# James Tursa

Interested in external applications, mex & engine applications with Fortran, C, C++, etc, particularly in the areas of speed improvement and memory management efficiencies.

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**Content Feed**

What does 'li' mean?

1i is used to ensure it isn't confused with a variable named i that you might have in the workspace. I.e., 1i is always sqrt(-1...

circa 22 ore ago | 0

I have matrix A and I need to find (e^(At)) where t is the sampling time. How to find that? Also what is the difference between exp(A) and expm(A)?

exp(A) just takes the exp( ) of the scalar elements individually. I.e., exp(A(1,1)), exp(A(1,2)), etc. expm(A) takes the matri...

circa 23 ore ago | 0

help with my 4th order runge kutta code

My guess without seeing the rest of your code is that phi is a column vector, so when you add phi*h to the row vector Y(i,:) it ...

4 giorni ago | 0

| accepted

how to change from for loop to while loop

You never change i or Errors within the while loop, so how is the loop ever supposed to exit? Maybe you want to add this line: ...

4 giorni ago | 0

Is mxCopyPtrToInteger4() not compatible with Interleaved Complex API ?

See this post: https://www.mathworks.com/matlabcentral/answers/93860-why-does-the-fortran-mex-api-function-mxcopyptrtointeger4-...

13 giorni ago | 0

| accepted

What should go in a next-generation MATLAB X?

Symmetric variables and Hermitian variables. MATLAB could implement bit flags in the mxArray header to indicate this and they c...

15 giorni ago | 0

Why indexing vs not indexing the 2D matrix will lead to different result?

Some links related to this known behavior: https://www.mathworks.com/matlabcentral/answers/599818-computational-complexity-of-m...

15 giorni ago | 0

Properly resizing mxCreateStructMatrix post-2018a?

TMW doesn't publish the complete rules for how the data areas of structs are organized, but historically they have always put ea...

22 giorni ago | 0

How to get the ceofficients of a expression derived by ODE or integral ? For exampli,C1 and C2 in u

Typically these would be obtained by combining the solution with initial values for u. Do you have initial values?

circa 2 mesi ago | 0

Conversion Wolfram Mathematica to Matlab (Do loop)

result = sum(1:100);

circa 2 mesi ago | 0

Is it possible to vectorize this 'for loop' involving multiple matrix inversions

You might look at this FEX submission by Bruno Luong: https://www.mathworks.com/matlabcentral/fileexchange/24260-multiple-same-...

3 mesi ago | 0

| accepted

Mulitplying 3D matrix by 3D matrix - result being 4D.

result = pagemtimes(matrix1,reshape(matrix2,3,512,1,512));

3 mesi ago | 0

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Matrix dimensions must agree error problem

Use element-wise division with the dot: Y=acosd(sin(altitude)*sin(latitude)-sin(declination)./(cos(altitude)*cos(latitude))); ...

3 mesi ago | 0

Combining two Matrices every other row

You could do direct assignment. E.g., [m,n] = size(A); C = zeros(2*m,n); C(1:2:end,:) = A; C(2:2:end,:) = B;

3 mesi ago | 0

Gauss-Elimination method

See the rank( ) function: https://www.mathworks.com/help/matlab/ref/rank.html?s_tid=doc_ta

3 mesi ago | 1

Looping num2str

x(i) references a single element of x. However, the expression [num2str(y(i)),'%'] generates multiple characters. In essence, ...

3 mesi ago | 1

Mean computaion in matlab

You can specify the dimension to use with the mean( ) function. E.g., maybe this is the computation you want: data = your 3D m...

3 mesi ago | 0

What is the mean?

B' is the complex conjugate transpose of B. If B is real, then it is equivalent to just the tranpose and the assignment simply ...

3 mesi ago | 0

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How can I model equation of motion (2nd order ODE) when system matrices are in terms of state variables?

Yes you can have derivative functions that depend on state variables. This is quite common. E.g., define a 4x1 state vector y th...

3 mesi ago | 0

empty double matrix in case of nul value

Do you mean you want to do something like this: if( isempty(malicious) ) malicious = 0; end

3 mesi ago | 0

To allocate character array='2a'

'2a' is a two element character string. You can't assign two elements to a single element. I.e., you can't assign a 1x2 vector...

4 mesi ago | 0

How to get the desired quaternion representation from a rotation matrix?

As you already know, both q and -q represent the same rotation. Which sign to pick is entirely up to the underlying algorithm. ...

4 mesi ago | 0

| accepted

Implementation of Runge Kutta Numerical Solution for system of ODE's

The main technical problem (other than your syntax not being MATLAB) is that you are trying to solve your four 1st order equatio...

4 mesi ago | 0

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How to convert complex float to complex integer in MEX gateway function?

I would guess you can just use the appropriate data types. E.g., mxComplexInt32* Data1 = mxGetComplexInt32s(prhs[0]); mxComp...

4 mesi ago | 0

| accepted

Can you help me with this loop?

Can you use num2str? Message = [num2str(TotalMinutes) ' minutes are equal to ' num2str(Hours) ' hours and ' num2str(Minutes) ' ...

4 mesi ago | 0

How to separate a string(1x1 cell) into a 1x4 cell

If the hex codes are always 8 characters, why can't you just pick off the characters you want? E.g., {result(1:8),result(10:17...

4 mesi ago | 1

how to generate a code for solving 2st order differential equations using improved runge kutta 4th order method

The posted code actually has a bug. Also it is almost set up properly for vector solutions, but not quite. Make these changes:...

4 mesi ago | 0

Computer crashed when calculating matrix (mixed with sparse and full) multiplication and summation

Remember that each element of the B*C result is simply the dot product of a row of B with a column of C. If that column of C ha...

4 mesi ago | 0

| accepted

Fast vector reshaping/permutation

Don't do the permute( ) operation. Just use pagemtimes( ) downstream in your code with the appropriate 'transpose' option. Thi...

4 mesi ago | 1

Can someone explain this Loop?

Of the numbers 1 ... 10 that get produced by the randperm( ) call, only three of them are less than 4: 1, 2, 3. 1 will cause n...

4 mesi ago | 0

| accepted