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How to access a field of a struct by indexing?

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Rightia Rollmann
Rightia Rollmann 2017-2-26
评论： Voss 2023-12-19
I have a 1-by-1 struct that possesses 3 fields named B, C, and D. Is there any way to call D by its index (i.e., D is the third field of struct A, so call the third field of struct A without mentioning the field name D) rather than its name (i.e, A.D)?
A.B = 1;
A.C = 2;
A.D = 3;

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采纳的回答

Jan
Jan 2017-2-26
编辑：Jan 2017-2-26
A_cell = struct2cell(A);
D = A_cell{3}
Keep in mind that the order of the fields of structs is not necessarily constant:
A.B = 1;
A.C = 2;
A.D = 3;
B.B = 1;
B.D = 3;
B.C = 2;
isequal(A, B) % >> TRUE!
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Richard Crozier
Richard Crozier 2019-8-1
If struct2cell results in copying the data in the structure, Guillaume's answer below is superior.
James Tursa
James Tursa 2019-8-1
struct2cell creates shared-data-copies of the field variables. So, while there is overhead involved in creating the variable header info, the data itself is not copied.

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Guillaume
Guillaume 2017-2-26
Yes, there is a way to get the nth field directly:
fns = fieldnames(A);
A.(fns{3})
But be aware that the order of the fields depends solely on the order in which they were created. As Jan pointed out, two structures may be indentical, yet have different field order.
Usually, you would only access fields by their index when you're doing some structure metaprogramming
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Vaishnavi
Vaishnavi 2023-12-19
To add on to this question, can someone explain why A.(subsref(fieldnames(A),substruct('{}',{:}))) would not work?
Voss
Voss 2023-12-19
Ran in:
@Vaishnavi: One reason that doesn't work is that you need single quotes around the colon in substruct. But even then, it may not do what you expect. Here's what it does (gets the value of the first field of A):
A = struct('field1',1,'field2',2)
A = struct with fields:
field1: 1 field2: 2
A.(subsref(fieldnames(A),substruct('{}',{':'})))
ans = 1
In order to know whether that "works", one would need to know what you expected it to do.

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