source: trunk/MagicSoft/slalib/smat.c@ 2360

Last change on this file since 2360 was 732, checked in by tbretz, 25 years ago
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1#include "slalib.h"
2#include "slamac.h"
3void slaSmat ( int n, float *a, float *y, float *d, int *jf, int *iw )
4/*
5** - - - - - - - -
6** s l a S m a t
7** - - - - - - - -
8**
9** Matrix inversion & solution of simultaneous equations.
10**
11** (single precision)
12**
13** For the set of n simultaneous equations in n unknowns:
14** a.y = x
15**
16** where:
17** a is a non-singular n x n matrix
18** y is the vector of n unknowns
19** x is the known vector
20**
21** slaSmat computes:
22** the inverse of matrix a
23** the determinant of matrix a
24** the vector of n unknowns
25**
26** Arguments:
27**
28** symbol type dimension before after
29**
30** n int no. of unknowns unchanged
31** *a float [n][n] matrix inverse
32** *y float [n] vector solution
33** *d float - determinant
34** > *jf int - singularity flag
35** *iw int [n] - workspace
36**
37**
38** > jf is the singularity flag. If the matrix is non-singular,
39** jf=0 is returned. If the matrix is singular, jf=-1 & d=0.0 are
40** returned. In the latter case, the contents of array a on return
41** are undefined.
42**
43** Algorithm:
44** Gaussian elimination with partial pivoting.
45**
46** Speed:
47** Very fast.
48**
49** Accuracy:
50** Fairly accurate - errors 1 to 4 times those of routines optimized
51** for accuracy.
52**
53** Example call (note handling of "adjustable dimension" 2D array):
54**
55** float a[MP][MP], v[MP], d;
56** int j, iw[MP];
57** :
58** slaSmat ( n, (float *) a, v, &d, &j, iw );
59**
60** Last revision: 17 August 1999
61**
62** Copyright P.T.Wallace. All rights reserved.
63*/
64
65#define TINY 1e-20f
66
67{
68 int k, imx, i, j, ki;
69 float amx, t, yk;
70
71/* Pointers to beginnings of rows in matrix a[n][n] */
72
73 float *ak, /* row k */
74 *ai, /* row i */
75 *aimx; /* row imx */
76
77 *jf = 0;
78 *d = 1.0f;
79
80 for ( k = 0, ak = a; k < n; k++, ak += n ) {
81 amx = (float) fabs ( (double) ak[k] );
82 imx = k;
83 aimx = ak;
84 if ( k != n ) {
85 for ( i = k + 1, ai = ak + n; i < n; i++, ai += n ) {
86 t = (float) fabs ( (double) ai[k] );
87 if ( t > amx ) {
88 amx = t;
89 imx = i;
90 aimx = ai;
91 }
92 }
93 }
94 if ( amx < TINY ) {
95 *jf = -1;
96 } else {
97 if ( imx != k ) {
98 for ( j = 0; j < n; j++ ) {
99 t = ak[j];
100 ak[j] = aimx[j];
101 aimx[j] = t;
102 }
103 t = y[k];
104 y[k] = y[imx];
105 y[imx] = t;
106 *d = - *d;
107 }
108 iw[k] = imx;
109 *d *= ak[k];
110 if ( fabs ( *d ) < TINY ) {
111 *jf = -1;
112 } else {
113 ak[k] = 1.0f / ak[k];
114 for ( j = 0; j < n; j++ ) {
115 if ( j != k ) {
116 ak[j] *= ak[k];
117 }
118 }
119 yk = y[k] * ak[k];
120 y[k] = yk;
121 for ( i = 0, ai = a; i < n; i++ , ai += n ) {
122 if ( i != k ) {
123 for ( j = 0; j < n; j++ ) {
124 if ( j != k ) {
125 ai[j] -= ai[k] * ak[j];
126 }
127 }
128 y[i] -= ai[k] * yk;
129 }
130 }
131 for ( i = 0, ai = a; i < n; i++ , ai += n ) {
132 if ( i != k )
133 ai[k] *= - ak[k];
134 }
135 }
136 }
137 }
138 if ( *jf != 0 ) {
139 *d = 0.0f;
140 } else {
141 for ( k = n; k-- > 0; ) {
142 ki = iw[k];
143 if ( k != ki ) {
144 for ( i = 0, ai = a; i < n; i++ , ai += n ) {
145 t = ai[k];
146 ai[k] = ai[ki];
147 ai[ki] = t;
148 }
149 }
150 }
151 }
152}
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