to elegantly ignore some return values of a MATLAB function?.
However, if you really want or need to do this, the only feasible way I can think of would be to create your own function secondreturnvalue. Here's a more general example called nth_output:
function value = nth_output(N,fcn,varargin)
[value{1:N}] = fcn(varargin{:});
value = value{N};
end
And you would call it by passing as inputs 1) the output argument number you want, 2) a function handle to myfunc, and 3) whatever input arguments you need to pass to myfunc:
abs(nth_output(2,#myfunc,x))
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.