extract from structure data with a field with the same value

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Giuela
Giuela 2021-1-22
评论： Giuela 2021-1-25
I have a structure with several fields and I want to extract only the structure elements having the same value for a particular field.
For example:
A = struct('type', '', 'par1', 0, 'par2', 0);
A(1).type = 'type1';
A(1).par1 = 10;
A(1).par2 = 20;
A(2).type = 'type2';
A(2).par1 = 11;
A(2).par2 = 22;
A(3).type = 'type3';
A(3).par1 = 33;
A(3).par2 = 40;
.
.
.
A(10).type = 'type2';
A(10).par1 = -1;
A(10).par2 = -1;
I need to extract ONLY the elements with type = 'type1'; the only way I know to do this is to do a loop like this:
indx = [];
for ii = 1:length(A)
if strcmp(A(ii).type,'type1')
indx = [ind ii];
end
end
Actually, A is bigger than the example; is there an easier an faster way to do this?
Thank you

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采纳的回答

Jan
Jan 2021-1-22
编辑：Jan 2021-1-22
match = strcmp({A.type}, 'type1'); % Logical indices
idx = find(match) % Indices
value = [A(match).par1] % Faster with logical indexing
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Jan
Jan 2021-1-23
Your apporach has some points for improvements:
idx = find(match1);
Index vectors are more expensive than addressing the elements by logical indexing. So prefer to use match1 directly and omit the find(). The find() for getting match2 is not needed also.
[B.par1] is nices and faster than cell2mat({B.par1}). Together:
match1 = strcmp({A.type}, 'type1');
B = A(match1);
match2 = ([B.par1] == 10);
value = [B(match2).par2];
Creating the partial copy B consumes a lot of ressources. It is cheaper to create the final index without this intermediate copy:
match = strcmp({A.type}, 'type1') & ([A.par1] == 10);
value = [A(match).par2]
Giuela
Giuela 2021-1-25
Thank you Jan, I've guessed this solution, but I wasn't able to join these two conditions in the correct way...

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