Is there an alternative to eval in a struct?

Question

Yasir Çatal 2021-10-21

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回答： Steven Lord 2021-10-22

Hi,

I know that eval is evil, but in this particular case, I couldn't find any other way to do it, so I wanted to ask you if anyone comes up with a more efficient way.

The problem is, I need to define a struct, and structs inside that struct, and more structs inside that struct. I think it would be better to describe it (simplified) this way:

subject = {'subj_1','subj_2','subj_3'};
runs = {'rest','task1','task2','task3'};
roigroups = {'a','b','c'};
a = {'dlpfc','amyg'};
b = {'insula','postcg'};
c = {'thalamus','nucl_ruber'};
subjects = struct();
% The dirty way, assume that other structs defined previously:
for i = 1:length(subject)
    for p = 1:length(runs)
        for x = 1:length(roigroups)
            for y = 1:eval(strcat('length(',roigroups{x},')'))
                subjects.(subject{i}).(runs{p}).(roigroups{x}).(eval(strcat(eval('roigroups{x}'),'{y}'))) = struct();
            end
        end
    end
end
Unrecognized function or variable 'tasks'.

It works really well, manually checked, but it's a pain to write and read. Is there a workaround for this type of task?

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Akira Agata 2021-10-22

Still not working.

As shown below, error occurs in the line "subjects.(subjects{i})~" .

subjects = {'subj_1','subj_2','subj_3'};
tasks = {'rest','task1','task2','task3'};
roigroups = {'a','b','c'};
a = {'dlpfc','amyg'};
b = {'insula','postcg'};
c = {'thalamus','nucl_ruber'};
% The dirty way, assume that other structs defined previously:
for i = 1:length(subjects)
    for p = 1:length(tasks)
        for x = 1:length(roigroups)
            for y = 1:eval(strcat('length(',roigroups{x},')'))
                subjects.(subjects{i}).(tasks{p}).(roigroups{x}).(eval(strcat(eval('roigroups{x}'),'{y}'))) = struct();
            end
        end
    end
end
Unable to perform assignment because dot indexing is not supported for variables of this type.

Yasir Çatal 2021-10-22

Sorry, I edited now so that it works. Should've checked before, sorry about that.

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Answer 1

Matt J 2021-10-22

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编辑：Matt J 2021-10-22

Your data organization seems like it should be reconsidered. In a struct, it doesn't make sense if different fields have essentially the same kind of contents. What I think you really want is a 2D or 3D struct array, something like

subject = {'subj_1','subj_2','subj_3'};
tasks = {'rest','task1','task2','task3'};
M=numel(subject);
N=numel(tasks);
a = struct('dlpfc',[],'amyg',[]);
b = struct('insula',[],'postcg',[]);
c = struct('thalamus',[],'nucl_ruber',[]);
[Subjects(1:M,1:N).a]=deal(a);
[Subjects(1:M,1:N).b]=deal(b);
[Subjects(1:M,1:N).c]=deal(c)
Subjects = 3×4 struct array with fields:
    a
    b
    c

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Matt J 2021-10-22

编辑：Matt J 2021-10-22

Okay, but is there an intuitive way to see which task/roi etc. belong to which subject?

In your post, all subjects share the same tasks and roigroups. That's why I think an array makes more sense.

for example, I need to exclude some subjects, and when I do that, I don't want every subsequent subject to go up one row and mess everything.

You would have to show an example of "mess everything". Why couldn't you extract a subset of Subjects using integer indices? e.g.,

subset=Subjects([1,5,7],:)

Yasir Çatal 2021-10-22

Thanks to everyone that involved, I guess I should go with subsets.

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Answer 2

Akira Agata 2021-10-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1569153-is-there-an-alternative-to-eval-in-a-struct#answer_813868

I believe it's better to arrange the data as a table rather than the deeply nested structure. How about the following?

subject = {'subj_1','subj_2','subj_3'};
runs = {'rest','task1','task2','task3'};
roigroups = {'a','b','c'};
a = {'dlpfc','amyg'};
b = {'insula','postcg'};
c = {'thalamus','nucl_ruber'};
[p,q,r] = ndgrid(subject, runs, [a,b,c]);
T = table(p(:),q(:),r(:),'VariableNames',{'subject','runs','roi'});
T.roigroups = repelem(roigroups',24,1);
T = movevars(T,'roigroups','Before','roi');
disp(T)
     subject        runs       roigroups         roi      
    __________    _________    _________    ______________

    {'subj_1'}    {'rest' }      {'a'}      {'dlpfc'     }
    {'subj_2'}    {'rest' }      {'a'}      {'dlpfc'     }
    {'subj_3'}    {'rest' }      {'a'}      {'dlpfc'     }
    {'subj_1'}    {'task1'}      {'a'}      {'dlpfc'     }
    {'subj_2'}    {'task1'}      {'a'}      {'dlpfc'     }
    {'subj_3'}    {'task1'}      {'a'}      {'dlpfc'     }
    {'subj_1'}    {'task2'}      {'a'}      {'dlpfc'     }
    {'subj_2'}    {'task2'}      {'a'}      {'dlpfc'     }
    {'subj_3'}    {'task2'}      {'a'}      {'dlpfc'     }
    {'subj_1'}    {'task3'}      {'a'}      {'dlpfc'     }
    {'subj_2'}    {'task3'}      {'a'}      {'dlpfc'     }
    {'subj_3'}    {'task3'}      {'a'}      {'dlpfc'     }
    {'subj_1'}    {'rest' }      {'a'}      {'amyg'      }
    {'subj_2'}    {'rest' }      {'a'}      {'amyg'      }
    {'subj_3'}    {'rest' }      {'a'}      {'amyg'      }
    {'subj_1'}    {'task1'}      {'a'}      {'amyg'      }
    {'subj_2'}    {'task1'}      {'a'}      {'amyg'      }
    {'subj_3'}    {'task1'}      {'a'}      {'amyg'      }
    {'subj_1'}    {'task2'}      {'a'}      {'amyg'      }
    {'subj_2'}    {'task2'}      {'a'}      {'amyg'      }
    {'subj_3'}    {'task2'}      {'a'}      {'amyg'      }
    {'subj_1'}    {'task3'}      {'a'}      {'amyg'      }
    {'subj_2'}    {'task3'}      {'a'}      {'amyg'      }
    {'subj_3'}    {'task3'}      {'a'}      {'amyg'      }
    {'subj_1'}    {'rest' }      {'b'}      {'insula'    }
    {'subj_2'}    {'rest' }      {'b'}      {'insula'    }
    {'subj_3'}    {'rest' }      {'b'}      {'insula'    }
    {'subj_1'}    {'task1'}      {'b'}      {'insula'    }
    {'subj_2'}    {'task1'}      {'b'}      {'insula'    }
    {'subj_3'}    {'task1'}      {'b'}      {'insula'    }
    {'subj_1'}    {'task2'}      {'b'}      {'insula'    }
    {'subj_2'}    {'task2'}      {'b'}      {'insula'    }
    {'subj_3'}    {'task2'}      {'b'}      {'insula'    }
    {'subj_1'}    {'task3'}      {'b'}      {'insula'    }
    {'subj_2'}    {'task3'}      {'b'}      {'insula'    }
    {'subj_3'}    {'task3'}      {'b'}      {'insula'    }
    {'subj_1'}    {'rest' }      {'b'}      {'postcg'    }
    {'subj_2'}    {'rest' }      {'b'}      {'postcg'    }
    {'subj_3'}    {'rest' }      {'b'}      {'postcg'    }
    {'subj_1'}    {'task1'}      {'b'}      {'postcg'    }
    {'subj_2'}    {'task1'}      {'b'}      {'postcg'    }
    {'subj_3'}    {'task1'}      {'b'}      {'postcg'    }
    {'subj_1'}    {'task2'}      {'b'}      {'postcg'    }
    {'subj_2'}    {'task2'}      {'b'}      {'postcg'    }
    {'subj_3'}    {'task2'}      {'b'}      {'postcg'    }
    {'subj_1'}    {'task3'}      {'b'}      {'postcg'    }
    {'subj_2'}    {'task3'}      {'b'}      {'postcg'    }
    {'subj_3'}    {'task3'}      {'b'}      {'postcg'    }
    {'subj_1'}    {'rest' }      {'c'}      {'thalamus'  }
    {'subj_2'}    {'rest' }      {'c'}      {'thalamus'  }
    {'subj_3'}    {'rest' }      {'c'}      {'thalamus'  }
    {'subj_1'}    {'task1'}      {'c'}      {'thalamus'  }
    {'subj_2'}    {'task1'}      {'c'}      {'thalamus'  }
    {'subj_3'}    {'task1'}      {'c'}      {'thalamus'  }
    {'subj_1'}    {'task2'}      {'c'}      {'thalamus'  }
    {'subj_2'}    {'task2'}      {'c'}      {'thalamus'  }
    {'subj_3'}    {'task2'}      {'c'}      {'thalamus'  }
    {'subj_1'}    {'task3'}      {'c'}      {'thalamus'  }
    {'subj_2'}    {'task3'}      {'c'}      {'thalamus'  }
    {'subj_3'}    {'task3'}      {'c'}      {'thalamus'  }
    {'subj_1'}    {'rest' }      {'c'}      {'nucl_ruber'}
    {'subj_2'}    {'rest' }      {'c'}      {'nucl_ruber'}
    {'subj_3'}    {'rest' }      {'c'}      {'nucl_ruber'}
    {'subj_1'}    {'task1'}      {'c'}      {'nucl_ruber'}
    {'subj_2'}    {'task1'}      {'c'}      {'nucl_ruber'}
    {'subj_3'}    {'task1'}      {'c'}      {'nucl_ruber'}
    {'subj_1'}    {'task2'}      {'c'}      {'nucl_ruber'}
    {'subj_2'}    {'task2'}      {'c'}      {'nucl_ruber'}
    {'subj_3'}    {'task2'}      {'c'}      {'nucl_ruber'}
    {'subj_1'}    {'task3'}      {'c'}      {'nucl_ruber'}
    {'subj_2'}    {'task3'}      {'c'}      {'nucl_ruber'}
    {'subj_3'}    {'task3'}      {'c'}      {'nucl_ruber'}

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Answer 3

Steven Lord 2021-10-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1569153-is-there-an-alternative-to-eval-in-a-struct#answer_814408

For most of what you're trying to do you could use setfield.

subject = {'subj_1','subj_2','subj_3'};
runs = {'rest','task1','task2','task3'};
roigroups = {'a','b','c'};
a = {'dlpfc','amyg'};
b = {'insula','postcg'};
c = {'thalamus','nucl_ruber'};
subjects = struct();
subjects = setfield(subjects, subject{1}, runs{2}, roigroups{3}, c{1})
subjects = struct with fields:
    subj_1: [1×1 struct]
% Checking
subjects.subj_1
ans = struct with fields:
    task1: [1×1 struct]
subjects.subj_1.task1
ans = struct with fields:
    c: 'thalamus'

For that last input probably I'd make a cell array of cell arrays out of a, b, and c.

D = {a, b, c};
subjects = setfield(subjects, subject{2}, runs{1}, roigroups{2}, D{2}{2})
subjects = struct with fields:
    subj_1: [1×1 struct]
    subj_2: [1×1 struct]
subjects.subj_2.rest.b
ans = 'postcg'