It is log2 n, that is the number of binary bits (or the rank of the highest 1 bit) in the binary representation of n
And you made a typo, you probably mean
for(i=n,j=0 ; i>0 ; (i/=2), (j+=i));
So you start with i==n
then you halve it till 0 is reached (hence the log2 n number of loops).
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solved Number of terms in this code series? [closed]