/*
 * Contrived example of dynamically-allocated 1D array.
 * Also uses strtol to parse command-line arguments.
 *
 * Program takes two command-line arguments, seed and size, and uses
 * them to generate array of 1D "random" doubles in the range [0 .. 1],
 * for which it then computes variance.
 */
#include <stdio.h>
#include <stdlib.h>

double square(double d);
double variance(long size, double vals[]);

int main(int argc, char *argv[]) {
	if (argc < 3) {
		printf("usage %s seed size\n", argv[0]);
		return EXIT_FAILURE;
	}
	char *endptr;
	int seed = strtol(argv[1], &endptr, 10);
	if (*endptr != '\0') {
		printf("usage %s seed size\n", argv[0]);
		return EXIT_FAILURE;
	}
	long size = strtol(argv[2], &endptr, 10);
	if (*endptr != '\0') {
		printf("usage %s seed size\n", argv[0]);
		return EXIT_FAILURE;
	}
	double *vals = malloc(sizeof(*vals) * size);
	if (vals == NULL) {
		printf("could not allocate space for %ld values\n", size);
		return EXIT_FAILURE;
	}
	srand(seed);
	for (long i = 0; i < size; ++i) {
		vals[i] = ((double) rand()) / RAND_MAX;
	}
	double vals_variance = variance(size, vals);
	printf("variance is %g\n", vals_variance);

	free(vals);
	return EXIT_SUCCESS;
}

double square(double d) { return d*d; }

double variance(long size, double vals[]) {
	double sum = 0.0;
	for (long i = 0; i < size; i++) {
		sum += vals[i];
	}
	double average = sum/size;

	double variance_sum = 0.0;
	for (long i = 0; i < size; i++) {
		variance_sum += square(vals[i]-average);
	}
	return variance_sum/size;
}