MATLAB Answers

# how extract specific values from matrix and build separate arrays?

35 views (last 30 days)
Seba.V on 25 Apr 2020
Commented: Seba.V on 25 Apr 2020
Hi there!
I have a function that generates a 4x4 matrix with different values at every iteration (100 matrices all up). I then want to get the 4th coulmn 1st row values of all these matrices in one array and the 4th column 2nd row in another array.
I could only think of a for loop to go through every matrix one at the time and extract the elemts required in each array but no success
Here is an example of what I am trying to acheive.
Any help would be much appreciated!
T(95) =
0.4982 0 -0.8671 1.495
0.8671 0 0.4982 2.601
0 -1 0 0
0 0 0 1
T(96) =
0.4989 0 -0.8666 1.497
0.8666 0 0.4989 2.6
0 -1 0 0
0 0 0 1
T(97) =
0.4995 0 -0.8663 1.498
0.8663 0 0.4995 2.599
0 -1 0 0
0 0 0 1
T(98) =
0.4998 0 -0.8662 1.499
0.8662 0 0.4998 2.598
0 -1 0 0
0 0 0 1
T(99) =
0.4999 0 -0.8661 1.5
0.8661 0 0.4999 2.598
0 -1 0 0
0 0 0 1
T(100) =
0.5000 0 -0.8660 1.5
0.8660 0 0.5000 2.598
0 -1 0 0
0 0 0 1
for i=1:100
a1(i)=T(i)(4,1)
a2(i)=T(i)(4,2)
end
%resut:
a1=[......1.495 1.497 1.498 1.499 1.5 1.5]
a2=[[...... 2.601 2.6 2.599 2.598 2.598 2.598]

#### 2 Comments

Walter Roberson on 25 Apr 2020
T(95) =
0.4982 0 -0.8671 1.495
0.8671 0 0.4982 2.601
0 -1 0 0
0 0 0 1
Is that form, complete with the (95), being output to a text file? Or is that being output to the command window? Do you have access to the source code to be able to put the outputs into a cell array or a 3D array?
Seba.V on 25 Apr 2020
no that is out of the cooman window, and the function only give the matrix in that form.

Sign in to comment.

### Answers (1)

KALYAN ACHARJYA on 25 Apr 2020
Edited: KALYAN ACHARJYA on 25 Apr 2020
One Way: Indexing
%generates T in 3D
T(:,:,1)= [0.4982 0 -0.8671 1.495
0.8671 0 0.4982 2.601
0 -1 0 0
0 0 0 1]
T(:,:,2)= [0.4989 0 -0.8666 1.497
0.8666 0 0.4989 2.6
0 -1 0 0
0 0 0 1]
T(:,:,3)=[0.4995 0 -0.8663 1.498
0.8663 0 0.4995 2.599
0 -1 0 0
0 0 0 1]
T(:,:,4)= [0.4998 0 -0.8662 1.499
0.8662 0 0.4998 2.598
0 -1 0 0
0 0 0 1]
T(:,:,5)= [0.4999 0 -0.8661 1.5
0.8661 0 0.4999 2.598
0 -1 0 0
0 0 0 1]
T(:,:,6)= [0.5000 0 -0.8660 1.5
0.8660 0 0.5000 2.598
0 -1 0 0
0 0 0 1]
Then easy to extract the required data
>> whos T
Name Size Bytes Class Attributes
T 4x4x6 768 double
>> ar1=T(1,4,:)
ar1(:,:,1) =
1.4950
ar1(:,:,2) =
1.4970
ar1(:,:,3) =
1.4980
ar1(:,:,4) =
1.4990
ar1(:,:,5) =
1.5000
ar1(:,:,6) =
1.5000
Later you can go for 3D o 1D

#### 5 Comments

Show 2 older comments
Seba.V on 25 Apr 2020
I am using a free robotic toolbox (Peter Corke) function for forward kinematics called fkine()
Walter Roberson on 25 Apr 2020
The Peter Corke fkine() method returns an SE3 object. If you have that SE3 object, then you can use the R method to get a 3D array of rotations.
t = [0:.056:2]; % generate a time vector
q = jtraj(qz, qr, t); % compute the joint coordinate trajectory
T = p560.fkine(q);
Now
Rots = T.R;
Rots would be 3 x 3 x 36 where 36 is the number of poses (one for each t value in the above)
Seba.V on 25 Apr 2020
Amazing Walter...
Thank you!

Sign in to comment.

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by