Here we create a plot with limits 0-10 and 0-5 on the X and Y axes and then call the S-Lang version of PICKPOINTS. Since we only wanted to select one point, we did not supply an argument - or "()"'s - to the chips_pickpoints() call.

The screen output and behaviour is the same as if PICKPOINTS had been used. The difference is that the selected values are returned as a one-dimensional array; the variable pnts is used to store the data.

Here we ask for three points rather than 1 (and so need to use "()" around the argument). Note that the output array is still one-dimensional, with the values stored in (xi,yi) order.

Here we use the reshape() routine to change the format of the returned array from one-dimensional - with eight elements - to a two-dimensional array with 4x2 elements.

We then use the array-indexing capabilities of S-Lang to take a "slice" of this array to extract just the x (xpnts) and y (ypnts) values.

Alternatively we could have used

pos1 = pnts[0,*]

to create a two-element array pos1 which contains the x and y coordinates of the first point in the list.

Here we use a plot in which one of the axes (the Y axis) has log spacing. The return values for that axis are still in logarithmic units (which is why they are both less than 0). The writeascii() call writes out the y values of the selected points in the first column, and the corresponding linear values in the second column. We take advantage of S-Lang's array-indexing mechanism to restrict the output to just use the second and fourth elements (ie "[1,3]") of the lpnts array.

The previous examples have shown chips_pickpoints() being used from the ChIPS prompt. It can also be used from a S-Lang script, as the following example shows.

  unix% cat pick.sl
  % load the ChIPS functions
  require("chips");

  % turn off redrawing until the plot is finished
  () = chips_auto_redraw(0);

  % set the limits
  () = chips_set_xrange(0,10);
  () = chips_set_yrange(10,20);

  % plot some data
  variable x = [1:9];
  variable y = x + 10;
  chips.symbolstyle = _chips->none;
  chips.linestyle   = _chips->simpleline;
  () = curve(x,y);
  () = chips_auto_redraw(1);

  % call pickpoints
  variable out = chips_pickpoints(3);
  if ( andelse { out == NULL }
               { length(out) != 6 } )
    error( "Did not select 3 points from the plot" );

  print("\nYou selected the following values:");
  variable i;
  foreach ( [1:3] ) {
    i = ();
    () = printf( "  point %d  x = %4.2f  y = %5.2f\n",
                 i, out[2*(i-1)], out[2*(i-1)+1] );
  }
  % end of script

which, when run using slsh, will create output something like (depending on the actual points selected):

  unix% slsh pick.sl

  Click LMB or tap spacebar to pick point.
  Click RMB or type 'q' to quit picking points.

  Point picked: (3.468972, 15.146487)
  Point picked: (6.880163, 14.975967)
  Point picked: (9.093795, 18.173878)

  You selected the following values:
    point 1  x = 3.47  y = 15.15
    point 2  x = 6.88  y = 14.98
    point 3  x = 9.09  y = 18.17