I was given a problem in MatLab where I needed to write a single line of code that starts with
variableName =
and find the max value of a 2d array and it's index. I don't use matlab at all, and this seems infuriatingly simple in any language I know. I know to get the value & index of the result of max, you do something like
[M,I] = max(stuffToCheck)
I just dont understand how to assign the array that creates to a variable name. I've spent some time googling but this feels like a very weird constraint so I haven't found anything yet. How do I do this on one line?
use the variable you want to assign the result:
[variableName(:,1),variableName(:,2)] = max(stuffToCheck)
its the only way because in matlab if you write :
variable = function();
matlab return only the first output, to get other ouput you have to write:
[output1,output2,...] = function();
Related
When looking at Matlab code I have stumbled upon the following line of code:
[~,ui] = Unique(Day)
(Where Day is the vector containing a numeric value of day like so: 1,2,3, etc.)
What is it doing? I have noticed that it creates some kind of unique identifiers for the numeric value of the day (i.e. for 1 to 31) as well as a variable called Volume. What is Volume?
[~,ui] = Unique(Day) evaluates the function Unique with input argument Day.
This function has 2 outputs, and if you want to use both, you would write
[a,b]=Unique(Day). However, if you need only second output, you can put ~ instead of the first argument. So, your first output will not be saved.
It is impossible to answer, what Volume means, because you didn't provide the code of the function Unique.
I have multiple variables var_1, var_2, var_3....var_9 (they are named like that) that I want to pass in a function. All of the variables are saved in the workspace. The function takes 2 variables, and spits out an output. I want to compare var_1 with all the variables, including itself, so I prefer to automate it in a loop.
So I want to execute
function(var_1,var_1)--> display answer, function(var_1,var_2)--> display answer...function(var_1,var_9)-->display answer all at once in a loop. I've tried the following, with no luck:
for i=1:7
functionname(var_1,var_'num2str(i)')
end
Where did I go wrong?
You cannot make a dynamic variable name directly. But you can use the eval-function to evaluate an expression as a string. The string can be generated with sprintf and replaces %d with your value.
for i=1:7
eval(sprintf('functionname(var_1,var_%d)', i));
end
But: Whenever you can, you should avoid using the eval function. A much better solution is to use a cell array for this purpose. In the documentation of Matlab there is a whole article about the why and possible alternatives. To make it short, here is the code that uses a cell array:
arr = {val_1, val_2, val_3, val_4, val_5, val_6, val_7, val_8, val_9};
for i = 1:length(arr)
functionname(arr{1},arr{i})
end
I have to make a call to read in a matrix with a name that is created from a strcat, in Matlab. For example
person = 2;
Index_XNY = strcat('Index_X',num2str(person),'Y');
Big_Index = find(Index_XNY(1,:)==1);
This works if I replace Index_XNY with Index_X2Y for this example.
I have tried a number of different things to obtain Big_Index, but so far I have not been successful. Can this be done, as there is a large collection of data elements that need to be called.
You can use the eval function to evaluate a Matlab expression in a string:
expr = strcat('find(Index_X',num2str(person),'Y(1,:)==1)');
Big_Index = eval(expr);
Of course, a lot of alternative ways to do this with eval function exist, depending on how much you would like to put into the string and how much would be left out.
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 struct array called AnalysisResults, that may contain any MATLAB datatypes, including other struct arrays and cell arrays.
Then I have a string called IndexString, which is the index to a specific subfield of StructArray, and it may contain several indices to different struct arrays and cell arrays, for example:
'SubjectData(5).fmriSessions{2}.Stats' or 'SubjectData(14).TestResults.Test1.Factor{4}.Subfactor{3}'.
And then I have a variable called DataToBeEntered, which can be of any MATLAB datatype, usually some kind of struct array, cell array or matrix.
Using eval, it is easy to enter the data to the field or cell indexed by IndexString:
eval([ 'AnalysisResults.', IndexString, ' = DataToBeEntered;' ])
But is it possible to avoid using eval in this? setfield doesn't work for this.
Thank you :)
Well, eval surely is the easiest way, but also the dirtiest.
The "right" way to do so, I guess, would be to use subsasgn. You will have to parse the partial MATLAB command (e.g. SubjectData(5).fmriSessions{2}.Stats) into the proper representation for those functions. Part of the work can be done by substruct, but that is the lightest part.
So for example, SubjectData(5).fmriSessions{2}.Stats would need to be translated into
indexes = {'.' , 'SubjectData',
'()', {5},
'.' , 'fmriSessions',
'{}', {2},
'.' , 'Stats'};
indexStruct = substruct(indexes{:});
AnalysisResult = subsasgn(AnalysisResult, indexStruct, DataToBeEntered);
Where you have to develop the code such that the cell array indexes is made as above. It shouldn't be that hard, but it isn't trivial either. Last year I ported some eval-heavy code with similar purpose and it seemed easy, but it is quite hard to get everything exactly right.
You can use dynamic field names:
someStruct.(someField) = DataToBeEntered;
where someField is a variable holding the field name, but you will have to parse your IndexString to single field name and indices.