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3.3.1 Array creation ¶

There are many ways in MathGL how data arrays can be created and filled.

One can put the data in mglData instance by several ways. Let us do it for sinus function:

• one can create external array, fill it and put to mglData variable

    double *a = new double[50];
    for(int i=0;i<50;i++)   a[i] = sin(M_PI*i/49.);
  
    mglData y;
    y.Set(a,50);
  

• another way is to create mglData instance of the desired size and then to work directly with data in this variable

    mglData y(50);
    for(int i=0;i<50;i++)   y.a[i] = sin(M_PI*i/49.);
  

• next way is to fill the data in mglData instance by textual formula with the help of Modify() function

    mglData y(50);
    y.Modify("sin(pi*x)");
  

• or one may fill the array in some interval and modify it later

    mglData y(50);
    y.Fill(0,M_PI);
    y.Modify("sin(u)");
  

• finally it can be loaded from file

    FILE *fp=fopen("sin.dat","wt");   // create file first
    for(int i=0;i<50;i++)   fprintf(fp,"%g\n",sin(M_PI*i/49.));
    fclose(fp);
  
    mglData y("sin.dat");             // load it
  

  At this you can use textual or HDF files, as well as import values from bitmap image (PNG is supported right now).

• at this one can read only part of data

    FILE *fp-fopen("sin.dat","wt");   // create large file first
    for(int i=0;i<70;i++)   fprintf(fp,"%g\n",sin(M_PI*i/49.));
    fclose(fp);
  
    mglData y;
    y.Read("sin.dat",50);             // load it
  

Creation of 2d- and 3d-arrays is mostly the same. But one should keep in mind that class mglData uses flat data representation. For example, matrix 30*40 is presented as flat (1d-) array with length 30*40=1200 (nx=30, ny=40). The element with indexes {i,j} is a[i+nx*j]. So for 2d array we have:

  mglData z(30,40);
  for(int i=0;i<30;i++)   for(int j=0;j<40;j++)
    z.a[i+30*j] = sin(M_PI*i/29.)*sin(M_PI*j/39.);

or by using Modify() function

  mglData z(30,40);
  z.Modify("sin(pi*x)*cos(pi*y)");

The only non-obvious thing here is using multidimensional arrays in C/C++, i.e. arrays defined like mreal dat[40][30];. Since, formally these elements dat[i] can address the memory in arbitrary place you should use the proper function to convert such arrays to mglData object. For C++ this is functions like mglData::Set(mreal **dat, int N1, int N2);. For C this is functions like mgl_data_set_mreal2(HMDT d, const mreal **dat, int N1, int N2);. At this, you should keep in mind that nx=N2 and ny=N1 after conversion.