engPutVariable (C)

R2026b

Put variable into MATLAB engine workspace

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C Syntax

#include "engine.h"
int engPutVariable(Engine *ep, const char *name, const mxArray *pm);

Description

engPutVariable writes mxArray pm to the engine ep, giving it the variable name name. The function returns 0 if successful and 1 if an error occurs.

If the mxArray does not exist in the workspace, the function creates it. If an mxArray with the same name exists in the workspace, the function replaces the existing mxArray with the new mxArray.

The limit for the size of data transferred is 2 GB.

Do not use MATLAB® function names for variable names. Common variable names that conflict with function names include i, j, mode, char, size, and path. To determine whether a particular name is associated with a MATLAB function, use the which function.

The engine application owns the original mxArray and is responsible for freeing its memory. Although the engPutVariable function sends a copy of the mxArray to the MATLAB engine workspace, the engine application does not need to account for or free memory for the copy.

Note

The matlab::engine::MATLABEngine.setVariable function in the MATLAB Engine API for C++ is recommended over engPutVariable. The MATLAB Engine API for C++ includes modern C++ features for writing engine applications. For more information, see Call MATLAB from C++. There are no plans to remove engPutVariable or the Engine API for C.

Input Arguments

expand all

Pointer to engine, specified as Engine *.

Name of mxArray in the MATLAB engine workspace, specified as const char *.

Pointer to mxArray, specified as const mxArray *.

Examples

Put Variable into C Engine Workspace on Windows

This C code includes two examples of how to put a variable into the MATLAB engine workspace from a C engine application on a Windows® system.

/*
 *	engwindemo.c
 *
 *	This program illustrates how to call MATLAB
 *	Engine functions from a C program on Windows.
 *
 * Copyright 1984-2017 The MathWorks, Inc.
 */
#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#include <matrix.h>
#define BUFSIZE 256

static double Areal[6] = { 1, 2, 3, 4, 5, 6 };

int PASCAL WinMain (HINSTANCE hInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR     lpszCmdLine,
                    int       nCmdShow)

{
	Engine *ep;
	mxArray *T = NULL, *a = NULL, *d = NULL;
	char buffer[BUFSIZE+1];
	double *Dreal, *Dimag;
	double time[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

	/*
	 * Start MATLAB engine
	 */
	if (!(ep = engOpen(NULL))) {
		MessageBox ((HWND)NULL, (LPSTR)"Can't start MATLAB engine",
			(LPSTR) "Engwindemo.c", MB_OK);
		exit(-1);
	}

	/*
	 * PART I
	 *
	 * For the first half of this demonstration, we will send data
	 * to MATLAB, analyze the data, and plot the result.
	 */

	/*
	 * Create a variable from our data
	 */
	T = mxCreateDoubleMatrix(1, 10, mxREAL);
	/*
	 * Copy data from time to matrix T
	 */
	#if MX_HAS_INTERLEAVED_COMPLEX
		memcpy(mxGetDoubles(T), time, 10*sizeof(double));
	#else
		memcpy(mxGetPr(T), time, 10*sizeof(double));
	#endif
	/*
	 * Place the variable T into the MATLAB workspace
	 */
	engPutVariable(ep, "T", T);

	/*
	 * Evaluate a function of time, distance = (1/2)g.*t.^2
	 * (g is the acceleration due to gravity)
	 */
	engEvalString(ep, "D = .5.*(-9.8).*T.^2;");

	/*
	 * Plot the result
	 */
	engEvalString(ep, "plot(T,D);");
	engEvalString(ep, "title('Position vs. Time for a falling object');");
	engEvalString(ep, "xlabel('Time (seconds)');");
	engEvalString(ep, "ylabel('Position (meters)');");

	/*
	 * PART II
	 *
	 * Create another mxArray, put it into MATLAB,
	 * and calculate its eigen values.
	 * 
	 */

	a = mxCreateDoubleMatrix(3, 2, mxREAL);
	/*
	 * Copy data from Areal to matrix a
	 */
	#if MX_HAS_INTERLEAVED_COMPLEX
		memcpy(mxGetDoubles(a), Areal, 6*sizeof(double));
	#else
		memcpy(mxGetPr(a), Areal, 6*sizeof(double));
	#endif
	engPutVariable(ep, "A", a);

	/*
	 * Calculate the eigen value
	 */
	engEvalString(ep, "d = eig(A*A')");

	/*
	 * Use engOutputBuffer to capture MATLAB output. Ensure first that
	 * the buffer is always NULL terminated.
	 */
	buffer[BUFSIZE] = '\0';
	engOutputBuffer(ep, buffer, BUFSIZE);

	/*
	 * the evaluate string returns the result into the
	 * output buffer.
	 */
	engEvalString(ep, "whos");
	MessageBox ((HWND)NULL, (LPSTR)buffer, (LPSTR) "MATLAB - whos", MB_OK);

	/*
	 * Get the eigen value mxArray
	 */
	d = engGetVariable(ep, "d");
	engClose(ep);

	if (d == NULL) {
		MessageBox ((HWND)NULL, (LPSTR)"Get Array Failed", (LPSTR)"Engwindemo.c", MB_OK);
	}
	else {
		/*
		 * Using interleaved complex representation we can access complex number
		 * with a single pointer instead of using two separate pointers.
		 */
		#if MX_HAS_INTERLEAVED_COMPLEX
			if (mxIsComplex(d)) {
				sprintf(buffer,"Eigenval 2: %g+%gi",mxGetComplexDoubles(d)[1].real,mxGetComplexDoubles(d)[1].imag);
			}
			else {
				sprintf(buffer,"Eigenval 2: %g",mxGetDoubles(d)[1]);
			}
		#else
			Dreal = mxGetPr(d);
			Dimag = mxGetPi(d);
			if (Dimag) {
				sprintf(buffer,"Eigenval 2: %g+%gi",Dreal[1],Dimag[1]);
			}
			else {
				sprintf(buffer,"Eigenval 2: %g",Dreal[1]);
			}
		#endif
		MessageBox ((HWND)NULL, (LPSTR)buffer, (LPSTR)"Engwindemo.c", MB_OK);
		mxDestroyArray(d);
	} 

	/*
	 * We're done! Free memory, close MATLAB engine and exit.
	 */
	mxDestroyArray(T);
	mxDestroyArray(a);

	return(0);
}

/* LocalWords:  eigen eng Eigenval gi
 */

Put Variable into C Engine Workspace on Linux or macOS

This C code includes two examples of how to put a variable into the MATLAB engine workspace from a C engine application on a Linux® or macOS system.

/*
 *	engdemo.c
 *
 *	A simple program to illustrate how to call MATLAB
 *	Engine functions from a C program.
 *
 * Copyright 1984-2016 The MathWorks, Inc.
 * All rights reserved
 */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#define  BUFSIZE 256

int main()

{
	Engine *ep;
	mxArray *T = NULL, *result = NULL;
	char buffer[BUFSIZE+1];
	double time[10] = { 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 };

	/*
	 * Call engOpen with a NULL string. This starts a MATLAB process 
     * on the current host using the command "matlab".
	 */
	if (!(ep = engOpen(""))) {
		fprintf(stderr, "\nCan't start MATLAB engine\n");
		return EXIT_FAILURE;
	}

	/*
	 * PART I
	 *
	 * For the first half of this demonstration, send data
	 * to MATLAB, analyze the data, and plot the result.
	 */

	/* 
	 * Create a variable for the data
	 */
	T = mxCreateDoubleMatrix(1, 10, mxREAL);
	memcpy((void *)mxGetPr(T), (void *)time, sizeof(time));
	/*
	 * Place the variable T into the MATLAB workspace
	 */
	engPutVariable(ep, "T", T);

	/*
	 * Evaluate a function of time, distance = (1/2)g.*t.^2
	 * (g is the acceleration due to gravity)
	 */
	engEvalString(ep, "D = .5.*(-9.8).*T.^2;");

	/*
	 * Plot the result
	 */
	engEvalString(ep, "plot(T,D);");
	engEvalString(ep, "title('Position vs. Time for a falling object');");
	engEvalString(ep, "xlabel('Time (seconds)');");
	engEvalString(ep, "ylabel('Position (meters)');");

	/*
	 * use fgetc() to pause long enough to be
	 * able to see the plot
	 */
	printf("Hit return to continue\n\n");
	fgetc(stdin);
	/*
	 * We're done for Part I! Free memory, close MATLAB figure.
	 */
	printf("Done for Part I.\n");
	mxDestroyArray(T);
	engEvalString(ep, "close;");


	/*
	 * PART II
	 *
	 * For the second half of this demonstration, we will request
	 * a MATLAB string, which should define a variable X.  MATLAB
	 * will evaluate the string and create the variable.  We
	 * will then recover the variable, and determine its type.
	 */
	  
	/*
	 * Use engOutputBuffer to capture MATLAB output, so we can
	 * echo it back.  Ensure first that the buffer is always NULL
	 * terminated.
	 */

	buffer[BUFSIZE] = '\0';
	engOutputBuffer(ep, buffer, BUFSIZE);
	while (result == NULL) {
	    char str[BUFSIZE+1];
	    char *input = NULL;
	    /*
	     * Get a string input from the user
	     */
	    printf("Enter a MATLAB command to evaluate.  This command should\n");
	    printf("create a variable X.  This program will then determine\n");
	    printf("what kind of variable you created.\n");
	    printf("For example: X = 1:5\n");
	    printf(">> ");

	    input = fgets(str, BUFSIZE, stdin);
	  
	    /*
	     * Evaluate input with engEvalString
	     */
	    engEvalString(ep, str);
	    
	    /*
	     * Echo the output from the command.  
	     */
	    printf("%s", buffer);
	    
	    /*
	     * Get result of computation
	     */
	    printf("\nRetrieving X...\n");
	    if ((result = engGetVariable(ep,"X")) == NULL)
	      printf("Oops! You didn't create a variable X.\n\n");
	    else {
		printf("X is class %s\t\n", mxGetClassName(result));
	    }
	}

	/*
	 * We're done! Free memory, close MATLAB engine and exit.
	 */
	printf("Done!\n");
	mxDestroyArray(result);
	engClose(ep);
	
	return EXIT_SUCCESS;
}

Version History

Introduced before R2006a

See Also

Classes