Questions about functions - function
True or False: When a function is called, the calling program suspends until
the function completes.
True or False: When you call a function with a list as a parameter, you could
change the original calling program’s list from within the function
True or False: When you call a function with a dictionary as a parameter, you could change the original calling program’s dictionary from within the
function.
Im very fuzzy on these questions and what they mean, could someone help explain these?
1.True
the original program will stop and going to run the function. It will back to the place where it stop and start to execute the next line command after function done.
2.False
call by value. it wont change the original variable's value.
3.True
call by pointer or call by reference.
Sometime yes and sometimes no. It depends whether you are calling it synchronously or asynchronously, see this answer for the distinction: Asynchronous vs synchronous execution, what does it really mean?
In most cases yes, because you are just passing by reference, i.e you are passing the location in memory of the param you are passing. However, you can also pass by value, i.e passing a copy of the object. See this question for more detail: What's the difference between passing by reference vs. passing by value?
same answer as your previous question. Whether you are passing an integer, string, array, list, dictionary, it doesn't matter, it all depends on whether you are passing by reference or by value. Which one of these happens by default depends on which programming language you use. You can determine which one is happening pretty easily with some experimentation: Define a function with a dictionary variable, add a key/value pair call another function with the dict as a param, and modify it in the called function, then print it out in the calling function once the called function has returned. If it has been modified, you know you are passing by reference. If it has the original key/value you set in the caller and is not modified, you know you are passing by value.
Related
Check if a function takes specified arguments in Rust
Is there a way in a where clause in Rust to check if a function passed as a parameter takes specified arguments?
If you're defining a function that takes a function argument, that argument has a very specific type associated with it that dictates the arguments. You cannot call that function with something that doesn't match, it just won't compile. If you're thinking in terms of dynamic languages where the arguments are somewhat subjective, you're assuming you can make a mistake here and call it incorrectly. You can't. It's strictly disallowed.
What is the memory allocation when you return a function in Golang?
Here is a simplified code
func MyHandler(a int) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.WriteCode(a)
})
}
Whenever a http request comes MyHandler will be called, and it will return a function which will be used to handle the request. So whenever a http request comes a new function object will be created. Function is taken as the first class in Go. I'm trying to understand what actually happened when you return a function from memory's perspective. When you return a value, for example an integer, it will occupy 4 bytes in stack. So how about return a function and lots of things inside the function body? Is it an efficient way to do so? What's shortcomings?
If you're not used to closures, they may seem a bit magic. They are, however, easy to implement in compilers: The compiler finds any variables that must be captured by the closure. It puts them into a working area that will be allocated and remain allocated as long as the closure itself exists. The compiler then generates the inner function with a secret extra parameter, or some other runtime trickery,1 such that calling the function activates the closure. Because the returned function accesses its closure variables through the compile-time arrangement, there's nothing special needed. And since Go is a garbage-collected language, there's nothing else needed either: the pointer to the closure keeps the closure data alive until the pointer is gone because the function cannot be called any more, at which point the closure data evaporates (well, at the next GC). 1GCC sometimes uses trampolines to do this for C, where trampolines are executable code generated at runtime. The executable code may set a register or pass an extra parameter or some such. This can be expensive since something treated as data at runtime (generated code) must be turned into executable code at runtime (possibly requiring a system call and potentially requiring that some supervisory code "vet" the resulting runtime code). Go does not need any of this because the language was defined with closures in mind, so implementors don't, er, "close off" any easy ways to make this all work. Some runtime ABIs are defined with closures in mind as well, e.g., register r1 is reserved as the closure-variables pointer in all pointer-to-function types, or some such.
Actual function size is irrelevant. When you return a function like this, memory will be allocated for the closure, that is, any variables in the scope that the function uses. In this case, a pointer will be returned containing the address of the function and a pointer to the closure, which will contain a reference to the variable a.
How to pass a function AND its parameters into another function
My use case is this: a function called 'time' that will return how long it takes to run any function you give it. So the time function needs to know all the parameters to pass into the function when it calls it. I know how to pass a function into another function, but how can I pass all its parameters, without knowing in advance how many and what type they are, so they can be used when calling the function? For example, if I pass in an array of all the parameters I need to send, is there some Dart way to call a function by expanding an array into a list of parameters? Or perhaps there's another way to capture and pass a function call, including all parameters, as one executable object? I'm also interested in knowing if there's a more Dartful way to accomplish what I'm trying to do re: timing function calls.
I believe using a List of parameters with the apply method is the most common way and practical of doing this and I have seen something similar used to pass parameters for JS interop. As far as I know, there isn't a way to expand an array into a list of parameters like you can for javascript. You could of course create your own object to pass arguments, but I think it would add unnecessary complexity and end up being more difficult. Example of passing parameters to function in dart:js here.
Naming parameters the same as the variable passed to them?
Are there any rules (or will I run into any problems) if I name the parameters of a function the same as the variable I will pass into them? For example in Python: def foo(param): pass param = 2 foo(param) In the fairly limited programming I've done, I have not ran into any problems doing this. Will I get problems in certain languages? Is this okay to do, or is it a practice to be avoided?
The "problem" in this particular case is that the function parameter name will shadow the outer param variable; i.e. you cannot (implicitly) refer to the global param anymore because inside your function param is defined as a local variable. But, this is really as it should be. Your function should only worry about the parameters it declares locally, not about implicit global variables. Conversely, a caller of a function should not have to worry about anything that goes on inside a function. Naming a variable the same as a parameter to a function is of no consequence to the caller, and should be of no consequence to the function itself. So, no, there's absolutely no issue here.
'Invalid Handle object' when using a timer inside a function in MatLab
I am using a script in MatLab that works perfectly fine by itself, but I need to make a function out of it.
The script read a .csv file, extract all values, start a timer, and at each tick displays the corresponding coordinates extracted from the .csv, resulting in a 3D animation of my graph.
What I would like is to give it the location of the .csv, so that it starts displaying the graphs for this csv.
Here is what I have come up with:
function handFig(fileLoc)
csv=csvread(fileLoc,1,0);
both = csv(:,2:19);
ax=axes;
set(ax,'NextPlot','replacechildren');
Dt=0.1; %sampling period in secs
k=1;
hp1=text(both(k,1),both(k,2),both(k,3),'thumb'); %get handle to dot object
hold on;
hp2=text(both(k,4),both(k,5),both(k,6),'index');
hp3=text(both(k,7),both(k,8),both(k,9),'middle');
hp4=text(both(k,10),both(k,11),both(k,12),'ring');
hp5=text(both(k,13),both(k,14),both(k,15),'pinky');
hp6=text(both(k,16),both(k,17),both(k,18),'HAND');
L1=plot3([both(k,1),both(k,16)],[both(k,2),both(k,17)],[both(k,3),both(k,18)]);
L2=plot3([both(k,4),both(k,16)],[both(k,5),both(k,17)],[both(k,6),both(k,18)]);
L3=plot3([both(k,7),both(k,16)],[both(k,8),both(k,17)],[both(k,9),both(k,18)]);
L4=plot3([both(k,10),both(k,16)],[both(k,11),both(k,17)],[both(k,12),both(k,18)]);
L5=plot3([both(k,13),both(k,16)],[both(k,14),both(k,17)],[both(k,15),both(k,18)]);
hold off;
t1=timer('TimerFcn','k=doPlot(hp1,hp2,hp3,hp4,hp5,hp6,L1,L2,L3,L4,L5,both,t1,k)','Period', Dt,'ExecutionMode','fixedRate');
start(t1);
end
And the doplot function used:
function k=doPlot(hp1,hp2,hp3,hp4,hp5,hp6,L1,L2,L3,L4,L5,pos,t1,k)
k=k+1;
if k<5000%length(pos)
set(hp1,'pos',[pos(k,1),pos(k,2),pos(k,3)]);
axis([0 255 0 255 0 255]);
set(hp2,'pos',[pos(k,4),pos(k,5),pos(k,6)]);
set(hp3,'pos',[pos(k,7),pos(k,8),pos(k,9)]);
set(hp4,'pos',[pos(k,10),pos(k,11),pos(k,12)]);
set(hp5,'pos',[pos(k,13),pos(k,14),pos(k,15)]);
set(hp6,'pos',[pos(k,16),pos(k,17),pos(k,18)]);
set(L1,'XData',[pos(k,1),pos(k,16)],'YData',[pos(k,2),pos(k,17)],'ZData',[pos(k,3),pos(k,18)]);
set(L2,'XData',[pos(k,4),pos(k,16)],'YData',[pos(k,5),pos(k,17)],'ZData',[pos(k,6),pos(k,18)]);
set(L3,'XData',[pos(k,7),pos(k,16)],'YData',[pos(k,8),pos(k,17)],'ZData',[pos(k,9),pos(k,18)]);
set(L4,'XData',[pos(k,10),pos(k,16)],'YData',[pos(k,11),pos(k,17)],'ZData',[pos(k,12),pos(k,18)]);
set(L5,'XData',[pos(k,13),pos(k,16)],'YData',[pos(k,14),pos(k,17)],'ZData',[pos(k,15),pos(k,18)]);
else
k=1;
set(hp1,'pos',[pos(k,1),pos(k,2),pos(k,3)]);
axis([0 255 0 255 0 255]);
set(hp2,'pos',[pos(k,4),pos(k,5),pos(k,6)]);
set(hp3,'pos',[pos(k,7),pos(k,8),pos(k,9)]);
set(hp4,'pos',[pos(k,10),pos(k,11),pos(k,12)]);
set(hp5,'pos',[pos(k,13),pos(k,14),pos(k,15)]);
set(hp6,'pos',[pos(k,16),pos(k,17),pos(k,18)]);
end
However, when I run handFig('fileName.csv'), I obtain the same error everytime:
??? Error while evaluating TimerFcn for timer 'timer-7'
Invalid handle object.
I figured that it might come from the function trying to create a new 'csv' and 'both' everytime, so I tried removing them, and feeding the function the data directly, without results.
What is exactly the problem? Is there a solution?
Thanks a lot!
I think it's because when you call doPlot in the timer for the first time, you pass in t1 as an argument, and it might not exist the first time.
Does doPlot need t1 at all? I'd suggest modifying it so it's not used, and then your call to:
t1=timer('TimerFcn','k=doPlot(hp1,hp2,hp3,hp4,hp5,hp6,L1,L2,L3,L4,L5,both,k)','Period', Dt,'ExecutionMode','fixedRate');
Note the missing t1 in the doPlot call.
Either that, or initialise your t1 before you create the timer so it has some value to pass in.
Update (as an aside, can you use pause(Dct) in a loop instead? seems easier)
Actually, now I think it's a problem of scope.
It took a bit of digging to get to this, but looking at the Matlab documentation for function callbacks, it says:
When MATLAB evaluates function handles, the same variables are in scope as when the function handle was created. (In contrast, callbacks specified as strings are evaluated in the base workspace.)
You currently give your TimerFcn argument as a string, so k=doPlot(...) is evaluated in the base workspace. If you were to go to the matlab prompt, run handFig, and then type h1, you'd get an error because h1 is not available in the global workspace -- it's hidden inside handFig.
That's the problem you're running into.
However, the workaround is to specify your function as a function handle rather than a string (it says function handles are evaluated in the scope in which they are created, ie within handFig).
Function handles to TimerFcn have to have two arguments obj and event (see Creating Callback Functions). Also, that help file says you have to put doPlot in its own m-file to have it not evaluate in the base Matlab workspace.
In addition to these two required input arguments, your callback
function can accept application-specific arguments. To receive these
input arguments, you must use a cell array when specifying the name of
the function as the value of a callback property. For more
information, see Specifying the Value of Callback Function Properties.
It goes through an example of what you have to do to get this working. Something like:
% create timer
t = timer('Period', Dt,'ExecutionMode','fixedRate');
% attach `k` to t so it can be accessed within doPlot
set(t,'UserData',k);
% specify TimerFcn and its extra arguments:
t.TimerFcn = { #doPlot, hp1, hp2, hp3, ...., both };
start(t)
Note -- the reason k is set in UserData is because it needs to be somehow saved and modified between calls to doPlot.
Then modify your doPlot to have two arguments at the beginning (which aren't used), and not accept the k argument. To extract k you do get(timer_obj,'UserData') from within doPlot:
function k=doPlot(timer_obj, event, hp1,hp2,hp3,.....)
k = get(timer_obj,'UserData');
.... % rest of code here.
% save back k so it's changed for next time!
set(timer_obj,'UserData',k);
I think that's on the right track - play around with it. I'd highly recommend the mathworks forums for this sort of thing too, those people are whizzes.
This thread from the mathworks forum was what got me started and might prove helpful to you.
Good luck!