Index summation of 'for' loop

Why does these snippets give different results? I know the first one is correct, but what happens in the second case?

Sample 1

int i=0;
for(;i<5;i+=3){i=i*i;cout<<i;}
cout<<i;

Output:

0

9

12

Sample 2

for(int i=0;i<5;i+=3){i=i*i;cout<<i;}
cout<<i;

Output:

0

9

9