/*
 * tinv(p,dof) returns the inverse t-distribution value for probability
 * p and dof degrees of freedom.  It does this by looking up the appropriate
 * value in the array tinv_array.  Only dof between 1 and 20 are exact.
 * For dof > 20, tinv() returns approximations.  Also, only probabilities
 * of 0.75, 0.9, 0.95, 0.975, 0.99, 0.995, and 0.9995 are provided.
 *
 * If p is not one of the 7 provided values, or if dof is not greater than or
 * equal to 1, tinv() causes the program to exit.
 */

double tinv(p,dof)
double p;
int    dof;
{
   int    dofindex,pindex;

   double tinv_array[31][7] = {
   {1.0000, 3.0777, 6.3138, 12.7062, 31.8207, 63.6574, 636.6192}, /* 1 */
   {0.8165, 1.8856, 2.9200, 4.3027, 6.9646, 9.9248, 31.5991}, /* 2 */
   {0.7649, 1.6377, 2.3534, 3.1824, 4.5407, 5.8409, 12.9240}, /* 3 */
   {0.7407, 1.5332, 2.1318, 2.7764, 3.7649, 4.6041, 8.6103},  /* 4 */
   {0.7267, 1.4759, 2.0150, 2.5706, 3.3649, 4.0322, 6.8688},  /* 5 */
   {0.7176, 1.4398, 1.9432, 2.4469, 3.1427, 3.7074, 5.9588},  /* 6 */
   {0.7111, 1.4149, 1.8946, 2.3646, 2.9980, 3.4995, 5.4079},  /* 7 */
   {0.7064, 1.3968, 1.8595, 3.3060, 2.8965, 3.3554, 5.0413},  /* 8 */
   {0.7027, 1.3830, 1.8331, 2.2622, 2.8214, 3.2498, 4.7809},  /* 9 */
   {0.6998, 1.3722, 1.8125, 2.2281, 2.7638, 1.1693, 4.5869},  /* 10 */
   {0.6974, 1.3634, 1.7959, 2.2010, 2.7181, 3.1058, 4.4370},  /* 11 */
   {0.6955, 1.3562, 1.7823, 2.1788, 2.6810, 3.0545, 4.3178},  /* 12 */
   {0.6938, 1.3502, 1.7709, 2.1604, 2.6403, 3.0123, 4.2208},  /* 13 */
   {0.6924, 1.3450, 1.7613, 2.1448, 2.6245, 2.9768, 4.1405},  /* 14 */
   {0.6912, 1.3406, 1.7531, 2.1315, 2.6025, 2.9467, 4.0728},  /* 15 */
   {0.6901, 1.3368, 1.7459, 2.1199, 2.5835, 2.9208, 4.0150},  /* 16 */
   {0.6892, 1.3334, 1.7396, 2.1098, 2.5669, 2.8982, 3.9651},  /* 17 */
   {0.6884, 1.3304, 1.7341, 2.1009, 2.5524, 2.8784, 3.9216},  /* 18 */
   {0.6876, 1.3277, 1.7291, 2.0930, 2.5395, 2.8609, 3.8834},  /* 19 */
   {0.6870, 1.3253, 1.7247, 2.0860, 2.5280, 2.8453, 3.8495},  /* 20 */
   {0.6844, 1.3163, 1.7081, 2.0595, 2.4851, 2.7874, 3.7251},  /* 25 */
   {0.6828, 1.3104, 1.6973, 2.0423, 2.4573, 2.7500, 3.6460},  /* 30 */
   {0.6816, 1.3062, 1.6896, 2.0301, 2.4377, 2.7238, 3.5911},  /* 35 */
   {0.6807, 1.3031, 1.6839, 2.0211, 2.4233, 2.7045, 3.5510},  /* 40 */
   {0.6800, 1.3006, 1.6794, 2.0141, 2.4121, 2.6896, 3.5203},  /* 45 */
   {0.6794, 1.2987, 1.6759, 2.0086, 2.4033, 2.6778, 3.4960},  /* 50 */
   {0.6786, 1.2958, 1.6706, 2.0003, 2.3901, 2.6603, 3.4602},  /* 60 */
   {0.6780, 1.2938, 1.6669, 1.9944, 2.3808, 2.6479, 3.4350},  /* 70 */
   {0.6776, 1.2922, 1.6641, 1.9901, 2.3739, 2.6387, 3.4163},  /* 80 */
   {0.6772, 1.2910, 1.6620, 1.9867, 2.3685, 2.6316, 3.4019},  /* 90 */
   {0.6770, 1.2901, 1.6602, 1.9840, 2.3642, 2.6259, 3.3905}}; /* 100 */

   if (dof >= 1 && dof <= 20) dofindex = dof-1;
   else if (dof < 25) dofindex = 19;
   else if (dof < 30) dofindex = 20;
   else if (dof < 35) dofindex = 21;
   else if (dof < 40) dofindex = 22;
   else if (dof < 45) dofindex = 23;
   else if (dof < 50) dofindex = 24;
   else if (dof < 60) dofindex = 25;
   else if (dof < 70) dofindex = 26;
   else if (dof < 80) dofindex = 27;
   else if (dof < 90) dofindex = 28;
   else if (dof < 100) dofindex = 29;
   else if (dof >= 100) dofindex = 30;
   else
   {
      printf("tinv: degrees of freedom must be at least 1\n");
      exit(1);
   }

   if (p == .75)        pindex = 0;
   else if (p == .9)    pindex = 1;
   else if (p == .95)   pindex = 2;
   else if (p == .975)  pindex = 3;
   else if (p == .99)   pindex = 4;
   else if (p == .995)  pindex = 5;
   else if (p == .9995) pindex = 6;
   else
   {
      printf("tinv: probability not tabulated\n");
      exit(1);
   }

   return(tinv_array[dofindex][pindex]);
}