User`s guide

4 Guidelines for Writing C MEX S-Functions
4-36
matlabroot/simulink/src/lookup_index.c
/* File : lookup_index.c
* Abstract:
*
* Contains a routine used by the S-function sfun_directlookup.c to
* compute the index in a vector for a given data value.
*
* Copyright (c) 1990-1998 by The MathWorks, Inc. All Rights Reserved.
* $Revision: 1.3 $
*/
#include “tmwtypes.h”
/*
* Function: GetDirectLookupIndex ==============================================
* Abstract:
* Using a bisection search to locate the lookup index when the x-vector
* isn’t evenly spaced.
*
* Inputs:
* *x : Pointer to table, x[0] ....x[xlen-1]
* xlen : Number of values in xtable
* u : input value to look up
*
* Output:
* idx : the index into the table such that:
* if u is negative
* x[idx] <= u < x[idx+1]
* else
* x[idx] < u <= x[idx+1]
*/
int_T GetDirectLookupIndex(const real_T *x, int_T xlen, real_T u)
{
int_T idx = 0;
int_T bottom = 0;
int_T top = xlen-1;
/*
* Deal with the extreme cases first:
*
* i] u <= x[bottom] then idx = bottom
* ii] u >= x[top] then idx = top-1
*
*/
if (u <= x[bottom]) {
return(bottom);
} else if (u >= x[top]) {
return(top);
}
/*
* We have: x[bottom] < u < x[top], onward
* with search for the appropriate index ...