# Glossary of computer science

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This glossary of computer science terms is a list of definitions about computer science, its sub-disciplines, and related fields.

## H

• Hard problemComputational complexity theory focuses on classifying computational problems according to their inherent difficulty, and relating these classes to each other. A computational problem is a task solved by a computer. A computation problem is solvable by mechanical application of mathematical steps, such as an algorithm.
• Hash function – is any function that can be used to map data of arbitrary size to data of a fixed size. The values returned by a hash function are called hash values, hash codes, digests, or simply hashes. Hash functions are often used in combination with a hash table, a common data structure used in computer software for rapid data lookup. Hash functions accelerate table or database lookup by detecting duplicated records in a large file.
• Hash table – In computing, a hash table (hash map) is a data structure that implements an associative array abstract data type, a structure that can map keys to values. A hash table uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.
• Heap – is a specialized tree-based data structure which is essentially an almost complete[87] tree that satisfies the heap property: if P is a parent node of C, then the key (the value) of P is either greater than or equal to (in a max heap) or less than or equal to (in a min heap) the key of C.[88] The node at the "top" of the heap (with no parents) is called the root node.
• Heapsort – is a comparison-based sorting algorithm. Heapsort can be thought of as an improved selection sort: like that algorithm, it divides its input into a sorted and an unsorted region, and it iteratively shrinks the unsorted region by extracting the largest element and moving that to the sorted region. The improvement consists of the use of a heap data structure rather than a linear-time search to find the maximum.[89]
• Human-computer interaction – (HCI) researches the design and use of computer technology, focused on the interfaces between people (users) and computers. Researchers in the field of HCI both observe the ways in which humans interact with computers and design technologies that let humans interact with computers in novel ways. As a field of research, human–computer interaction is situated at the intersection of computer science, behavioral sciences, design, media studies, and several other fields of study.

## References

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• for an object [variable or constant], causes storage to be reserved for that object;
• for a function, includes the function body;
• for an enumeration constant, is the (only) declaration of the identifier;
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89. ^ Skiena, Steven (2008). "Searching and Sorting". The Algorithm Design Manual. Springer. p. 109. doi:10.1007/978-1-84800-070-4_4. ISBN 978-1-84800-069-8. [H]eapsort is nothing but an implementation of selection sort using the right data structure.

## Notes

1. ^ The function may be stored as a reference to a function, such as a function pointer.