为什么setw不工作？

Question

#include<fstream.h>
#include<iomanip.h>
#include<iostream.h>
using namespace std;
ifstream f("atestat.in");
ofstream g("atestat.out");
int n,i,nr=0;
float v[100];
void a(int n)
{
    for(i=1;i<=n;i++)
        f>>v[i];
    for(i=1;i<=n;i++)
    cout<<v[i]<<" ";
}
int main()
{
    f>>n;
    a(n);
    cout<<endl;
    float s=0;
    for(i=1;i<=n;i++)
    {
        if(v[i]<0)
        {   
        s=s+v[i];
        nr++;
        }
    }
    cout<<setw(2)<<s/nr<<endl;
}

my "atestat.in" file contains: 6 -56.765 2.3 4.56 -1.2 -1.8 3

The program first displays all the numbers on the second line of the "atestat.in" file, through the use of an array, and then it is supposed to display the arithmetic mean of all the negative numbers inside that array, with a precision of 2 numbers after the decimal mark. For some reason, setw(2) does nothing at all, as my cout<<setw(2)<<s/nr<<endl; displays "19.9217" instead of "19.92"...could anyone tell me why? Am i using it improperly somehow?

Answer 1

with a precision of 2 numbers after the decimal mark

For this purpose, you need:

std::cout << std::fixed;
std::cout << std::setprecision(2) << f << '\n'; //assume f the number you wanna print

std::setw does not serve for this purpose:

When used in an expression out << setw(n) or in >> setw(n), sets the width parameter of the stream out or in to exactly n.