You should maintain counters for range hits, e.g.
#include <iostream>
using namespace std;
int main()
{
int pres;
int inrange[2]={0};
string range[2] = {" 0- 50 ", " 51-100 "};
for (int i =0; i<7; i++)
{
cin >> pres;
if ( (pres>= 0) && (pres <= 50) )
++inrange[0];
else if ( (pres > 50) && (pres <=100))
++inrange[1];
else
{
}
}
for (int i=0;i<2; ++i)
{
cout << range[i];
for (int j=0; j<inrange[i]; ++j)
cout << "*";
cout << endl;
}
}