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Running time function for an algorithm with while and for loop
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whilewithloopfunctiontimealgorithmrunningforand
Problem
I have trouble determining the running time function for algorithm below. I know that there are initially two assignment operations,
Also, I know that the while loop is represented by $\lfloor \log_2n \rfloor + 1$, but the for loop depends on the value of $i$ which decrements by half.
So, the first time the for loop performs $n$ times, the second time the for loop performs $n/2$ times, and so on. How can I represent this behavior?
I suppose that, at the end, the result is the product of both behaviors.
int i=n and int s=0.Also, I know that the while loop is represented by $\lfloor \log_2n \rfloor + 1$, but the for loop depends on the value of $i$ which decrements by half.
So, the first time the for loop performs $n$ times, the second time the for loop performs $n/2$ times, and so on. How can I represent this behavior?
I suppose that, at the end, the result is the product of both behaviors.
int i = n; int s = 0;
while(i > 0) {
for(j = 1; j <= i; ++j) s++;
i /= 2;
}Solution
For each value of $i$, the inner loop runs $i$ times, and so the running time of the body of the outer loop with a given value
$$ \Theta(n + n/2 + n/4 + \cdots + 1) = \Theta(n). $$
Here we use the fact that the infinite series $1 + 1/2 + 1/4 + \cdots$ converges (to $2$).
i is in $\Theta(i)$. The values of i, as you mention, are $n, \lfloor n/2 \rfloor, \lfloor n/4 \rfloor, \cdots, 1$, and so the overall running time is$$ \Theta(n + n/2 + n/4 + \cdots + 1) = \Theta(n). $$
Here we use the fact that the infinite series $1 + 1/2 + 1/4 + \cdots$ converges (to $2$).
Context
StackExchange Computer Science Q#44366, answer score: 2
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