2.3.1 Array creation

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 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 float dat[40][30];. Since, formaly this arrays element 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(float **dat, int N1, int N2);. For C this is functions like mgl_data_set_float2(HMDT d, const float **dat, int N1, int N2);. At this, you should keep in mind that nx=N2 and ny=N1 after conversion.