In computing, a one-pass algorithm or single-pass algorithm is a streaming algorithm which reads its input exactly once.[1] It does so by processing items in order, without unbounded buffering; it reads a block into an input buffer, processes it, and moves the result into an output buffer for each step in the process.[2] A one-pass algorithm generally requires O(n) (see 'big O' notation) time and less than O(n) storage (typically O(1)), where n is the size of the input.[3] An example of a one-pass algorithm is the Sondik partially observable Markov decision process.[4]
Example problems solvable by one-pass algorithms
Given any list as an input:
Count the number of elements.
Given a list of numbers:
Find the k largest or smallest elements, k given in advance.
Find the modes (This is not the same as finding the most frequent symbol from a limited alphabet).
Sort the list.
Count the number of items greater than or less than the mean. However, this can be done in constant memory with two passes: Pass 1 finds the average and pass 2 does the counting.
The two-pass algorithms above are still streaming algorithms but not one-pass algorithms.
