MATLAB Answers

## How to divide a structure into sub-structures based on a condition

Asked by SS

### SS (view profile)

on 24 Jul 2019
Latest activity Answered by Joel Handy

### Joel Handy (view profile)

on 25 Jul 2019
Accepted Answer by Walter Roberson

### Walter Roberson (view profile)

Hello.
I am new to MATLAB and expecting some help. I have a structure array (1 x 50,000) with 30 fields. I want to divide this structure into multiple structures based on a condition.
I tried sometheing like this. Let the main structure be vel and it has 30 fields. Based on the values of the parameters in the field y of the structure vel, I want to create sub-structures.
for i=1:length(vel)
if y<=20
vel_sub1(i)=vel(i) % creating a new structure
else
vel_sub(i)=[] % null
end
if y>=20 && y<=100
vel_sub2(i)=vel(i) % creating another sub-structure
else
vel_sub(2)=[] % null
end
end
But, unfortunatley I am getting some empty fields in the sub-structure, is there any smart way to implement this without any empty rows in the fields?

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SS

### SS (view profile)

on 25 Jul 2019
Hi. Thanks for the reply. Actually, it is the latter. I want to pull out certain fields and form a new structure.
Walter Roberson

### Walter Roberson (view profile)

on 25 Jul 2019
Is the desired result 1 x 50000 struct with two fields, each of which is a scalar struct with 30 fields?
Or is the desired result a scalar struct with two fields, one of which is a 1 x something struct array of 30 fields and the other is 1 x (50000 minus something) struct array of 30 fields?
SS

### SS (view profile)

on 25 Jul 2019
Hi. It is the second one.

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## 2 Answers

### Walter Roberson (view profile)

Answer by Walter Roberson

### Walter Roberson (view profile)

on 25 Jul 2019
Accepted Answer

mask = [vel.y] <= 20;
result.vel_sub1 = vel(mask) ;
result.vel_sub2 = vel(~mask) ;

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### Joel Handy (view profile)

Answer by Joel Handy

### Joel Handy (view profile)

on 25 Jul 2019

I think this is what you are looking for. You will end up with two struture arrays both smaller than the original structure array in the number of elements and the number of fields.
sub1Fields = {'Field1', 'Field3', 'Field5'};
sub2Fields = {'Field2', 'Field4', 'Field6'};
vel_sub1 = vel([vel,y] < 20);
vel_sub1 = rmfield(vel_sub1,sub2Fields);
vel_sub2 = vel([vel,y] >= 20);
vel_sub2 = rmfield(vel_sub1,sub1Fields);

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