#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double dsum( double* numbers, int numNumbers ) {
  double retval = 0.0;
  int x; 
  for( x = 0; x < numNumbers; x++ ) {
    retval += numbers[ x ];
  }
  return retval;
}

double average( double* numbers, int numNumbers ) {
  return dsum( numbers, numNumbers ) / numNumbers;
}

double stddev( double* numbers, int numNumbers ) {
  int x;
  double summation = 0.0;
  double avg = average( numbers, numNumbers );
  for( x = 0; x < numNumbers; x++ ) {
    double minus = numbers[ x ] - avg;
    summation += minus * minus;
  }

  return sqrt( ( 1 / ( (double)numNumbers - 1 ) ) * summation );
}

// reads in numbers from the user until we encounter
// a non-number
double* readNumbers( int* numNumbersRead ) {
  double* retval = NULL;
  double input;
  *numNumbersRead = 0;
  
  while( scanf( "%lf", &input ) == 1 ) {
    (*numNumbersRead)++; // NOT THE SAME AS `*numNumbersRead++`!!
    retval = realloc( retval, *numNumbersRead * sizeof( double ) );
    retval[ *numNumbersRead - 1 ] = input;
  }

  return retval;
}

int main() {
  double* numbers;
  int numNumbers;

  numbers = readNumbers( &numNumbers );
  printf( "Standard deviation: %lf\n", stddev( numbers, numNumbers ) );
  free( numbers );
  return 0;
}