/* -------------------------------------------------------------------- 
//  Figure 15.28: Torque program: The first step.      
//  Figure 15.36: Torque program, step 2: The try_once() function.                               
// -------------------------------------------------------------------- 
*/
#include "tools.h"     

#define TRIALS 1000 
void try_once( stream s );

void main( void )             
{   
    stream fout;       /* For the expermental data.*/
    int k;             /* Loop counter for the process repetition. */ 

    say( "Measuring Torque" );
    say( "\n Opening file theta.out for output." );
    fout = fopen( "theta.out", "w" ); 
    if (fout == NULL) fatal( "Cannot open theta.out." ); 

    srand( time(NULL) );        /* seed random number generator */

    say( "\n Ready to begin torque experiment." );
    for (k = 0; k < TRIALS; ++k)  try_once( fout ); 
    
    bye();                                            
}

/* -----------------------------------------------------------------------------
// Carry out one experimental trial.
// Report an error is result is outside of expected range, otherwise 
//     write the force and resulting twist angle to a file.
*/
#define  MINF  10.0 
#define  MAXF  30.0 
#define  EPS  .1            /* permitted variation from theoretical value */      

#define  G   12E06          /* psi, modulus of rigidity for steel */ 
#define  R   0.125          /* inches, radius of shaft */
#define  L   12.0           /* inches, length of shaft */
#define  b   3.0            /* inches, effective length of handle */ 

void 
try_once( stream s )             
{   
    const double J = PI * pow( R,4 ) / 2;        /* polar moment of inertia. */
    double F;               /* Force: randomly selected input for experiment.*/ 
    double theta;           /* Twist angle; experimental output. */
    double theta_t;         /* Twist angle; theoretical value. */

    F = randy() * (MAXF-MINF) + MINF;           /* Get and scale random force.*/

    printf( "%g\n", F);                         /* Send force to apparatus.*/ 
    scanf( "%lf", &theta);                      /* Receive measured result.*/ 

    theta_t = (F * b * L) / (J * G);    
    if (theta > theta_t - EPS && theta < theta_t + EPS)  
        fprintf( s, "%f %f\n", F, theta ); 
    else  
        say( " Angle %g outside possible range for force %g.", theta, F );
}