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OOP: Select Data with conditional

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Nycholas Maia
Nycholas Maia on 27 Nov 2018
Edited: Matt J on 29 Nov 2018
I would like to filter a array of objects (animal class) and get a new array of objects that fits in 2 properties conditions simultaneally.
Example:
I want to get all animals that have more than 5 years old and less than 10 inches of height.
My animal class is:
classdef animal
properties
age % years
height % inches
name % string
end
end
So I can create animals objects like
dog1 = animal();
dog1.age = 10;
dog1.height = 15;
dog1.name = "Bob";
cat1 = animal();
cat1.age = 2;
cat1.height = 10;
cat1.name = "Alice";
And than put all animals objects in same array, like:
my_array = {dog1, cat1, ...};
How can I "filter" my_array using 2 conditionals (age and height)?

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Accepted Answer

Luna
Luna on 27 Nov 2018
Hi Nycholas,
Try this code below. Filtered array will return to you Jessie and Kayle which meets the conditions.
dog1 = animal();
dog1.age = 10;
dog1.height = 15;
dog1.name = 'Bob';
cat1 = animal();
cat1.age = 2;
cat1.height = 10;
cat1.name = 'Alice';
dog2 = animal();
dog2.age = 10;
dog2.height = 8;
dog2.name = 'Jessie';
cat2 = animal();
cat2.age = 6;
cat2.height = 3;
cat2.name = 'Kayle';
my_array = {dog1, cat1,dog2,cat2};
idx = logical(zeros(1,numel(my_array)));
for i = 1:numel(my_array)
if my_array{i}.age > 5 && my_array{i}.height < 10
idx(i) = true;
end
end
filteredArray = my_array(idx);

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Luna
Luna on 28 Nov 2018
If you want to get that kind of operation, you should create a vector of ages and heights of your animals.
AgesVector = [10 5 3 6 20];
HeightsVector = [20 15 3 5 6];
idx = AgesVector > 5 & HeightsVector < 10;
filteredArray = my_array(idx);
How did you create your my_array? If it has a hundred thousand element, I assume it is not possible to create it with: my_array = {dog1, cat1, etc ... };
So maybe we can find an easy way to create vector of your animal's ages and heights. Creating table would help I guess.
Nycholas Maia
Nycholas Maia on 28 Nov 2018
Sure Luna, you are right!
In the end of the day, I'm thinking to use vectors (or a big matrix) to do these operations above.
I was just trying to implement the same idea using OOP, because, in my opinion, the code stays more "clear"...but, any way...
I put here the "animal class" just to abstract my real problem: Read/Import all musical notes from a digital music score/sheet and use MATLAB to analysis the music score in mathematics way...
So, my read/import process is a "for loop" statement that runs on each note of my music score file. Inside this "for loop" I was creating a "musical note" object from the class below:
classdef musical_note
properties
midi_note % Scalar value, like: 60
duration % in seconds, like: 0.5
bar % or measure, like: 1578
instrument % Instrumen who is playing this note, like: "Piano"
Other1 % Other parameter of the musical note...
Other2
Other3
OtherN
end
end
After the import process, I will have the 'my_array' filled with all music_notes object.
Now, I would like to filter this array, in some "magical way", like:
filtered_array = some_function(my_array, midi_note = 50, duration > 10, instrument = "Piano")
As you well said, I think that the best way to implement this is creanting a big matrix and not use OOP. Right?
Thank you for your quickly answer!
Luna
Luna on 29 Nov 2018
It does nothing to do with OOP, I think what you need is a kind of table structure for that magical way below.
filtered_array = some_function(my_array, midi_note = 50, duration > 10, instrument = "Piano")
You should set a good defined tables for what you need or be using a database.
(You can still continue to use OOP)

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More Answers (1)

Matt J
Matt J on 29 Nov 2018
Edited: Matt J on 29 Nov 2018
In this scenario, it is better to have a single object whose properties are arrays than an array of objects whose properties are scalars. However, the following should be faster than looping,
my_array = [my_array{:}]; %get rid of cell array storage
conditions = [my_array.age]>5 & [my_array.height]<10;
filtered_array = my_array(conditions);

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