Substring

From Wikipedia, the free encyclopedia

A subsequence, substring, prefix or suffix of a string is a subset of the symbols in a string, where the order of the elements is preserved. In this context, the terms string and sequence have the same meaning.

1 Subsequence
2 Substring
3 Prefix
4 Suffix
5 References

[edit] Subsequence

Main article subsequence

A subsequence of a string $T = t_1 t_2 \dots t_n$ is a string $\hat T = t_{i_1} \dots t_{i_m}$ such that $i_1 < \dots < i_m$ , where $m \leq n$ . Subsequence is a generalisation of substring, suffix and prefix. Finding the longest string which is equal to a subsequence of two or more strings is known as the longest common subsequence problem.

Example: The string anna is equal to a subsequence of the string banana:

banana
 || ||
 an na

[edit] Substring

A substring of a string $T = t_1 \dots t_n$ is a string $\hat T = t_{1+i} \dots t_{m+i}$ , where $0 \leq i$ and $m + i \leq n$ . A substring of a string is a prefix of a suffix of the string, and equivalently a suffix of a prefix. If $\hat T$ is a substring of $T$ , it is also a subsequence, which is a more general concept. Given a pattern $P$ , you can find its occurrences in a string $T$ with a string searching algorithm. Finding the longest string which is equal to a substring of two or more strings is known as the longest common substring problem.

Example: The string ana is equal to substrings (and subsequences) of banana at two different offsets:

banana
 |||||
 ana||
   |||
   ana

In the mathematical literature, substrings are also called subwords (in America) or factors (in Europe).

[edit] Prefix

A prefix of a string $T = t_1 \dots t_n$ is a string $\hat T = t_1 \dots t_{m}$ , where $m \leq n$ . A proper prefix of a string is not equal to the string itself and not empty^[1] ( $0 < m < n$ ). A prefix can be seen as a special case of a substring.

Example: The string ban is equal to a prefix (and substring and subsequence) of the string banana:

banana
|||
ban

[edit] Suffix

A suffix of a string $T = t_1 \dots t_n$ is a string $\hat T = t_{n-m+1} \dots t_{n}$ , where $m \leq n$ . A proper suffix of a string is not equal to the string itself and not empty^[2] ( $0 < m < n$ ). A suffix can be seen as a special case of a substring.

Example: The string nana is equal to a suffix (and substring and subsequence) of the string banana:

banana
  ||||
  nana

[edit] References

^ Gusfield, Dan [1997] (1999). Algorithms on Strings, Trees and Sequences: Computer Science and Computational Biology. USA: Cambridge University Press. ISBN 0-521-58519-8.

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Category: Algorithms on strings