# 18.1 Writing c functions in matlab (mex-files)  (Page 2/3)

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## Getting and creating data

The main MATLAB structure used for holding data in MEX-Files is the mxArray. This structure can hold real data, complex data, arrays, matrices, sparse-arrays, strings, and a whole host of other MATLAB data-structures.Using data from some of the basic structures is shown here, but refer to the MATLAB help for using other data structures.

## Get that data

Lets use my example from above (if you forgot: `[z0,z1] = jasonsFunction(x,y,z);` ). Assume that x is a 2-D matrix, y is a string, and z is an integer. Here we wills ee how to extractand use these different types of data.

We have access to the input paramter x by a pointer held in the array prhs. In C, when referencing an array by index, the variable is automatically dereferenced (ie: you dont need to use a star).For clarity, I will copy the variable x over to an mxArray pointer named xData (This does not need to be done for the code to work).

```//---Inside mexFunction--- //DeclarationsmxArray *xData; double *xValues;int i,j; int rowLen, colLen;double avg; //Copy input pointer xxData = prhs[0];//Get matrix x xValues = mxGetPr(xData);rowLen = mxGetN(xData); colLen = mxGetM(xData);//Print the integer avg of each col to matlab console for(i=0;i<rowLen;i++) {avg=0; for(j=0;j<colLen;j++) {avg += xValues[(i*colLen)+j]; //Another Method:// //avg += *xValues++;} avg = avg/colLen;printf("The average of row %d, is %d",i,(int)avg); }```

The function `mxGetPr` is used to get a pointer to the real data xData. This function takes a pointer to an mxArray as the intput paramter, and returns a pointer array of doubles. A similar function `mxGetPi` can be used for complex data. `mxGetN` and `mxGetM` return integers of the lengths of the row and column in the matrix. If this were an array of data, one of thesereturn values would be zero. MATLAB gives the matrix as rows first, then columns (if you were to traverse the matrix linearly) so to jump by position, (x,y) maps to x*colLen+y. MATLAB organizesits arrays this way to reduce cache misses when the row traversal is on the outside loop. It is good to code it this way if you are working for efficiency. `printf()` will print out to the MATLAB command prompt.

Getting a string is very similar, but has its own method. The example below shows the procedure for getting a string. Again, I will copy the input to a pointer called yData.

```//---Inside mexFunction--- //DeclarationsmxArray *yData; int yLength;char *TheString; //Copy input pointer yyData = prhs[1];//Make "TheString" point to the string yLength = mxGetN(yData)+1;TheString = mxCalloc(yLength, sizeof(char)); //mxCalloc is similar to malloc in C mxGetString(yData,TheString,yLength);```

This last example shows how to get a simple integer. This is the method that has always worked for me, but it seems kind of strange so I imagine there is another way to do this.

what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
how did you get the value of 2000N.What calculations are needed to arrive at it
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