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...Read Coordinates Entered by a User

In addition to writing out coordinate values generated by your program ([*]), you may also need to accept coordinates entered by a user, or perhaps read from a file. In this case, you will probably want to allow ``free-format'' input, so that the user has some flexibility in the format that can be used. You will probably also want to detect any typing errors.

Let's assume that you want to read a number of lines of text, each containing the world coordinates of a single point, and to split each line into individual numerical coordinate values. Using the FrameSet pointer ``wcsinfo'' obtained earlier ([*]), you could proceed as follows:

#include <stdio.h>
char *t;
char text[ MAXCHARS + 2 ];
double coord[ 10 ];
int iaxis, n, naxes;


/* Obtain the number of coordinate axes (if not already known). */
naxes = astGetI( wcsinfo, "Naxes" );

/* Loop to read each line of input text, in this case from the
   standard input stream (your programming environment will probably
   provide a better way of reading text than this). Set the pointer
   "t" to the start of each line read. */
while ( t = fgets( text, MAXCHARS + 2, stdin ) ) {

/* Attempt to read a coordinate for each axis. */
   for ( iaxis = 1; iaxis <= naxes; iaxis++ ) {
      n = astUnformat( wcsinfo, iaxis, t, &coord[ iaxis - 1 ] );

/* If nothing was read and this is not the first axis or the
   end-of-string, try stepping over a separator and reading again. */
   if ( !n && ( iaxis > 1 ) && *t )
      n = astUnformat( wcsinfo, iaxis, ++t, &coord[ iaxis - 1 ] );

/* Quit if nothing was read, otherwise move on to the next coordinate. */
      if ( !n ) break;
      t += n;

/* Test for the possible errors that may occur... */

/* Error detected by AST (a message will have been issued). */
   if ( !astOK ) {

/* Error in input data at character t[n]. */
   } else if ( *t || !n ) {
      <handle the error, or report your own message here>

   } else {
      <coordinates were read OK>

This algorithm has the advantage of accepting free-format input in whatever style is appropriate for the world coordinates in use (under the control of the FrameSet whose pointer you provide). For example, wavelength values might be read as floating point numbers (e.g. ``1.047'' or ``4787''), whereas celestial positions could be given in sexagesimal format (e.g. ``12:34:56'' or ``12 34.5'') and would be converted into radians. Individual coordinate values may be separated by white space and/or any non-ambiguous separator character, such as a comma.

For more information on reading coordinate values using the astUnformat function, see [*]. For details of how sexagesimal formats are handled, and the forms of input that may be used for celestial coordinates, see [*].

next up previous 264
Next: ...Create a New WCS Calibration
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Previous: ...Display Coordinates as they are Transformed

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
25th February 2013

Copyright (C) 2014 Science \& Technology Facilities Council