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Task/Statistics-Normal-distribution/C/statistics-normal-distribution.c

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+/*
+ * RosettaCode example: Statistics/Normal distribution in C
+ *
+ * The random number generator rand() of the standard C library is obsolete
+ * and should not be used in more demanding applications. There are plenty
+ * libraries with advanced features (eg. GSL) with functions to calculate
+ * the mean, the standard deviation, generating random numbers etc.
+ * However, these features are not the core of the standard C library.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+#include <string.h>
+#include <time.h>
+
+
+#define NMAX 10000000
+
+
+double mean(double* values, int n)
+{
+    int i;
+    double s = 0;
+
+    for ( i = 0; i < n; i++ )
+        s += values[i];
+    return s / n;
+}
+
+
+double stddev(double* values, int n)
+{
+    int i;
+    double average = mean(values,n);
+    double s = 0;
+
+    for ( i = 0; i < n; i++ )
+        s += (values[i] - average) * (values[i] - average);
+    return sqrt(s / (n - 1));
+}
+
+/*
+ * Normal random numbers generator - Marsaglia algorithm.
+ */
+double* generate(int n)
+{
+    int i;
+    int m = n + n % 2;
+    double* values = (double*)calloc(m,sizeof(double));
+    double average, deviation;
+
+    if ( values )
+    {
+        for ( i = 0; i < m; i += 2 )
+        {
+            double x,y,rsq,f;
+            do {
+                x = 2.0 * rand() / (double)RAND_MAX - 1.0;
+                y = 2.0 * rand() / (double)RAND_MAX - 1.0;
+                rsq = x * x + y * y;
+            }while( rsq >= 1. || rsq == 0. );
+            f = sqrt( -2.0 * log(rsq) / rsq );
+            values[i]   = x * f;
+            values[i+1] = y * f;
+        }
+    }
+    return values;
+}
+
+
+void printHistogram(double* values, int n)
+{
+    const int width = 50;
+    int max = 0;
+
+    const double low   = -3.05;
+    const double high  =  3.05;
+    const double delta =  0.1;
+
+    int i,j,k;
+    int nbins = (int)((high - low) / delta);
+    int* bins = (int*)calloc(nbins,sizeof(int));
+    if ( bins != NULL )
+    {
+        for ( i = 0; i < n; i++ )
+        {
+            int j = (int)( (values[i] - low) / delta );
+            if ( 0 <= j  &&  j < nbins )
+                bins[j]++;
+        }
+
+        for ( j = 0; j < nbins; j++ )
+            if ( max < bins[j] )
+                max = bins[j];
+
+        for ( j = 0; j < nbins; j++ )
+        {
+            printf("(%5.2f, %5.2f) |", low + j * delta, low + (j + 1) * delta );
+            k = (int)( (double)width * (double)bins[j] / (double)max );
+            while(k-- > 0) putchar('*');
+            printf("  %-.1f%%", bins[j] * 100.0 / (double)n);
+            putchar('\n');
+        }
+
+        free(bins);
+    }
+}
+
+
+int main(void)
+{
+    double* seq;
+
+    srand((unsigned int)time(NULL));
+
+    if ( (seq = generate(NMAX)) != NULL )
+    {
+        printf("mean = %g, stddev = %g\n\n", mean(seq,NMAX), stddev(seq,NMAX));
+        printHistogram(seq,NMAX);
+        free(seq);
+
+        printf("\n%s\n", "press enter");
+        getchar();
+        return EXIT_SUCCESS;
+    }
+    return EXIT_FAILURE;
+}