- Computationally expensive
A computationally expensive
algorithmis one that, for a given input size, requires a relatively large number of steps to complete; in other words, one with high computational complexity. The "cost" of an algorithm relative to the size of its input is often represented using Big O notation.
For some expensive algorithms, there are less expensive counterparts. A traditional example in which multiple algorithms exist to achieve the same end is the class of
sorting algorithms used to order a one-dimensional list. From the point of view of implementation, the so-called bubble sortis one of the simplest of these algorithms. It is, however, relatively computationally expensive, requiring more computation steps for a list of given size than almost any other standard algorithm. As such, bubble sort is virtually never used in practice. For real-life sorting problems, much more efficient algorithms are used, such as Quicksort; these, however, are somewhat more complicated in their implementation.
Often, the more general an algorithm, the more computationally expensive it is. For example, algorithms used for manipulating a generic matrix will work on a
sparse matrix, but algorithms designed specifically for sparse matrices will be less expensive.
Additionally, the cost of a given action can apply to what kind of I/O is involved; fetching data from memory is usually considered inexpensive, whereas having to fetch memory from disk -- a far slower medium -- is considered expensive.
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