Question

I am not able to find the bug in this small piece of code. what is wrong with it?

string f,s;
f[0] = 'd';
s.append(f);
cout<<f.length()<<"  "<<f<<"  "<<f[0]<<endl;
cout<<s.length()<<"  "<<s<<"  "<<s[0]<<endl;  
Output is : 
0    d
0    d

Even if I change s.length to s.size, the result is the same. Why is s[0] = 'd' and s.size() = 0;?

Answer 1

Both the strings are empty when you create them, they contain no characters. f[0] is out of bounds, and accessing any element of an empty container is undefined behaviour, so anything can legally happen.

You need to do

string f(1, 'd'), s; // creates f with 1 repetition of 'd'
s.append(f);
...

Or

string f, s;
f += 'd'; // or f.push_back('d'), or f.append('d'), or...
s.append(f);
...