I'm interested in the general problem of accessing a field which may be buried an arbitrary number of levels deep in a containing structure. A concrete example using two levels is below.
Say I have a structure toplevel, which I define from the MATLAB command line with the following:
midlevel.bottomlevel = 'foo';
toplevel.midlevel = midlevel;
I can access the midlevel structure by passing the field name as a string, e.g.:
fieldnameToAccess = 'midlevel';
value = toplevel.(fieldnameToAccess);
but I can't access the bottomlevel structure the same way -- the following is not valid syntax:
fieldnameToAccess = 'midlevel.bottomlevel';
value = toplevel.(fieldnameToAccess); %# throws ??? Reference to non-existent field 'midlevel.bottomlevel'
I could write a function that looks through fieldnameToAccess for periods and then recursively iterates through to get the desired field, but I am wondering if there's some clever way to use MATLAB built-ins to just get the field value directly.
You would have to split the dynamic field accessing into two steps for your example, such as:
>> field1 = 'midlevel';
>> field2 = 'bottomlevel';
>> value = toplevel.(field1).(field2)
value =
foo
However, there is a way you can generalize this solution for a string with an arbitrary number of subfields delimited by periods. You can use the function TEXTSCAN to extract the field names from the string and the function GETFIELD to perform the recursive field accessing in one step:
>> fieldnameToAccess = 'midlevel.bottomlevel';
>> fields = textscan(fieldnameToAccess,'%s','Delimiter','.');
>> value = getfield(toplevel,fields{1}{:})
value =
foo
Related
I need to pass a part of a structure's name into a function.
Examples of a available structs:
systems.system1.stats.equityCurve.relative.exFee
systems.system1.stats.equityCurve.relative.inFee
systems.system2.stats.equityCurve.relative.exFee
systems.system2.stats.equityCurve.relative.inFee
systems.system1.returns.aggregated.exFee
systems.system1.returns.aggregated.inFee
systems.system2.returns.aggregated.exFee
systems.system2.returns.aggregated.inFee
... This goes on...
Within a function, I loop through the structure as follows:
function mat = test(fNames)
feeString = {'exFee', 'inFee'};
sysNames = {'system1', 'system2'};
for n = 1 : 2
mat{n} = systems.(sysNames{n}).stats.equityCurve.relative.(feeString{n});
end
end
What I like to handle in a flexible way within the loop is the middle part, i.e. the part after systems.(sysNames{n}) and before .(feeString{n}) (compare examples).
I am now looking for a way to pass the middle part as an input argument fNames into the function. The loop should than contain something like
mat{n} = systems.(sysNames{n}).(fName).(feeString{n});
How about using a helper function such as
function rec_stru = recSA(stru, field_names)
if numel(field_names) == 1
rec_stru = stru.(field_names{1});
else
rec_stru = recSA(stru.(field_names{1}), field_names(2:end));
end
This function takes the intermediate field names as a cell array.
This would turn this statement:
mat{n} = systems.(sysNames{n}).stats.equityCurve.relative.(feeString{n});
into
mat{n} = recSA(systems.(sysNames{n}), {'stats', 'equityCurve', 'relative', feeString{n}});
The first part of the cell array could then be passed as an argument to the function.
This is one of those cases where matlab is a bit unhelpful in the documentation. There is a way to use the fieldnames function in matlab to get the list of all the fields and iterate over that using dynamic fields.
systems.system1.stats.equityCurve.relative.exFee='T'
systems.system1.stats.equityCurve.relative.inFee='E'
systems.system2.stats.equityCurve.relative.exFee='S'
systems.system2.stats.equityCurve.relative.inFee='T'
systems.system1.returns.aggregated.exFee='D'
systems.system1.returns.aggregated.inFee='A'
systems.system2.returns.aggregated.exFee='T'
systems.system2.returns.aggregated.inFee='A'
dynamicvariable=fieldnames(systems.system1)
This will return a cell matrix of the field names which you can use to iterate over.
systems.system1.(dynamicvariable{1})
ans =
equityCurve: [1x1 struct]
Ideally you would have your data structure fixed in such a way that you know how many levels of depth are in your data structure.
I have a long list of variables in my workspace.
First, I'm finding the potential variables I could be interested in using the who function. Next, I'd like to loop through this list to find the size of each variable, however who outputs only the name of the variables as a string.
How could I use this list to refer to the values of the variables, rather than just the name?
Thank you,
list = who('*time*')
list =
'time'
'time_1'
'time_2'
for i = 1:size(list,1);
len(i,1) = length(list(i))
end
len =
1
1
1
If you want details about the variables, you can use whos instead which will return a struct that contains (among other things) the dimensions (size) and storage size (bytes).
As far as getting the value, you could use eval but this is not recommended and you should instead consider using cell arrays or structs with dynamic field names rather than dynamic variable names.
S = whos('*time*');
for k = 1:numel(S)
disp(S(k).name)
disp(S(k).bytes)
disp(S(k).size)
% The number of elements
len(k) = prod(S(k).size);
% You CAN get the value this way (not recommended)
value = eval(S(k).name);
end
#Suever nicely explained the straightforward way to get this information. As I noted in a comment, I suggest that you take a step back, and don't generate those dynamically named variables to begin with.
You can access structs dynamically, without having to resort to the slow and unsafe eval:
timestruc.field = time;
timestruc.('field1') = time_1;
fname = 'field2';
timestruc.(fname) = time_2;
The above three assignments are all valid for a struct, and so you can address the fields of a single data struct by generating the field strings dynamically. The only constraint is that field names have to be valid variable names, so the first character of the field has to be a letter.
But here's a quick way out of the trap you got yourself into: save your workspace (well, the relevant part) in a .mat file, and read it back in. You can do this in a way that will give you a struct with fields that are exactly your variable names:
time = 1;
time_1 = 2;
time_2 = rand(4);
save('tmp.mat','time*'); % or just save('tmp.mat')
S = load('tmp.mat');
afterwards S will be a struct, each field will correspond to a variable you saved into 'tmp.mat':
>> S
S =
time: 1
time_1: 2
time_2: [4x4 double]
An example writing variables from workspace to csv files:
clear;
% Writing variables of myfile.mat to csv files
load('myfile.mat');
allvars = who;
for i=1:length(allvars)
varname = strjoin(allvars(i));
evalstr = strcat('csvwrite(', char(39), varname, '.csv', char(39), ', ', varname, ')');
eval(evalstr);
end
I have a Matlab structure like this:
Columns.T21=6;
Columns.ws21=9;
Columns.wd21=10;
Columns.u21=11;
Is there some elegant way I can give the value and return the field name? For instance, if I give 6 and it would return 'T21.' I know that fieldnames() will return all the field names, but I want the fieldname for a specific value. Many thanks!
Assuming that the structure contains fields with scalar numeric values, you can use this struct2array based approach -
search_num = 6; %// Edit this for a different search number
fns=fieldnames(Columns) %// Get field names
out = fns(struct2array(Columns)==search_num) %// Logically index into names to find
%// the one that matches our search
Goal:
Construct two vectors from your struct, one for the names of fields and the other for their respective values. This has analogy to the dict in Python or map in C++, where you have unique keys being mapped to possibly non-unique values.
Simple Solution:
You can do this very simply using the various functions defined for struct in Matlab, namely: struc2cell() and cell2mat()
For the particular element of interest, say 1 of your struct Columns, get the names of all fields in the form of a cell array, using fieldnames() function:
fields = fieldnames( Columns(1) )
Similarly, get the values of all the fields of that element of Columns, in the form of a matrix
vals = cell2mat( struct2cell( Columns(1) ) )
Next, find the field with the corresponding value, say 6 here, using the find function and convert the resulting 1x1 cell into a char using cell2mat() function :
cell2mat( fields( find( vals == 6 ) ) )
which will yield:
T21
Now, you can define a function that does this for you, e.g.:
function fieldname = getFieldForValue( myStruct, value)
Advanced Solution using Map Container Data Abstraction:
You can also choose to define an object of the containers.map class using the field-names of your struct as the keySet and values as valueSet.
myMap = containers.Map( fieldnames( Columns(1) ), struct2cell( Columns(1) ) );
This allows you to get keys and values using corresponding built-in functions:
myMapKeys = keys(myMap);
myMapValues = values(myMap);
Now, you can find all the keys corresponding to a particular value, say 6 in this case:
cell2mat( myMapKeys( find( myMapValues == 6) )' )
which again yields:
T21
Caution: This method, or for that matter all methods for doing so, will only work if all the fields have the values of the same type, because the matrix to which we are converting vals to, need to have a uniform type for all its elements. But I assume from your example that this would always be the case.
Customized function/ logic:
struct consists of elements that contain fields which have values, all in that order. An element is thus a key for which field is a value. The essence of "lookup" is to find values (which are non-unique) for specific keys (which are unique). Thus, Matlab has a built-in way of doing so. But what you want is the other way around, i.e. to find keys for specific values. Since its not a typical use case, you need to write up your own logic or function for it.
Suppose your structure is called S. First extract all the field names into an array:
fNames=fieldnames(S);
Now define a following anonymous function in your code:
myfun=#(yourArray,desiredValue) yourArray==desiredValue;
Then you can get the desired field name as:
desiredFieldIndex=myfun(structfun(#(x) x,S),3) %desired value is 3 (say)
desiredFieldName=fNames(desiredFieldIndex)
Alternative using containers.Map
Assuming each field in the structure contains one scalar value as in the question (not an array).
Aim is to create a Map object with the field values as keys and the field names as values
myMap = containers.Map(struct2cell(Columns),fieldnames(Columns))
Now to get the fieldname for a value index into myMap with the value
myMap(6)
ans =
T21
This has the advantage that if the structure doesn't change you can repeatedly use myMap to find other value-field name pairs
I have a variable called 'int' with alot of data in it. I would like to find a way to programically rename this variable with a user input. So I can query the user indentifcation information about the data, say the response is 'AA1', I want either rename the variable 'int' to 'AA1' or make 'AA1' a variable that is identical to int.
A problem using the input command arises because it allows the user to assign a value to an already created varialbe, instead of actually creating a variable name. Would using the eval function, or a variation of it, help me achieve this? Or is there an easier way?
Thanks!
Just for the record, int is a rather poor variable name choice.
That aside, you can do what you want as follows
say foo is the variable that holds a string that the user input. You can do the following:
% eliminate leading/trailing whitespace
foo = strtrim(foo);
a = regexp('[a-zA-Z][a-zA-Z0-9_]*',foo));
if numel(a) == 0
fprintf('Sorry, %s is not a valid variable name in matlab\n', foo);
elseif a ~= 1
fprintf('Sorry, %s is not a valid variable name in matlab\n', foo);
elseif 2 == exist(foo,'var')
fprintf('Sorry, %s already in use as a variable name.');
else
eval([foo,' = int']);
end
Assuming int (and now foo) is a structure with field named bar, you can read bar as follows:
barVal = eval([foo,'.bar']);
This is all somewhat clunky.
An alternative approach, that is far less clunky is to use an associative array, and let the user store various values of int in the array. The Matlab approach for associative arrays is Maps. That would be my preferred approach to this problem. Here is an example using the same variables as above.
nameValueMap = containers.Map;
nameValueMap(foo) = int;
The above creates the association between the name stored in foo with the data in the variable int.
To get at the data, you just do the following:
intValue = nameValueMap(foo);
How can i access the following structure path with dynamic fieldnames:
var = 'refxtree.CaseDefinition.FlowSheetObjects.MaterialStreamObjects{8}.MaterialStreamObjectParams.Pressure.Value.Text';
fields = textscan(var,'%s','Delimiter','.');
refxtree.(fields{:}) does not work because MaterialStreamObjects contains a cell array of which I want to access the 8th cell and then continue down the structure path.
In the end I want to get and set the fieldvalues.
You need to build the appropriate input to subsref, possibly using substruct. Look at the MATLAB help.
You can define an anonymous function to navigate this particular kind of structure of the form top.field1.field2.field3{item}.field4.field5.field6.field7 (as an aside: is it really necessary to have such a complicated structure?).
getField = #(top,fields,item)top.(fields{1}).(fields{2}).(fields{3}){item}.(fields{4}).(fields{5}).(fields{6}).(fields{7})
setField = #(top,fields,item,val)subsasgn(top.(fields{1}).(fields{2}).(fields{3}){item}.(fields{4}).(fields{5}).(fields{6}),struct('type','.','subs',fields{7}),val);
You use the functions by calling
fieldValue = getField(refxtree,fields,8);
setField(refxtree,fields,8,newFieldValue);
Note that fields is required to have seven elements. If you want to generalize the above, you will have to dynamically create the above functions
In this case, it is easier to just use EVAL:
str = 'refxtree.CaseDefinition.FlowSheetObjects.MaterialStreamObjects{8}.MaterialStreamObjectParams.Pressure.Value.Text';
%# get
x = eval(str)
%# set
evalc([str ' = 99']);