# Nesting depth and the error "Expected one output from a curly brace or dot indexing expression, but there were x results."

5 views (last 30 days)
z8080 on 18 Aug 2022
Commented: Stephen23 on 19 Aug 2022
Consider this toy example:
a(1).x.y=1
a(2).x.y=2
[a.x.y]
Why does this produce the infamous error
Expected one output from a curly brace or dot indexing expression, but there were 2 results.
..instead of just listing all values from across all indexed structure elements, as in this other example where the nesting is at level 2 instead of level 3:
a(1).x=1
a(2).x=2
[a.x]
>> [a.x]
ans =
1 2
Stephen23 on 19 Aug 2022

Bruno Luong on 19 Aug 2022
Edited: Bruno Luong on 19 Aug 2022
a work around if you insist on oneline
a(1).x.y=1
a = struct with fields:
x: [1×1 struct]
a(2).x.y=2
a = 1×2 struct array with fields:
x
axy = [struct([a.x]).y]
axy = 1×2
1 2

Jan on 19 Aug 2022
R2022a creates a different error:
a(1).x.y=1;
a(2).x.y=2;
[a.x.y]
Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations.
"instead of just listing all values from across all indexed structure elements"
Think twice. [a.x] is an array already with 2 elements. The dot operator cannot handle an array as input, but a scalar struct only. This is plausible. Consider, that there is no logical decision for the dimensions of the output. It is also unclear, what you call "just listing all values".
Stephen23 on 19 Aug 2022
"The dot operator cannot handle an array as input, but a scalar struct only."
???
a(1).x.y=1;
a(2).x.y=2;
tmp = [a.x] % array struct, not scalar struct
tmp = 1×2 struct array with fields:
y
[tmp.y] % dot indexing accepts an array without any problem
ans = 1×2
1 2

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