/********************************************************
*      Program to demonstrate Chi-square Statistic      *
* ----------------------------------------------------- *
*    Reference: BASIC Scientific Subroutines, Vol. II   *
*    By F.R. Ruckdeschel, BYTE/McGRAWW-HILL, 1981 [1].  *
*                                                       *
*                     C++ version by J-P Moreau, Paris  *
*                          (www.jpmoreau.fr)            *
* ----------------------------------------------------- *
* SAMPLE RUN:                                           *
*                                                       *
*  CHI-SQUARE CUMULATIVE DISTRIBUTION APPROXIMATION     *
*                                                       *
*   P(X)        X2       REAL X2                        *
*  -----------------------------                        *
*  0.05     124.5347     124.3                          *
*  0.10     118.6326     118.5                          *
*  0.15     114.5999                                    *
*  0.20     111.4342                                    *
*  0.25     108.7913     109.1                          *
*  0.30     106.5055                                    *
*  0.35     104.4821                                    *
*  0.40     102.6610                                    *
*  0.45     101.0018                                    *
*  0.50      99.1944      99.3                          *
*  0.55      97.6863                                    *
*  0.60      96.0812                                    *
*  0.65      94.3595                                    *
*  0.70      92.4934                                    *
*  0.75      90.4428      90.1                          *
*  0.80      88.1446                                    *
*  0.85      85.4893                                    *
*  0.90      82.2522      82.4                          *
*  0.95      77.7867      77.9                          *
*  1.00       0.0000                                    *
*  -----------------------------                        *
*                                                       *
********************************************************/
#include <stdio.h>
#include <math.h>

double x,y;
int    i,m;
char   *x1[21];



/***************************************************
* Chi-square cumulative distribution approximation *
* Good for m > 100.                                *
* Reference: Statistics Manual, Crow, Maxfield and *
* Davis (Dover, 1960).                             *
* The input value is y, the probability, the output* 
* value is the corresponding Chi-square statistic. *
***************************************************/
void  Chi_Square()  {
  //Labels: e100, e200, e210
  double z;
  x = y;
  //Guard against zero discontinuity
  if (x <= 0) x = exp(-100);
  if (x > 0.5) goto e100;
  x = -log(x);
  //Regressed table correction
  z = -0.803 + 1.312 * x - 0.2118 * x * x + 0.016 * x * x * x;
  goto e200;
e100: x = 1.0 - x;
  //Guard against zero discontinuity
  if (x <= 0) goto e210;
  x = -log(x);
  z = 0.803 - 1.312 * x + 0.2118 * x * x - 0.016 * x * x * x ;
e200: x = 2.0 / (9.0 * m);
  x = 1.0 - x + z * sqrt(x);
e210: x = m * x * x * x;

}


void main()  {

  for (i=0; i<21; i++) x1[i] = "";
  x1[1]  = "124.3"; x1[2]  = "118.5";
  x1[5]  = "109.1"; x1[10] = " 99.3";
  x1[15] = " 90.1"; x1[18] = " 82.4"; x1[19] = " 77.9";
  printf(" CHI-SQUARE CUMULATIVE DISTRIBUTION APPROXIMATION\n");
  printf(" P(X)        X2       REAL X2 \n");
  printf(" ---------------------------- \n");
  m = 100; i = 0;
  y = 0.05;
  while (y < 1.01)  {
    i++;
    Chi_Square();
	printf("%5.2f     %8.4f     %s\n",y,x,x1[i]);
    y += 0.05;
  }
  printf(" ---------------------------- \n");

}

// End of file chisqa.cpp