(algorithm)

**Definition:**
Calculate an *associative* function, f, on all prefixes of an n-element array, that is, s[0], f(s[0], s[1]), f(s[0], f(s[1], s[2])), …, f(s[0], f(s[1], … f(s[n-2], s[n-1])…)), using *Θ(n)* processors in Θ(log n) time. The algorithm is

for j := 0 to lg(n)-1 dowhere lg is the logarithm base 2, and parallel-do does the innermost computations in parallel.

for i := 2^{j}to n-1 parallel-do

s[i] := f(s[i-2^{j}], s[i])

*Note:
In particular, this calculates any associative function, such as sum, maximum, or concatenate, over a list of values in logarithmic time. Note the write-after-read hazards in the parallel-do loop: old values of s[2^{j}] to s[n-1-2^{j}] must be read before being overwritten. Since this algorithm overwrites the initial values, the n processors can copy input values to a result array in parallel in one additional step. *

From Yair Tuaff (r56409@email.sps.mot.com), 29 December 1999.

Author: PEB

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Entry modified 25 January 2010.

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Cite this as:

Paul E. Black, "parallel prefix computation", in
*Dictionary of Algorithms and Data Structures* [online], Paul E. Black, ed. 25 January 2010. (accessed TODAY)
Available from: https://www.nist.gov/dads/HTML/parallelPrefix.html