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What is the time complexity of binary sum, the sum of two binary numbers done like in elementary school?

Say one number is F and his length is $s$ bits, and another number is H and his length is $t$. (Like Time complexity of binary multiplication? , but with sum).

Mohsen Shahriari
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ben
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    Welcome to Math.SE! Does "binary sum" mean you are adding numbers represented in base two, or does it mean you are adding two numbers? Or both? It would be helpful if you could add some context to the Question, not least because it would help Answers be written at a level that is useful to you. – hardmath Dec 02 '13 at 11:37
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    @hardmath binary sum mean you are adding numbers represented in base two – ben Dec 02 '13 at 11:39

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The short answer is that adding two numbers by the "elementary school" algorithm has linear complexity. That is, given binary representations F and H of respective lengths $s$ and $t$, the number of steps needed is $O(s+t)$.

This should be intuitively clear. After arranging the longer number over the shorter one, starting at the right hand side and adding-with-carry from right to left will generate a sum of length at most $\max(s,t)+1$. The computation required for each bit of the result is $O(1)$ or constant, since this merely combines the two respective bits of F and H with a possible carry from the previous step. (There are at most eight possibilities for such a step.)

hardmath
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  • and i have another question is like this question: if ןnstead of sum, i do mult, the time complexity will be O(s*t)? – ben Dec 02 '13 at 12:55
  • Yes, which is roughly what the earlier Question you linked to says. We might think in terms of doing the multiplication as shift-and-add, based on the bits of the shorter multiplicand, so that you need to add (at most) $s$ shifted copies of the $t$ bit length number, which amounts to $(s-1)$ additions of (at most) $s+t-1$ bit length representations. I leave it to you to verify $sO(s+t) = O(st)$ when $s \le t$. – hardmath Dec 02 '13 at 13:36
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    But in http://math.stackexchange.com/questions/226394/time-complexity-of-binary-multiplication , he tell that it's O(n^2).. so who wrong? – ben Dec 02 '13 at 13:50
  • In that Question $n=s=t$, and we are just taking the large size to fit both multiplicands. The Accepted Answer there mentions using two different sizes, one of $n$ bits and one of $m$ bits. But these are exercises for you to think through! – hardmath Dec 02 '13 at 13:59
  • Can we say ,in the sum, that is O(n) too when n is the length of the bigger number? – ben Dec 02 '13 at 15:03
  • Yes, that would be accurate. Keep in mind $O(n)$ is the same as $O(2n)$. – hardmath Dec 02 '13 at 15:23
  • And it is right about the mult too? O(n^2) – ben Dec 02 '13 at 16:18
  • It is correct about multiplication. – hardmath Dec 02 '13 at 17:29
  • thank you very much, you helped me a lot!!!! – ben Dec 02 '13 at 17:51