Concatenate

Concatenate is a verb meaning to link things together in a chain or series. In mathematics and computer science, concatenation refers to forming a new object by appending one sequence to another. This operation applies to strings, lists, arrays, and other ordered sequences.

In computing, string concatenation is a common task in which characters from two strings are joined to produce a longer string. For example, concatenating "Hello" and "World" yields "HelloWorld" in many programming languages. The operation is typically represented by symbols such as the plus sign in languages like Python, Java, and JavaScript, by dedicated operators in others, or by functions like CONCAT in SQL.

Concatenation is associative, meaning that when concatenating multiple sequences, the grouping of operations does not affect the final result. It is generally not commutative; replacing the order of the inputs usually changes the output (e.g., concatenating "A" with "B" yields "AB," whereas "BA" yields a different string).

Beyond strings, concatenation also applies to lists or arrays, where the result is a longer list combining the elements of the operands in order. In formal language theory, concatenation of languages L1 and L2 consists of all strings that can be formed by taking a string from L1 followed by a string from L2.

Performance considerations arise with immutable strings: repeated concatenation can be inefficient, as it may create intermediate objects. Many languages offer optimized approaches, such as string builders, join operations, or mutable buffers, to improve efficiency for large or repeated concatenations.