converting string to int from files

[ROMMEL_HSQ]

hi
I'd like to convert a string which contains many numbers into int and then store them in an array.
the file.txt looks like this:
54 42 432 121
0 23 435 32
43 343 654 45
12 5 65 23

the procedure that I'm doing is this:
ifstream f81;
ofstream f81r;
f81.open("f81.txt");
f81r.open("f81r.txt");
int i;
string line;
while(!f81.eof())
{
getline(f81,line);
for(int k=0;k<32;k++) //my matrix size is 32x32
{
f[i][k]=?????? // help needed here.
}
i++;
}
thanks in advance.