# Example 1 - one-dimensional example varying the number of knots (C)

```#include <stdio.h>
#include <ncarg/ncargC.h>
#include <ncarg/gks.h>
#include <ncarg/ngmath.h>

/*
*  Function prototypes for plotting backgrounds and curves.
*/
void c_bkgft1(float, char *, float, float);
void c_drwft1(int, float [], float [], int, float [], float [],
float [], float []);

/*
*  The number of input data points.
*/
#define NDATA  10

/*
*  The number of output data points.
*/
#define NPTS  101

/*
*  The GKS workstation type (NCGM).
*/
#define IWTYPE  1

/*
*  The GKS workstaton identifier.
*/
#define WKID    1

/*
* This example illustrates the effects of using differing numbers
* of knots in calls to c_csa1s with the same input data.
*/
main ()
{

/*
*  Set up the output arrays.
*/
float xo[NPTS],*yo4,*yo7,*yo9,xinc;

/*
*  Define the input data.
*/
float xi[] = {0.0, 0.1,  0.2,  0.3, 0.5,  0.6, 0.65,  0.8,  0.9, 1.};
float yi[] = {0.0, 0.8, -0.9, -0.9, 0.9,  1.0, 0.90, -0.8, -0.8, 0.};

int i,knots,ier;

/*
*  Create the output X coordinate array.
*/
xinc = 1./ (float) (NPTS-1);
for (i = 0; i < NPTS; i++) {
xo[i] = (float) i * xinc;
}

/*
*  Calculate the approximated function values using differing
*  number of knots.
*/
knots = 4;
yo4 = c_csa1s(NDATA,xi,yi,knots,NPTS,xo,&ier);
if (ier != 0) {
printf("Error return from c_csa1s: %d\n",ier);
exit(1);
}
knots = 7;
yo7 = c_csa1s(NDATA,xi,yi,knots,NPTS,xo,&ier);
if (ier != 0) {
printf("Error return from c_csa1s: %d\n",ier);
exit(1);
}
knots = 9;
yo9 = c_csa1s(NDATA,xi,yi,knots,NPTS,xo,&ier);
if (ier != 0) {
printf("Error return from c_csa1s: %d\n",ier);
exit(1);
}

/*
*  Draw plot.
*/
c_drwft1(NDATA,xi,yi,NPTS,xo,yo4,yo7,yo9);

}

void c_drwft1(int n, float x[], float y[], int m, float xo[],
float curve1[], float curve2[], float curve3[])
{
/*
*  This function uses NCAR Graphics to plot three curves on
*  the same picture showing the results from calling c_csa1x with
*  differing number of knots.  The values for the curves are
*  contained in arrays curve1, curve2, and curve3.
*/
int   i;
float yb, yt, ypos_top = 0.88;

/*
*  Declare variables used in GKS calls.
*/
Gcolr_rep rgb;
Gpoint plist[NDATA];
Gpoint_list pmk;

/*
*  Open GKS, open and activate a workstation.
*/
gopen_gks("stdout",0);
gopen_ws(WKID, NULL, IWTYPE);
gactivate_ws(WKID);

/*
* Define a color table.
*/
rgb.rgb.red = rgb.rgb.green = rgb.rgb.blue = 1.;
gset_colr_rep(WKID,0,&rgb);
rgb.rgb.red = rgb.rgb.green = rgb.rgb.blue = 0.;
gset_colr_rep(WKID,1,&rgb);
rgb.rgb.red = 1.;
rgb.rgb.green = rgb.rgb.blue = 0.;
gset_colr_rep(WKID,2,&rgb);
rgb.rgb.red = rgb.rgb.green = 0.;
rgb.rgb.blue = 1.;
gset_colr_rep(WKID,3,&rgb);

/*
* Plot the main title.
*/
gset_clip_ind(0);
c_plchhq(.5,.95,":F21:Demo for c_csa1s",0.035,0.,0.);

/*
*  Draw a background grid for the first curve.
*/
yb = -1.2;
yt =  1.2;
c_bkgft1(ypos_top,"knots = 4",yb,yt);
c_gridal(5,5,4,1,1,1,10,0.0,yb);

/*
*  Graph the approximated function values for knots=4.
*/
c_curve(xo,curve1,m);

/*
*  Mark the input data points.
*/
for (i = 0; i < n; i++) {
plist[i].x = x[i];
plist[i].y = y[i];
}
gset_marker_size(2.2);
gset_marker_colr_ind(3);
pmk.num_points = n;
pmk.points = plist;
gpolymarker(&pmk);

/*
*  Graph the approximated function values for knots=7.
*/
yb = -1.2;
yt =  1.2;
c_bkgft1(ypos_top-0.3,"knots = 7",yb,yt);
c_gridal(5,5,4,1,1,1,10,0.0,yb);
c_curve(xo,curve2,m);

/*
*  Mark the input data points.
*/
for (i = 0; i < n; i++) {
plist[i].x = x[i];
plist[i].y = y[i];
}
gset_marker_size(2.2);
gset_marker_colr_ind(3);
pmk.num_points = n;
pmk.points = plist;
gpolymarker(&pmk);

/*
*  Graph the approximated function values for knots=9.
*/
yb = -1.2;
yt =  1.2;
c_bkgft1(ypos_top-0.6,"knots = 9",yb,yt);
c_gridal(5,5,4,1,1,1,10,0.0,yb);
c_curve(xo,curve3,m);

/*
*  Mark the input data points.
*/
for (i = 0; i < n; i++) {
plist[i].x = x[i];
plist[i].y = y[i];
}
gset_marker_size(2.2);
gset_marker_colr_ind(3);
pmk.num_points = n;
pmk.points = plist;
gpolymarker(&pmk);

c_frame();

/*
*  Deactivate and close workstation, close GKS.
*/
gdeactivate_ws(WKID);
gclose_ws(WKID);
gclose_gks();
}

/*
*  Draw a background.
*/
void c_bkgft1(float ypos, char *label, float yb, float yt) {
c_set(0.,1.,0.,1.,0.,1.,0.,1.,1);

/*
*   Plot the curve label using font 21 (Helvetica).
*/
c_pcseti("fn",21);
c_plchhq(.25,ypos-0.03,label,0.025,0.,-1.0);

/*
*  Draw a horizontal line at Y=0. using color index 2.
*/
c_set(0.13,0.93,ypos-0.2,ypos,0.0,1., yb, yt, 1);
gset_line_colr_ind(2);
c_line(0.,0.,1.,0.);
c_sflush();
gset_line_colr_ind(1);

/*
*  Set Gridal parameters.
*
*
*   Set lty to indicate that the Plotchar routine PLCHHQ should be used.
*/
c_gaseti("lty",1);

/*
*   Size and format for X axis labels.
*/
c_gasetr("xls",0.02);
c_gasetc("xlf","(f3.1)");

/*
*   Size and format for X axis labels.
*/
c_gasetr("yls",0.02);
c_gasetc("ylf","(f5.1)");

/*
*  Length of major tick marks for the X and Y axes.
*/
c_gasetr("xmj",0.02);
c_gasetr("ymj",0.02);
}
```

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