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mathematical calculation on 2D matrix(Implementation of 2 nested for loop )

asked 2015-06-16 22:23:00 -0600

Fi gravatar image

updated 2015-06-17 07:43:35 -0600

Hi all; As I understand, the 2D matrix in the ROS in not available until now and for this reasn, a definition of a msg that its variable is a msg of 1D should be done. But, I would like to know how I can implement the following code: 1-define a 2D matrix named mat[m][n]; (m*n)

I write the following messages: array 2d.msg and array 1d.msg. Is it a problem in my definition?

array 1d[] mat;

array 1d.msg 
int8[] a;

2- Implementation of 2 nested for loop The one of simple code for do some mathematical calculation on matrix is presented in the following:

for (i=starti; i<m, i++)  
  for (j=starti; j<n, j++)

Clearly, what I want to do is to do some mathematical functions on elements of matrix named mat. I write the simplest code of mathematical calculation in the previous lines. It is your kindness if you translate these lines to ROS code to give an idea to how to implement mathematical calculation with elements of 2D matrix.

Thanks for your help;


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answered 2015-06-17 08:02:42 -0600

updated 2015-06-19 14:58:57 -0600

The multi-array messages in std_msgs are very useful for sending matrices. All of these message definitions include a MultiArrayLayout message and just a simple array. For example, check out the definition of the Float32MultiArray. The MultiArrayLayout describes the size and shape of the matrix. This allows a node receiving one of these messages to properly unpack the array into a multidimensional array without requiring a priori knowledge of the matrix structure.

If you are trying to do mathematical calculations on those matrices in C++, it's probably worth checking into using a standard package for matrix math such as Eigen.


As requested, I've written a few simple demos. The demo consists of a Python script that fills out a 3x3 matrix using random integers in [0,10) and publishes that matrix as a Float32MultiArray on the /sent_matrix topic. There is then a C++ receiver that listens to that topic. In the callback I first access a few elements of the message using the formula in the MultiArrayLayout documentation, and I then convert the entire message into an Eigen::MatrixXf . Once I have that Eigen matrix I do a few simple matrix math operations and print them to the terminal. Finally I attempt to implement the nested for loop that was described in the original question. The entire ROS package is available at this gist:

The relevant Python code for filling out the matrix is below:

mat = Float32MultiArray()
mat.layout.dim[0].label = "height"
mat.layout.dim[1].label = "width"
mat.layout.dim[0].size = 3
mat.layout.dim[1].size = 3
mat.layout.dim[0].stride = 3*3
mat.layout.dim[1].stride = 3
mat.layout.data_offset = 0 = [0]*9

dstride1 = mat.layout.dim[1].stride
offset = mat.layout.data_offset
while not rospy.is_shutdown():
    for i in range(3):
        for j in range(3):
            num = random.randrange(0,10)
  [offset + i + dstride1*j] = num

The relevant C++ code for performing the original question could be boiled down to:

void matrixcb(const std_msgs::Float32MultiArray::ConstPtr& msg)
    float h = msg->layout.dim[0].size;
    float w = msg->layout.dim[1].size;
    std::vector<float> data = msg->data;
    Eigen::Map<Eigen::MatrixXf> mat(, h, w);
    Eigen::MatrixXf newmat = mat;
    for (int i=0; i<h-1; i++) 
        for (int j=0; j<w; j++) 
            newmat(i,j) = mat(i+1,j)+2;
    std::cout << "newmat = " << std::endl << newmat << std::endl;

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Thanks Jarvi Would you please write the code for simple aboving example code by MultiArray to have a better understand of it?

Fi gravatar image Fi  ( 2015-06-17 13:38:30 -0600 )edit

Thanks Javri; Would you please give me an example of using Eigen to work with matrix?

Fi gravatar image Fi  ( 2015-06-18 14:42:00 -0600 )edit

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Asked: 2015-06-16 22:23:00 -0600

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Last updated: Jun 19 '15