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<H2><A NAME="SECTION0004130000000000000000">SLA_PERMUT - Next Permutation</A>
<A NAME="xref_SLA_PERMUT"> </A><A NAME="SLA_PERMUT"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Generate the next permutation of a specified number of items.
<DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_PERMUT (N, ISTATE, IORDER, J)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>N</EM></TD>
<TH ALIGN="LEFT"><B>I</B></TH>
<TD ALIGN="LEFT" NOWRAP>number of items: there will be N! permutations</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>ISTATE</EM></TD>
<TD ALIGN="LEFT"><B>I(N)</B></TD>
<TD ALIGN="LEFT" NOWRAP>state, ISTATE(1)=-1 to initialize</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>ISTATE</EM></TD>
<TH ALIGN="LEFT"><B>I(N)</B></TH>
<TD ALIGN="LEFT" NOWRAP>state, updated ready for next time</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>IORDER</EM></TD>
<TD ALIGN="LEFT"><B>I(N)</B></TD>
<TD ALIGN="LEFT" NOWRAP>next permutation of numbers 1,2,...,N</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>J</EM></TD>
<TD ALIGN="LEFT"><B>I</B></TD>
<TD ALIGN="LEFT" NOWRAP>status:</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> -1 = illegal N (zero or less is illegal)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 0 = OK</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> +1 = no more permutations available</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>This routine returns, in the IORDER array, the integers 1 to N
inclusive, in an order that depends on the current contents of
the ISTATE array. Before calling the routine for the first
time, the caller must set the first element of the ISTATE array
to -1 (any negative number will do) to cause the ISTATE array
to be fully initialized.
<DT>2.
<DD>The first permutation to be generated is:
<BLOCKQUOTE>IORDER(1)=N, IORDER(2)=N-1, ..., IORDER(N)=1
</BLOCKQUOTE>
This is also the permutation returned for the ``finished'' (J=1) case.
The final permutation to be generated is:
<BLOCKQUOTE>IORDER(1)=1, IORDER(2)=2, ..., IORDER(N)=N
</BLOCKQUOTE>
<DT>3.
<DD>If the ``finished'' (J=1) status is ignored, the routine continues
to deliver permutations, the pattern repeating every N! calls.
</DL></DL>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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