For your code to work the index tn4 must be scalar, but the error message tells us that it is not.
Compare:
S(1).blah = 1:3;
S(2).blah = 5:7;
S(1).blah(1) % index into S is scalar
S(1:2).blah(1) % index into S is NOT scalar
In both cases the indexing into S creates a comma-separated list (of one and two variables respectively):
The error occurs because the indexing at the end is ambiguous when applied to two or more variables of a comma-separated list, it is only applied without error when the comma-separated list returns exactly one array.
So you need to figure out:
- if the assumption of only one index value for tn4 is appropriate or not,
- how to handle the cases where zero, two or more indices are found (e.g. throw an error, or handle only one index, or special case handling, or use default data, or skip an iteration, etc.)
