I wrote a main function in Matlab that calls other functions, each of which produces plots in a new figure. Though each function produces different plots (different colors, number of subplots, and so on) they all share some features (font, fontsize, Linewidth etc.).
In order to make it easier to modify the aforementioned shared parameter for all the MATLAB figures, I have defined in the preamble of the main function a structure variable as follows:
var.font = 'Arial Unicode MS';
var.fontsize = 13;
var.interpreter = 'none' ;
and so on for the other fields. When I call the function in this way (providing the structure as input):
function plot1( var , ... )
fig = gcf
fig.Position(3) = var.Position3
fig.Position(4) = var.Position4
end
everything works fine and the functions apply the feature to each figure. But if I try to provide a variable number of input to the functions using varargin, in this way
function plot1( varargin )
fig = gcf;
temp = varargin(1);
fig.Position(3) = temp.Position3;
fig.Position(4) = temp.Position4;
end
The following error message is prompted "Struct contents reference from a non-struct array object."
You're indexing the cell array incorrectly (this is easily done).
Round parentheses ( ) give you a cell output when indexing a cell array - i.e. your temp is a 1x1 cell with the struct inside it.
You need curly braces { } to extract the contents of a cell array.
Fix: use curly braces:
temp = varargin{1};
I sometimes think of cell arrays as a group of boxes, since each element (or "box" in this analogy) can basically contain anything. To extract a subset of boxes, use round parentheses. To extract the contents of the boxes, use braces. This analogy extends also to tables, where the notation is consistent.
Here's some docs on indexing cell arrays, where the difference is described in more detail:
https://uk.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html
Related
I have 40 structures in my Workspace. I Need to write a script to calculate the directional derivatives of all the elements. Here is the code :
[dx,dy] = gradient(Structure_element_1.value);
dxlb = min(min(dx));
dxub = max(max(dx));
dylb = min(min(dy));
dyub = max(max(dy));
[ddx,ddy] = gradient(gradient(Structure_element_1.value));
ddxlb = min(min(ddx));
ddxub = max(max(ddx));
ddylb = min(min(ddy));
ddyub = max(max(ddy));
This is the code for one element. I Need to find out the same for all the 40 elements and then use it later. Can anyone help with this.
To answer your literal question, you should store the variables in a structure array or at least a cell array. If all of your structures have the same fields, you can access all of them by indexing a single array variable, say Structure_element:
for i = 1:numel(Structure_element)
field = Structure_element(i).value
% compute gradients of field
end
Now to address the issue of the actual gradient computation. The gradient function computes an approximation for , where is your matrix of data. Normally, a MATLAB function is aware of how many output arguments are requested. When you call gradient(gradient(F)), the outer gradient is called on the first output of the inner gradient call. This means that you are currently getting an approximation for .
I suspect that you are really trying to get . To do this, you have to get both outputs from the inner call to gradient, pass them separately to the
outer call, and choose the correct output:
[dx,dy] = gradient(F);
[ddx, ~] = gradient(dx);
[~, ddy] = gradient(dy);
Note the separated calls. The tilde was introduced as a way to ignore function arguments in MATLAB Release 2009b. If you have an older version, just use an actual variable named junk or something like that.
I have 40 structures in my Workspace. I Need to write a script to calculate the directional derivatives of all the elements. Here is the code :
[dx,dy] = gradient(Structure_element_1.value);
dxlb = min(min(dx));
dxub = max(max(dx));
dylb = min(min(dy));
dyub = max(max(dy));
[ddx,ddy] = gradient(gradient(Structure_element_1.value));
ddxlb = min(min(ddx));
ddxub = max(max(ddx));
ddylb = min(min(ddy));
ddyub = max(max(ddy));
This is the code for one element. I Need to find out the same for all the 40 elements and then use it later. Can anyone help with this.
To answer your literal question, you should store the variables in a structure array or at least a cell array. If all of your structures have the same fields, you can access all of them by indexing a single array variable, say Structure_element:
for i = 1:numel(Structure_element)
field = Structure_element(i).value
% compute gradients of field
end
Now to address the issue of the actual gradient computation. The gradient function computes an approximation for , where is your matrix of data. Normally, a MATLAB function is aware of how many output arguments are requested. When you call gradient(gradient(F)), the outer gradient is called on the first output of the inner gradient call. This means that you are currently getting an approximation for .
I suspect that you are really trying to get . To do this, you have to get both outputs from the inner call to gradient, pass them separately to the
outer call, and choose the correct output:
[dx,dy] = gradient(F);
[ddx, ~] = gradient(dx);
[~, ddy] = gradient(dy);
Note the separated calls. The tilde was introduced as a way to ignore function arguments in MATLAB Release 2009b. If you have an older version, just use an actual variable named junk or something like that.
Suppose I have a struct array arr, where each element has a bunch of fields, including one called val. I'd like to increment each element's val field by some constant amount, like so:
for i = 1:length(arr)
arr(i).val = arr(i).val + 3;
end
This obviously works, but I feel there should be a way to do this in just one line of code (and no for loop). The best I've come up with is two lines and requires a temp variable:
newVals = num2cell([arr.val] + 3);
[arr.val] = deal(newVals{:});
Any ideas? Thanks.
Just a note, the deal isn't necessary there:
[arr.val] = newVals{:}; % achieves the same as deal(newVals{:})
The only other way I know how to do this (without the foor loop) is using arrayfun to iterate over each struct in the array:
% make a struct array
arr = [ struct('val',0,'id',1), struct('val',0,'id',2), struct('val',0,'id',3) ]
% some attempts
[arr.val]=arr.val; % fine
[arr.val]=arr.val+3; % NOT fine :(
% works !
arr2 = arrayfun(#(s) setfield(s,'val',s.val+3),arr)
That last command loops over each struct in arr and returns a new one where s.val has been set to s.val=3.
I think this is actually less efficient than your previous two-liner and the for loop though, because it returns a copy of arr as opposed to operating in-place.
(It's a shame Matlab doesn't support layered indexing like [arr.val]=num2cell([arr.val]+3){:}).
I like Carl's and mathematical.coffee's original ideas.
I have multiple similar lines to express, so for concision of my mainline code,
I went ahead and made the generic subfunction
function varargout = clist(in)
varargout = {in{:}};
end
then I could express each such line in a fairly readable way
[arr.var] = clist(num2cell([arr.var]+3));
[arr.var2] = clist(num2cell([arr2.var]/5+33));
Are all the fields in that struct scalar, or the same size? If so, the idiomatic Matlab way to do this is to rearrange your struct to be a scalar struct with arrays in each of its fields, instead of an array of structs with scalar values in the fields. Then you can do vectorized operations on the fields, like arr.val = arr.val + 3;. See if you can rearrange your data. Doing it this way is much more efficient in both time and memory; that's probably why Matlab doesn't provide convenient syntax for operating over fields of arrays of structs.
if the struct array you are trying to set is a set of graphics objects (line handles, figure handles, axes handles, etc), then you need to use the function set:
x = (1:10)';
Y = rand(10,5);
l = plot(x,Y,'-k'); % returns an array of line handles in l
set(l,'Color','r'); % sets the property 'Color' for all the five lines in l
I am working with a 400x1200 imported table (readtable generated from an .xls) which contains strings, doubles, dates, and NaNs. Each column is typed consistently. I am looking for a way to locate all instances in the table of any given string ('Help me please') and replace them all with a double (1). Doing this in Matlab will save me loads of work making changes to the approach used on the rest of this project.
Unfortunately, all of the options I've looked at (regexp, strrep, etc) can only take a string as a replacement. Strfind was similarly unhelpful, because of the typing across the table. The lack of cellfun has also made this harder than it should be. I know the solution should have something to do with finding the indices of the strings I want and then just looping DataFile{subscript} = [1], but I can't find a way to do it.
First you should transform your table at a cell array.
Then, you can use the strrep along with str2num, e.g.
% For a given cell index
strrep(yourCellIndexVariable, "Help me please", "1");
str2num(yourCellIndexVariable);
This will replace the string "Help me please" with the string "1" (the strrep function) and the str2num will change the cell index to the double value according to the string.
By yourCellIndexVariable I mean an element from the cell array. There are several ways to get all cells from a cell array, but I think that you have solved that part already.
What you can do is as follows:
[rows, cols] = size(table); % Get the size of your table
YourString = 'Help me please'; % Create your string
Strmat = repmat(YourString,rows,cols); % Stretch to fill a matrix of table size
TrueString = double(strcmp(table,Strmat)); % Compares all entries with one another
TrueString now contains logicals, 1 where the string 'Help me please' is located, and 0 where it is not.
If you have a table containing multiple classes it might be handy to switch to cells though.
Thank you very much everyone for helping think through to a solution. Here's what I ended up with:
% Reads data
[~, ~, raw] = xlsread ( 'MyTable.xlsx');
MyTable = raw;
% Makes a backup of the data in table form
MyTableBackup = readtable( 'MyTable.xlsx' );
% Begin by ditching 1st row with variable names
MyTable(1,:) = [];
% wizard magic - find all cells with strings
StringIndex = cellfun('isclass', MyTable, 'char');
% strrep goes here to recode bad strings. For example:
MyTable(StringIndex) = strrep(MyTable(StringIndex), 'PlzHelpMe', '1');
% Eventually, we are done, so convert back to table
MyTable = cell2table(MyTable);
% Uses backup Table to add variable names
% (the readtable above means the bad characters in variable names are already escaped!)
MyTable.Properties.VariableNames = MyTableBackup.Properties.VariableNames;
This means the new values exist as strings ('1', not 1 as a double), so now I just str2double when I access them for analysis. My takeaway - Matlab is for numbers. Thanks again all!
I was reading that in MatLab, if you are going to fill a larger matrix, its more computational efficient to declare it's size from before with the use of the cell command; E.g.
X = cell(500,90);
but when I try to add values to it, like
X(i;) = x
where i is vector of double of length 90, and i an integer, I get
conversion from cell to double is not possible
Is my understanding of the cell function correct?
Cell contents are being addressed using curly braces, for example:
X{1,1}=1:8;
cell command creates an empty array:
C = cell(3,4,2);
% Or alternatively:
C{3,4,2} = [];
What you do with the cell array is up to you. But most likely it is not what you want - see Rasman's comment.
Have a look at some more examples either at MathWorks or other tutorials.