I have an array of structures.
I'm trying to select several records from the array, that match some condition.
I know there's this option: (For example array A with field f1):
A([A.f1]==5)
Which would return all the records that have f1 = 5.
But I want to do it for several different fields, in a loop. I saved the fields names in a cell array, but I don't know how to do the same with a dynamic field name.
I know there's the 'getfield' function, but it only selects a field from a single structure.
Is there a way to do it?
Thanks!
To access dynamically a field of a structure:
% Create example structure
s.a = 1;
s.b = 2;
% Suppose you retrieve the fieldnames (or hardcode them fnames = {'a','b'})
fnames = fieldnames(s);
The you can retrieve e.g. the second one:
s.(fnames{2})
In a loop
for f = 1:numel(fnames)
s.(fnames{f})
end
In your case:
A([A.(fnames{ii})] == n)
This code will run through the first 5 records of your dynamical names
for i=1:5
eval(['A([A.' cell_array{i} ']==5)'])
end
Related
I have use a 428×1 cell array called data
data = {[1×1 struct]
[1×1 struct]
....
[1x1 struct]}
These struct all have the same structure
data {2,1}
>>struct with fields
additional_model_information: 'H Series,S3150-S0-AW-04-02-C-F421,'
ceiling_fan_size_diameters_in_inches: '60'
airflow_efficiency_cfm_watt_low: '727'
airflow_efficiency_cfm_watt_high: '392'
The structure fields are the same for all 428 cells, and the fields values change in each struct.
How could i create a table that puts all of the structs together so that it shows all of the field values for each field? Pehaps using a nested for loop?
I eventually want to export this data to excel.
Let's say you've got your nx1 cell array of structs, data. This method doesn't assume you have 4 fields in your data or any knowledge of what they are called. You could even use it with structs with different fieldnames if you included some try/catch logic.
You can get the field names of a representative struct using fieldnames
fnames = fieldnames(data{1});
Then set up some output cell array out of the correct size
out = cell(numel(fnames), numel(data));
Then use nested loops to 1. loop through items in data and 2. loop through field names.
for n = 1:numel(data)
for field = 1:numel(fnames)
temp = data{n};
out{field, n} = temp.(fnames{field});
end
end
Finally, append the field names as the first column
out = [fnames, out];
And write the cell array to your Excel file
% File location, cell array, sheet, cell
xlswrite('C:\...\test.xlsx', out, 1, 'A1');
How about
T = cell(length(data) , 4);
for k = 1 : length(data)
T(k , :) = {data{k}.additional_model_information , ...
data{k}.ceiling_fan_size_diameters_in_inches, ...
data{k}.airflow_efficiency_cfm_watt_low, ...
data{k}.airflow_efficiency_cfm_watt_high};
end
Then you can use xlswrite to export to excel.
I am currently post-processing a lot of .mat files (300+) created with a long simulation (3days+).
Each of these mat files contain several variables, each named after their position on a grid (yes, they've been created with eval).
I thus created a script that opens each of the files sequentially
s = what(pwd);
s = s.mat;
for i=1:length(s)
data = open(sprintf('%s',s{i,:}));
% here goes what I would like to do
end
What I am trying to do now is to extract the current name of the data component that fits a certain pattern.
Specifically, I know that in data there is a vector named coef_%i_%i and I would like to isolate it and assign it to a multi-dimensional array.
The second part I know how to do it, I can scan the variable name for the _ characters, isolate the integers and assign the vector to its appropriate location in the array.
My question is:
Is there in matlab a way to do something along the lines of vectorname = extractname('data.coef*');?
Assuming you have something like:
clear data
% create a dummy field
name = sprintf ( 'coef_%i_%i', randi(100,[2 1]) );
% a data struct with a field which starts "coef*"
data.(name) = rand;
% and your data field may contain some other fields?
data.other = [];
You can then extract out the fields which contain the coef string
function output = extractname ( data )
%extract the fieldnames from data:
fn = fieldnames(data);
% find all that have coef in them
index = cellfun(#isempty, strfind(fn,'coef'));
% remove any that dont have coef in them
fn(index) = [];
%You are then left with one(or more) that contain coef in the name:
output = fn;
end
If your data struct contains fields which may have "coef" elsewhere in the name you would need to go through them and check if coef is at the start. You should also check that at the end of your function that one and only one field has been found.
I'm working on a problem where I have an array A of 100 elements.
All these 100 elements are changing with time.
So in my workspace, I only get the final values of all these elements after the entire time cycle has run.
I'm trying to save the values with time in a separate file (.txt or .mat) so that I can access that file in order to check how the variable varies with time.
I'm trying the following command:
save('file.mat','A','-append');
But this command overwrites the existing values in my file.
Kindly suggest me a way to save these values without overwriting them and also guide me how to access them in MATLAB.
You can also change the output filename to be unique for each iteration:
for iter=1:n
A = rand(10);
save(sprintf('file%d.mat',iter), 'A');
end
That way each iteration creates one file.
The reason that saving to a file (even using the -append) flag didn't work is because the variable A already exists in the file and will be over-written each time through the loop. You would need to create a new file or new variable name every time through the loop in order for this to not happen.
Saving the results in a file is probably not the best way to store the time-varying values of A. You would be better off using a cell array to store all intermediate values of A.
A_over_time = cell();
for k = 1:n
%// Get A somehow
A_over_time{k} = A;
end
Depending on what A is, you could also store the values of A in a numeric array or matrix.
%// Using an array
A_over_time = zeros(N, 1);
for k = 1:N
A_over_time(k) = A;
end
%// Using a matrix
A_over_time = zeros(N, numel(A));
for k = 1:N
A_over_time(k,:) = A;
end
I have an array within an array and I am trying to name the variables using a for loop as there are a lot of variables. When I use the following simple code Time1 = dataCOMB{1,1}{1,1}(1:1024, 1); it opens the first cell in an array and proceeds to open the first cell in the following array and finally defines all the values in column 1 rows 1 to 1024 as Time1. However I have 38 of these different sets of data and when I apply the following code:
for t = 1:38
for aa = 1:38
Time(t) = dataCOMB{1,1}{1,aa}(1:1024, 1);
end
end
I get an error
In an assignment A(I) = B, the number of elements in B and I must be the same.
Error in Load_Files_working (line 39)
Time(t) = dataCOMB{1,1}{1,aa}(1:1024, 1);
Basically I am trying to get matlab to call the first column in each data set Time1, Time2, etc.
The problem:
1)You'd want to extract in a cell row...
2) ...the first 1024 numbers in the 1st column...
3) ...from each of the first 38 cells of a cell array.
The plan:
1) If one wants to get info from each element of a cell array (that is, an array accessed via {} indexing), one may use cellfun. Calling cellfun(some_function, a_cell_array) will aggregate the results of some_function(a_cell_array{k}) for all possible k subscripts. If the results are heterogeneous (i.e. not having the same type and size), one may use the cell_fun(..., 'UniformOutput', false) option to put them in an output cell array (cell arrays are good at grouping together heterogeneous data).
2) To extract the first 1024 numbers from the first column of an numeric array x one may use this anonymous function: #(x) x(1:1024,1). The x argument will com from each element of a cell array, and our anonymous function will play the role of some_function in the step above.
3) Now we need to specify a_cell_array, i.e. the cell array that contains the first 38 cells of the target. That would be, simply dataCOMB{1,1}(1,1:38).
The solution:
This one-liner implements the plan:
Time = cellfun(#(x) x(1:1024,1), dataCOMB{1,1}(1,1:38), 'UniformOutput', false);
Then you can access your data as in this example:
this_time = Time{3};
Your error is with Time(t). That's not how you create a new variable in matlab. To do exactly what you want (ie, create variables names Time1, Time2, etc...you'll need to use the eval function:
for aa = 1:38
eval(['Time' num2str(aa) '= dataCOMB{1,1}{1,aa}(1:1024,1);']);
end
Many people do not like recommending the eval function. Others wouldn't recommend moving all of your data out of a data structure and into their own independently-named variables. So, to address these two criticisms, a better alternative might be to pull your data out of your complicated data structure and to put it into a simpler array:
Time_Array = zeros(1024,38);
for aa = 1:38
Time_Array(:,aa) = dataCOMB{1,1}{1,aa}(1:1024,1);
end
Or, if you don't like that because you really like the names Time1, Time2, etc, you could create them as fields to a data structure:
Time_Data = [];
for aa = 1:38
fieldname = ['Time' num2str(aa)];
Time_Data.(fieldname) = dataCOMB{1,1}{1,aa}(1:1024,1);
end
And, in response to a comment below by the original post, this method can be extended to further unpack the data:
Time_Data = [];
count = 0;
for z = 1:2;
for aa = 1:38
count = count+1;
fieldname = ['Time' num2str(count)];
Time_Data.(fieldname) = dataCOMB{1,z}{1,aa}(1:1024,1);
end
end
I currently have a function which I am batch running. It outputs its results to a cell array. I would like to export each of the outputs from the cell array to their own variable.
As such I have a variable id which records the ID of each layer. This process is possible manually as follows:
>> output =
[300x300x2 double] [300x300x2 double] [300x300x2 double]
>> [a1,a2,a3]=deal(output{:});
Where the number after a represents the ID. Is it possible to automate this command, so that the prefix (in this case: a) can be set by the user and the id filled in automatically? As in, I could have variables set as below, and use these in the deal command to name my new variables?
>> id =
1 2 3
>> prefix =
a
Any ideas?
Thanks.
You can construct your own custom expression as a string and then evaluate it with eval() (or evalin() if it's in a function and you want to return the output to your workspace).
function deal_output(output, id, prefix)
id = id(:);
vars = strcat(prefix, cellstr(num2str(id)))';
myexpr = ['[', sprintf('%s,', vars{1:end-1}), vars{end}, '] = deal(output{:})'];
evalin('caller', myexpr)
>> output = num2cell(1:3);
>> id = 1:3;
>> prefix = 'a';
>> deal_output(output, id, prefix)
a1 =
4
a2 =
5
a3 =
6
Also check out the join.m file on FileExchange for avoiding the ugly sprintf.
Perhaps something like:
function dealinto(prefix, cellarray)
% DEALINTO
% USAGE
% dealinto('a', {'one', 'two', 'three'})
% Causes these variables to be set in the base workspace:
% a1: 'one'
% a2: 'two'
% a3: 'three'
for i=1:numel(cellarray)
assignin('base', [prefix num2str(i)], cellarray{i});
end
If you replace 'base' with 'caller' in the above, the variables will be written into the calling function's workspace. I don't recommend doing this, though, for the same reason that I would not recommend calling LOAD with no output arguments inside a function: arbitrarily writing into a running function's workspace isn't very safe.
If you need something like this for use inside of functions but don't want it just writing variables willy nilly, you could do the same thing that LOAD does, which is to give you a structure whose fields are the variables you would otherwise produce.
Do you really have to output them as completely separate variables? It would be much more efficient to use dynamic field names in a structure as this would avoid the use of the eval() statement. See the MATLAB blogs for avoiding eval()
In this case
m = length(output);
[a(1:m).data] = deal(output{:});
This will return a structure array, the length calculation is added so it'll work for different sizes of the output cell array.
Each array can be accessed individually with an id number.
a(1).data
a(2).data
a(3).data
I can't seem to add a comment so I'm adding an answer in the form of a question ^_^
Is it not better to pre-create the list of names and check them in the caller workspace using genvarname ?
Names = cell(1,numel(data))
for idx = 1:numel(data)
Names{idx} = [Prefix, num2str(idx)]
end
Names = genvarname(Names,who)
dealinto(Names, data)
would prevent any invalid names from being create in the existing work space, then dealto would need to be modified as
function dealinto(Names, Values)
for idx = 1:length(Names)
assignin('caller', Names(idx), Values(idx))
end