char num_c;
cout<<"please input a number"<<endl;
cin>>num_c;
if (num_c<=57 && num_c>=48)
cout<<"you have input a number:"<<num_c<<endl;
else
cout<<"it is not a numbber"<<endl;

#include <iostream>
#include <string>
using namespace std;

bool CheckNumber(string s);

int main()
{
while(1)
{
cout<<"please input a string (press \"q\" to quit)"<<endl;
string in;
cin>>in;
if (in=="q" || in=="Q" ) break;
if (CheckNumber(in)) cout<<"this is a number!"<<endl;
else cout<<"this is not a number !!!"<<endl;
}

return 0;
}

bool CheckNumber(string s)
{
const char *temp = s.c_str();

for(int i=0; i<s.length(); i++)
{
if(temp[i]>=57 || temp[i]<=48) return false;
}

return true;
}