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I am a little confused by the complexity proof of Radix Sort.

For counting sort, the complexity reported is $O(n+R)$, where $n$ is the number of items and $R$ is the range.

But this is not entirely correct, right? To do binary arithmetic on numbers as big as $R$, I need $\log_2 R$ operations for any deterministic hashing function. So the complexity of counting shot should be $O(\log_2 R \cdot (n+R))$.

Then the complexity of radix sort with base $b$ should be $O(\log_b R \cdot \log_2b \cdot (n+b))$, which is equivalent to $O(\log_2R \cdot n)$. For range $R = \Omega(n)$, this is $\Omega(n \log n)$ and thus not $O(n)$.

Can someone point out where I am going wrong in my proof? As far as I know, radix sort is $O(n)$ for $R = O(n^c)$.

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  • $\begingroup$ You can use Mathjax here. See: math.meta.stackexchange.com/questions/5020/… $\endgroup$
    – HEKTO
    Commented Sep 30, 2020 at 4:34
  • $\begingroup$ Thanks, I didn't realize that was enabled on this stack exchange. $\endgroup$
    – xyz
    Commented Sep 30, 2020 at 21:49

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We typically analyze algorithms in the so-called "RAM machine" or "transdichotomous model". In this model, operations on machine words take $O(1)$, and a machine word has length $O(\log n)$ bits, where $n$ is the size of the input (in bits), or any other polynomially related quantity.

If $R$ is polynomial in $n$ (in this case, the length of the array), then an integer in the range $\{1,\ldots,R\}$ can be stored in a single machine word, and so operations on such integers take constant time in this model.

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    $\begingroup$ Arbitrary precision arithmetic doesn't necessarily take $O(1)$, even in the transdichotomous model. The basic operations allowed by the model operate on machine words. $\endgroup$
    – Yuval Filmus
    Commented Sep 30, 2020 at 21:50
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    $\begingroup$ If $n < 2^{64}$ and $R = O(n^c)$ then radix sort is $O(1)$. $\endgroup$
    – Yuval Filmus
    Commented Sep 30, 2020 at 21:53
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    $\begingroup$ A machine word is defined in the transdichotomous model as a word of length $O(\log N)$ bits, where $N$ is the input size in bits. $\endgroup$
    – Yuval Filmus
    Commented Sep 30, 2020 at 21:54
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    $\begingroup$ If the word size is fixed, then we're in the bit complexity model. It is less useful than the transdichotomous model to predict the behavior of actual algorithms. $\endgroup$
    – Yuval Filmus
    Commented Sep 30, 2020 at 21:57
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    $\begingroup$ Real-world machines can only access $O(1)$ memory, so any algorithm takes $O(1)$. This improves on your $O(n\log n)$. $\endgroup$
    – Yuval Filmus
    Commented Sep 30, 2020 at 21:57

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