concatenates

Concatenation is the operation of joining two or more sequences end to start to produce a longer sequence. The verb form concatenates describes performing this operation. In computing, concatenation is commonly applied to strings, arrays, lists, or byte sequences. The result preserves the order of operands; the length equals the sum of the operands' lengths. In mathematics, sequence concatenation is similarly defined and is associative.

Most programming languages provide a way to concatenate. In many languages, a plus operator combines strings; in others, dedicated functions or methods exist, such as concat, join, or append. The exact semantics vary: strings may be immutable or mutable, and source code may optimize with temporary buffers. Repeatedly concatenating strings in languages with immutable strings can lead to quadratic time complexity; techniques such as using a StringBuilder, a list of fragments, or a join operation are preferred.

When concatenating, encoding matters for textual data. Unicode handling requires consistent encoding and normalization; concatenating characters from different scripts generally works, but normalization forms can affect results in some contexts.

Applications include constructing user messages, file or URL paths, CSV lines, and data serialization. In SQL, CONCAT or the || operator concatenates strings; in Python, 'a' + 'b' yields 'ab', and ''.join(list) is efficient for many elements; in JavaScript, the + operator or Array.prototype.concat accomplishes the task.

The concept underpins many data-processing tasks, including building keys, composing strings from parts, and merging sequences in data structures. See also: string manipulation, string builder, join.