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ident_c

 Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input Detailed_Output Parameters Exceptions Files Particulars Examples Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

```   ident_c ( Return the 3x3 identity matrix )

void ident_c ( SpiceDouble    matrix[3][3] )

```

#### Abstract

```   Return the 3x3 identity matrix.
```

```   None.
```

#### Keywords

```   MATRIX

```

#### Brief_I/O

```   VARIABLE  I/O  DESCRIPTION
--------  ---  --------------------------------------------------
matrix     O   The 3x3 identity matrix.
```

#### Detailed_Input

```   None.
```

#### Detailed_Output

```   matrix      is the 3x3 Identity matrix. That `matrix' is
the following

.-                 -.
|  1.0   0.0   0.0  |
|  0.0   1.0   0.0  |
|  0.0   0.0   1.0  |
`-                 -'
```

#### Parameters

```   None.
```

#### Exceptions

```   Error free.
```

#### Files

```   None.
```

#### Particulars

```   This is a utility routine for obtaining the 3x3 identity matrix
so that you may avoid having to write the loop or assignments
needed to get the matrix.
```

#### Examples

```   The numerical results shown for this example may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.

1) Define a 3x3 matrix and compute its inverse using the CSPICE
routine invert_c. Verify the accuracy of the computed inverse
using the mathematical identity

-1
m x m   - i = 0

where `i' is the 3x3 identity matrix.

Example code begins here.

/.
Program ident_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"

int main( )
{

/.
Local variables.
./
SpiceDouble          idmat  [3][3];
SpiceDouble          imat   [3][3];
SpiceDouble          mout   [3][3];
SpiceDouble          mzero  [3][3];

SpiceInt             i;

/.
Define a matrix to invert.
./
SpiceDouble          m      [3][3] = { {0.0,  -1.0, 0.0},
{0.5,   0.0, 0.0},
{0.0,   0.0, 1.0} };

printf( "Original Matrix:\n" );
for ( i = 0; i < 3; i++ )
{

printf( "%16.7f %15.7f %15.7f\n", m[i][0], m[i][1], m[i][2] );

}

/.
Invert the matrix, then output.
./
invert_c ( m, mout );

printf( "\n" );
printf( "Inverse Matrix:\n" );
for ( i = 0; i < 3; i++ )
{
printf( "%16.7f %15.7f %15.7f\n",
mout[i][0], mout[i][1], mout[i][2] );
}

/.
Check the `m' times `mout' produces the identity matrix.
./
ident_c ( idmat );
mxm_c   ( m, mout, imat );

vsubg_c ( (SpiceDouble *)imat, (SpiceDouble *)idmat,
9,                  (SpiceDouble *)mzero );

printf( "\n" );
printf( "Original times inverse minus identity:\n" );
for ( i = 0; i < 3; i++ )
{
printf( "%16.7f %15.7f %15.7f\n",
mzero[i][0], mzero[i][1], mzero[i][2] );
}

return ( 0 );
}

When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:

Original Matrix:
0.0000000      -1.0000000       0.0000000
0.5000000       0.0000000       0.0000000
0.0000000       0.0000000       1.0000000

Inverse Matrix:
0.0000000       2.0000000      -0.0000000
-1.0000000       0.0000000      -0.0000000
0.0000000      -0.0000000       1.0000000

Original times inverse minus identity:
0.0000000       0.0000000       0.0000000
0.0000000       0.0000000       0.0000000
0.0000000       0.0000000       0.0000000
```

#### Restrictions

```   None.
```

#### Literature_References

```   None.
```

#### Author_and_Institution

```   N.J. Bachman        (JPL)
J. Diaz del Rio     (ODC Space)
W.L. Taber          (JPL)
```

#### Version

```   -CSPICE Version 1.0.1, 02-JUN-2021 (JDR)

```   Get the 3x3 identity matrix
`Fri Dec 31 18:41:08 2021`