I would like to create a function that adds a field to a struct with a previously defined value.
I defined:
myValue = [1, 3, 5]
And I wrote this function:
function new_field(myStruct, myName, myValue)
myStruct.(myName) = myValue(:);
end
But I get the error:
"Unable to perform assignment because dot indexing is not supported for variables of this type."
Where did I go wrong?
Related
For one of my models, it is required to read the final value of all variables and set them as initial value for the next simulation. My real problem is due to types defined in OMShell. As can be seen in the scripting commands of OpenModelica, there are String type and "VariableName", "TypeName" types. To elaborate the difference:
// The following loads the Standard Modelica Library
>> loadModel(Modelica)
true
// The following throws an error
>> loadModel("Modelica")
The reason is that the loadModel function doesn't expect a string variable. It expects the name of the model. Returning my problem, I tried to use val function as in the following, so that I can give the value as the initial value of the next simulation.
>> final_time := 200;
>> myVarValue := val(myVar, final_time);
This can be done for each variable by using a for-loop, I thought as I have several variables and I realized the problem at this point.
// Get the list of all variables
>> myVarList := readSimulationResultVars(currentSimulationResult);
>> size(myVarList)
{724}
// The problem is that this list contains strings
>> myVarList[1]
"mySubSystem.myComponent.myPin.v"
// Following throws an error as it doesn't expect a string
>> val(myVarList[1], final_time)
How can I remove "" from the string that OMShell returns to me? I need to convert "myVar" to myVar so that I can use it as an input for the functions of OMShell.
You can use the stringVariableName function.
>>> vars:=OpenModelica.Scripting.readSimulationResultVars(currentSimulationResult)
{"r","time"}
>>> val(stringVariableName(vars[1]), 0.0)
1.0
I want to splice a list of arguments to pass to a function. For a vector I know that I can use num2cell and call the cell with curly braces (see this question), but in my case the list I want to splice originally has structs and I need to access one of their attributes. For example:
austen = struct('ids', ids, 'matrix', matrix);
% ... more structs defined here
authors = [austen, dickens, melville, twain];
% the function call I want to do is something like
tmp = num2cell(authors);
% myFunction defined using varargin
[a,b] = myFunction(tmp{:}.ids);
The example above does not work because Matlab expected ONE output from the curly braces and it's receiving 4, one for each author. I also tried defining my list of arguments as a cell array in the first place
indexes = {austen.ids, dickens.ids, melville.ids, twain.ids};
[a,b] = myFunction(indexes{:});
but the problem with this is that myFunction is taking the union and intersection of the vectors ids and I get the following error:
Error using vertcat
The following error occurred converting from double to struct:
Conversion to struct from double is not possible.
Error in union>unionR2012a (line 192)
c = unique([a;b],order);
Error in union (line 89)
[varargout{1:nlhs}] = unionR2012a(varargin{:});
What is the correct way for doing this? The problem is that I will have tens of authors and I don't want to pass al of them to myFunction by hand.
As #kedarps rightly pointed out I need to use struct2cell instead of num2cell. The following code does the trick
tmp = struct2cell(authors);
[a, b] = myFunction(tmp{1,:,:}); %ids is the first entry of the structs
I had never heard about struct2cell before! It doesn't even show up in the See also of help num2cell! It would be amazing to have an apropos function like Julia's....
Prompted by Lyndon's question earlier today:
a.
julia> function f1(x::Float64)
const y = x;
y = "This should throw an error since y is of constant type";
return y;
end
f1 (generic function with 1 method)
julia> f1(1.0)
"This should throw an error since y is of constant type"
Why does the const keyword not work as expected here? (i.e., disallow assigning a string to y which has been declared as const).
b.
julia> function f2(x::Float64)
show(x);
const x = 1.;
end
f2 (generic function with 1 method)
julia> f2(1.0)
ERROR: UndefVarError: x not defined
Stacktrace:
[1] f2(::Float64) at ./REPL[1]:2
Why does defining x as const on line 3 affect the value of x on line 2?
c.
In particular, this prevents me from doing:
function f(x::Float64)
const x = x; # ensure x cannot change type, to simulate "strong typing"
x = "This should throw an error";
end
I was going to offer this as a way to simulate "strong typing", with regard to Lyndon's "counterexample" comment, but it backfired on me, since this function breaks at line 2, rather than line 3 as I expected it to.
What is causing this behaviour? Would this be considered a bug, or intentional behaviour?
Naming conventions aside, is there a more acceptable way to prevent an argument passed into a function from having its type altered?
(as in, is there a defined and appropriate way to do this: I'm not after workarounds, e.g. creating a wrapper that converts x to an immutable type etc)
EDIT:
So far, this is the only variant that allows me to enforce constness in a function, but it still requires the introduction of a new variable name:
julia> function f(x::Float64)
const x_const::Float64 = x;
x_const = "helle"; # this will break, as expected
end
but even then, the error just complains of an "invalid conversion from string to float" rather than an "invalid redefinition of a constant"
Because const in local scope is not yet implemented:
https://github.com/JuliaLang/julia/issues/5148
I'm working on a project in Matlab in which I have to modify a pre-existing code.
Before starting to work, I'd like to understand how eval function is used.
First of all, there's the parameter struct definition (look at eval function):
function [ descriptor ] = parameter ( name, nominal, mode, range )
descriptor = struct (
'name', name, % Parameter name inside Simulink schematic
'nominal', nominal, % Nominal parameter value
'mode', mode, % Range mode name in { 'Range', 'Percentage', 'PlusMinus' }
'range', range % Range boundaries, 1x2 vector
);
eval([ name '=' num2str(nominal) ';' ]); % Initialize with nominal value
end
Just for completion, parameter cell array is defined as follows:
parameters = {}; % Simulation parameter declarations
parameters{end+1} = parameter( 'simulink_Imm(1,1)', simulink_Imm(1,1), 'Percentage', [-10, 10] );
parameters{end+1} = parameter( 'simulink_q0(1)', 0, 'Range', [-1, 1] );
etc...
Finally we have this for loop (always look at the eval function):
for i = 1 : numParameters
p = parameters(i); p = p{1};
value = urealvalue( p.nominal, p.mode, p.range ); % Generate random real value
eval([ p.name '=' num2str(value) ';' ]); % Assign the generated value
That function is used twice and honestly I don't get how it works. I understand what it does thanks to the comments, but I don't understand how it assigns values.
I also searched in the matlab documentation, but it doesn't help.
Can anybody shed some light on this?
eval evaluates a valid Matlab expression which is defined by a string.
Imagine the following:
name = 'myVar';
nominal = 42;
when you now call:
eval([ name '=' num2str(nominal) ';' ]);
which is the same like:
eval([ 'myVar = 42;' ]);
you get a variable myVar in your workspace which has the value 42.
The same happens, when you type in
myVar = 42;
directly. So instead of having a line of code in your script, you can just evaluate a code-string from wherever. In your case it is used to create a variable from two struct fields, the first the variable name and the second it's value.
Another example, you want a variable named after it's actual value. But you don't know its name before:
value = randi(10);
eval([ 'var' num2str(value) '=' num2str(value) ';' ]);
The only way to do this is by using eval as you need to create a code-line-string according to the random generated value. Eval then "writes" it. If for example value = 9, it is stored to a variable var9 in the next step.
I have a MATLAB file that contains a single top-level function, called sandbox. That function in turn contains two nested functions, mysum and myprod, which are identical in functionality and what parameters they allow except that one uses #sum internally and the other uses #prod internally. My goal is to create a wrapper function to use in both mysum and myprod that takes care of all the validation and input parsing. This function is called applyFunc.
Here's where it gets tricky. mysum and myprod come in two forms:
mysum(v) returns sum(v, 1).
mysum(v, 'imag') returns sum(v, 1) + 1i
Any other combinations of input should throw an error.
I'm having trouble using inputParser to parse these various combinations of input, specifically the optional string input. Here's the code:
function sandbox()
%% Data
v = [1 4; 3 3];
%% Calculations
s = mysum(v);
si = mysum(v, 'imag');
p = myprod(v);
pi = myprod(v, 'imag');
%% Accuracy tests
assert(isequal(s, [4 7]))
assert(isequal(si, [4+1i 7+1i]))
assert(isequal(p, [3 12]))
assert(isequal(pi, [3+1i 12+1i]))
function x = mysum(varargin)
x = applyFunc(#sum, varargin{:});
end
function x = myprod(varargin)
x = applyFunc(#prod, varargin{:});
end
end
function x = applyFunc(func, varargin)
p = inputParser();
p.addRequired('func', #(x) validateattributes(x, {'function_handle'}, {'scalar'}));
p.addRequired('v', #(x) validateattributes(x, {'double'}, {}, 'applyFunc:msg', 'v'));
p.addOptional('imag', '', #(x) validatestring(x, {'imag', ''})); % THIS LINE IS THE PROBLEM
p.parse(func, varargin{:});
f = p.Results.func;
v = p.Results.v;
strflag = p.Results.imag;
x = f(v);
if ~isempty(strflag)
validatestring(strflag, {'imag'});
x = x + 1i;
end
end
The line that's causing the problem is this one (as marked in the code above):
p.addOptional('imag', '', #(x) validatestring(x, {'imag', ''}));
The documentation for inputParser says that:
For optional string inputs, specify a validation function. Without a validation function, the input parser interprets valid string inputs as invalid parameter names and throws an error.
Unfortunately I don't have any idea how to do this. Is there something simple Im missing or what? If the 'imag' argument isn't passed at all (as in the assignment of s and p), the code works fine, but if I do pass it, I get this error:
Error using sandbox>applyFunc (line 32)
The value of 'imag' is invalid. It must satisfy the function:
#(x)validatestring(x,{'imag',''}).
Error in sandbox/mysum (line 18)
x = applyFunc(#sum, varargin{:});
Error in sandbox (line 7)
si = mysum(v, 'imag');
Any help?
The problem is that validatestring returns the matching string from the cell argument ({'imag',''}) rather than a Boolean indicating if it passes validation. Instead, use strcmp and any:
#(x) any(strcmp(x,{'imag', ''}))
Also, with validatestring, if the input string did not match either 'imag' or '' (actually just 'imag' since empty strings only match in R2014a+), it would throw an error rather than returning false so that the inputParser could return the appropriate error.
Another nice way to fix the problem is to change the syntax of applyFunc entirely so that instead of just 'imag' as an optional string input argument, use a Parameter-Value with 'imag' as the parameter and a validated boolean as the input.
The input definition suggested by Amro in the comments:
p.addParameter('imag', false, #(x)validateattributes(x, {'logical'}, {'scalar'}))
The usage:
mysum(x,'imag',true)
mysum(x) % default is equivalent to mysum(x,'imag',false)
This would simplify the rest of the code with p.Result.imag being a logical scalar. I would suggest:
x = f(v) + p.Result.imag*1i;
The problem is not inputParser, I think the issue is with validatestring.
1) First it does not match on empty strings:
>> x = ''
x =
''
>> validatestring(x, {'imag',''})
Expected input to match one of these strings:
imag,
The input did not match any of the valid strings.
Caused by:
Error using validatestring>checkString (line 85)
Expected input to be a row vector.
2) Second, if it successfully matches, it returns the resolved string (from one of the valid choice), instead of true/false. inputParser requires that the validation function either return a boolean, or nothing but throws error on failure.