Count the Number of Words in a String

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

int main()
{
    string s1;
    int i,n,a=0;
    s1="Hello World";
    n=s1.length();
    for(i=0; i<=n; i++)
    {
        if (s1[i]==' ')
            a=a+1;
    }
    cout<<"No. of words in string are: "<<a+1;
    return 0;
}