Matlab - Indexing an array by using string values

Question

I have a predefined array of 20 position which correspond to 20 joint out of my body. The joints are marked with string values (e.g. 'left_knee', 'head', 'left_eye', etc.).

I want to refer to a certain value within the array by using the attached string value. For example I want to store the position of the observed joints. Initially all the position within the array are (-1 , -1) and then if I spot a certain joint I want to do something like Joints('left_knee') = [100 200].

How can I do this in Matlab?

Rody Oldenhuis · Accepted Answer · 2012-11-08 11:37:05Z

13

How about structs?

Joints.left_knee = [100 200];
Joints.head      = [-1 -100];

Get all fields with fieldnames, refer to individual entries dynamically like so:

someVar = 'left_eye';
Joints.(someVar) = [200 250];

etc. If you happen to have multiple joints, all needing the same sort of data but they all belong to the same system, you can make multi-D structs, too:

Joints(1).left_knee = [100 200];
Joints(1).head      = [-1 -100];

Joints(2).left_knee = [200 450];
Joints(2).head      = [-10 -189];

Joints(3).left_knee = [-118 264];
Joints(3).head      = [+33  78];

Just to show you some techniques useful in the context of multi-D structs:

>> someVar = 'head';
>> Joints.(someVar)     % will be expanded cell-array, 1 entry for each dimension
ans =
    -1  -100
ans =
   -10  -189
ans =
    33    78
>> [Joints.(someVar)]   % will collect those data in regular array
ans =
    -1  -100   -10  -189    33    78
>> {Joints.(someVar)}   % will collect those data in cell array
ans = 
    [1x2 double]    [1x2 double]    [1x2 double]
>> [A,B,C] = deal(Joints.(someVar));  % will assign data to 3 separate vars
A =
    -1  -100
B =
   -10  -189
C =
    33    78

Type help struct for more info and learn about relevant functions.

edited Nov 8, 2012 at 11:37

answered Nov 8, 2012 at 9:29

Rody Oldenhuis

38.1k7 gold badges53 silver badges98 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Eitan T Over a year ago

+1 Nice, I didn't know you could refer to struct fields with a string variable.

petrichor · Accepted Answer · 2012-11-08 09:46:46Z

9

You can use the built-in map containers to map values to unique keys. Here's a quick example for you:

valueSet = {[-1, 1], [2, 3], [1,5], [1,8]};
keySet = {'left_knee', 'right_knee', 'left_eye', 'right_eye'};
Joints = containers.Map(keySet,valueSet);
Joints('left_knee')

ans =
    -1     1

You can get the values/keys of the map object as follows:

values(Joints)

ans = 
    [1x2 double]    [1x2 double]    [1x2 double]    [1x2 double]

keys(Joints)

ans = 
    'left_eye'    'left_knee'    'right_eye'    'right_knee'

See the documentation for more 'examples and how to'.

answered Nov 8, 2012 at 9:46

petrichor

6,5694 gold badges39 silver badges48 bronze badges

Comments

Gunther Struyf · Accepted Answer · 2012-11-08 09:54:49Z

2

To expand @RodyOldenhuis solution, you can also store your indexes in a struct array and use a plain matrix for the data. This way you can still apply normal matrix operation to all the data vectorwise:

idx.left_knee  = 1;
idx.right_knee = idx.left_knee + 1;
idx.left_elbow = idx.right_knee+ 1;
idx.right_elbow= idx.left_elbow+ 1;

joint_pos = rand(4,8);
t = linspace(0,1,8);
joint_vel = diff(joint_pos,2)./diff(t);

joint_vel(idx.right_knee,:)

In my example I set up a data structure for position and velocity of 4 joints at 8 points in time to illustrate my technique.

I find this easier to work with when using multi-dimensional datasets. Of course it depends on your application, if you for example just want to store constants, @RodyOldenhuis' solution obviously is much simpler.

edited Nov 8, 2012 at 9:54

answered Nov 8, 2012 at 9:43

Gunther Struyf

11.2k2 gold badges36 silver badges58 bronze badges

Collectives™ on Stack Overflow

Matlab - Indexing an array by using string values

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related