stddev.cpp


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#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

const char *argv0;

void usage(){
	cerr<<"Usage: "<<argv0<<endl
	    <<"Prints some statistical information about the data on stdin."<<endl
	    <<"Data is assumed to be a list of floating-point values."<<endl;
}

int main(int,char **argv){
	argv0=argv[0];

	vector<double> data;

	double total=0;
	while(true){
		double v;
		cin>>v;
		if(!cin)break;
		data.push_back(v);
		total+=v;
	}
	if(data.size()==0){
		cerr<<"No data"<<endl;
		return 0;
	}

	sort(data.begin(),data.end());

	const double s_mean=total/data.size();
	const double s_median=data[data.size()/2];

	total=0;
	for(double v : data){
		total+=(v-s_mean)*(v-s_mean);
	}
	const double s_stddev=data.size()==1?nan(""):sqrt(total/(data.size()-1));
	const double s_stderr=s_stddev/sqrt(data.size());

	cout<<"Mean:   "<<s_mean<<'\n'
	    <<"Stddev: "<<s_stddev<<'\n'
	    <<"Stderr: "<<s_stderr<<'\n'
	    <<"Median: "<<s_median<<endl;
}