"Intermediate dot indexing produced a comma-separated list.." error prevents extracting values from nested indexed fields
487 views (last 30 days)
Show older comments
The following minimum working example illustrates a very common situation in Matlab, and shows the error produced when trying to aggregate values from across indexed fields that are themselves nested within other indexed fields.
a.students(1).grades = [7 8 9];
a.students(2).grades = [6 7 8];
max(a.students(:).grades(:)) % obtain maximum of ALL grades, from ALL students
% gives the error:
% Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations.
The same error is produced if I simply try to print out those values, without applying max or any other function, and putting the expression in square brackets doesn't help:
[ a.students(:).grades(:) ]
The only way to go around this is to keep using for loops, which makes code inelegant and error-prone. This seems too basic a feature to not have a built-in solution in Matlab. Would appreciate any suggestions how this could be achieved.
1 Comment
Stephen23
on 18 May 2022
Edited: Stephen23
on 18 May 2022
Your code does not work because you are trying to apply one index onto multiple variables. MATLAB does not have such a command that applies one index to all members of a comma-separated list, nor is it likely to in the near future (that would be a major change in how commas and lists are interpreted).
a.students(1).grades = [7 8 9];
a.students(2).grades = [6 7 8];
a.students(:).grades % this returns multiple separate variables, not one array.
a.students(:).grades(:) % how can we apply one index to multiple arrays?
Basically your code is doing something a bit like this (pseudo-code):
[7 8 9],[6 7 8](:)
with the expectation that the colon index would be applied to both members of the comma-separated list.
Accepted Answer
See Also
Categories
Find more on Matrix Indexing in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!