mexErrMsgTxt mxCreateFull mxGetM mxGetN mxGetPr mxIsComplex mxIsSparse mxIsStringIn MATLAB, mexample accepts two inputs and returns one output. The inputs are a 2x2 matrix denoted as MATRIX_IN and a 2x1 vector denoted as VECTOR_IN. The function calculates the determinant of MATRIX_IN, multiplies each element of VECTOR_IN by the determinant, and returns this as the output, denoted by VECTOR_OUT. All inputs and outputs to this function are assumed to be real (not complex).
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/* First, include some basic header files. The header file "mex.h" is required for a MEX-file. Add any other header files that your function may need here. */ #include "mex.h" /* A C MEX-file generally consists of two sections. The first section is a function or set of functions which performs the actual mathematical calculation that the MEX-function is to carry out. In this example, the function is called workFcn(). The second section is a gateway between MATLAB and the first section, and consists of a function called mexFunction. The gateway is responsible for several tasks, including: I) error checking, II) allocating memory for return arguments, III) converting data from MATLAB into a format that the workFcn function can use, and vice versa. The first function to be written in this example, then, is workFcn: Since C and MATLAB handle two-dimensional arrays differently, we will explicitly declare the dimension of the variable theMatrix. The variables, theVector and theResult, are both one-dimensional arrays, and therefore do not need such rigid typing. The #ifdef clause is intended so that both ANSI and non-ANSI compilers can compile this MEX-file. */ #ifdef __STDC__ void workFcn( double theMatrix[2][2],/* matrix with data from MATRIX_IN */ double theVector[], /* vector with data from VECTOR_IN */ double theResult[] /* vector with data for VECTOR_OUT */ ) #else void workFcn(theMatrix,theVector,theResult) double theMatrix[2][2];/* matrix with data from MATRIX_IN */ double theVector[]; /* vector with data from VECTOR_IN */ double theResult[]; /* vector with data for VECTOR_OUT */ #endif /* __STDC__ */ { double determinant; /* determinant of theMatrix */ determinant = theMatrix[0][0]*theMatrix[1][1] -theMatrix[0][1]*theMatrix[1][0]; theResult[0] = theVector[0]*determinant; theResult[1] = theVector[1]*determinant; } /* Now, define the gateway function, i.e., mexFunction. Below is the standard, predeclared header to mexFunction. nlhs and nrhs are the number of left-hand and right-hand side arguments that mexample was called with from within MATLAB. In this example, nlhs equals 1 and nrhs should equal 2. If not, then the user has called mexample the wrong way and should be informed of this. plhs and prhs are arrays which contain the pointers to the MATLAB matrices, which are stored in a C struct called a Matrix. prhs is an array of length nrhs, and its pointers point to valid input data. plhs is an array of length nlhs, and its pointers point to invalid data (i.e., garbage). It is the job of mexFunction to fill plhs with valid data. First, define the following values. This makes it much easier to change the order of inputs to mexample, should we want to change the function later. In addition, it makes the code easier to read. */ #define MATRIX_IN prhs[0] #define VECTOR_IN prhs[1] #define VECTOR_OUT plhs[0] #ifdef __STDC__ void mexFunction( int nlhs, Matrix *plhs[], int nrhs, Matrix *prhs[] ) #else mexFunction(nlhs, plhs, nrhs, prhs) int nlhs, nrhs; Matrix *plhs[], *prhs[]; #endif { double twoDarray[2][2];/* 2 dimensional C array to pass to workFcn() */ int row,col; /* loop indices */ int m,n; /* temporary matrix size holders */ /* Step 1: Error Checking Step 1a: is nlhs 1? If not, generate an error message and exit mexample (mexErrMsgTxt does this for us!) */ if (nlhs!=1) mexErrMsgTxt("mexample requires one output argument."); /* Step 1b: is nrhs 2? */ if (nrhs!=2) mexErrMsgTxt("mexample requires two input arguments, MATRIX_IN and \ VECTOR_IN."); /* Step 1c: Is MATRIX_IN a 2x2 numeric (nonstring), full (non- sparse), real (noncomplex) matrix? */ if (mxGetM(MATRIX_IN)!=2 || mxGetN(MATRIX_IN)!=2 || mxIsString(MATRIX_IN) || mxIsSparse(MATRIX_IN) || mxIsComplex(MATRIX_IN)) mexErrMsgTxt("First argument to mexample must be a 2x2, full, \ numeric, real-valued matrix."); /* Step 1d: Is VECTOR_IN a 2x1 or 1x2 numeric, full, real-valued vector? */ m=mxGetM(VECTOR_IN); /* Assigning result of mxGetM and mxGetN to variables */ n=mxGetN(VECTOR_IN); /* with simpler names makes for easier code reading. */ if (!((m==2 && n==1) || (m==1 && n==2)) || mxIsString(VECTOR_IN) || mxIsSparse(VECTOR_IN) || mxIsComplex(VECTOR_IN)) mexErrMsgTxt("Second argument to mexample must be 2 element, \ full, numeric, real-valued vector."); /* Step 2: Allocate memory for return argument(s) */ VECTOR_OUT = mxCreateFull(2, 1, REAL); /* create a 2x1 full, numeric, real-valued matrix */ /* Step 3: Convert MATRIX_IN to a 2x2 C array MATLAB stores a two-dimensional matrix in memory as a one- dimensional array. If the matrix is size MxN, then the first M elements of the one-dimensional array correspond to the first column of the matrix, and the next M elements correspond to the second column, etc. The following loop converts from MATLAB format to C format: */ for (col=0; col < mxGetN(MATRIX_IN); col++) for (row=0; row < mxGetM(MATRIX_IN); row++) twoDarray[row][col] = (mxGetPr(MATRIX_IN))[row+col*mxGetM(MATRIX_IN)]; /* mxGetPr returns a pointer to the real part of the matrix MATRIX_IN. In the line above, it is treated as the one- dimensional array mentioned in the previous comment. */ /* Step 4: Call workFcn function */ workFcn(twoDarray,mxGetPr(VECTOR_IN), mxGetPr(VECTOR_OUT)); /* workFcn will fill VECTOR_OUT with the return values for mexample, so the MEX-function is done! To use it, compile it according to the instructions in the MATLAB External Interface Guide. Then, in MATLAB, type c=mexample([1 2; 3 4],[1 2]) This will return the same as c=det([1 2; 3 4]) * [1 2] */ }
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