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What is a Vector Iterator?

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  • Written By: Jessica Susan Reuter
  • Edited By: Allegra J. Lingo
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  • Last Modified Date: 03 December 2016
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A vector iterator is a computer language construct that allows a program to read data values contained in a specialized collection called a vector. Vectors are objects used to group related data values, similar to arrays and lists. Many different computer languages, most notably Java and C++, contain vectors and their related iterators. Each language uses different syntax for the iterators, but the underlying mechanism in all languages is individual access to each possible member of the vector group.

Moving through a vector using an iterator is often called traversing or iterating. Vector iterators can also be used to explicitly identify a particular object in the vector collection. When this identification is performed, the object is identified by its index in the vector, not by any characteristics of the object itself. Actions can be performed on the object if the iterator is dereferenced, granting the program explicit access to the object rather than the vector.

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Vector iterators have very little chance of encountering the out of bounds errors that can happen while iterating over arrays. Although vectors are essentially glorified arrays when deconstructed, vectors almost always have some type of bounds checking that ensures a vector iterator remains at the proper indices. When traversing a vector, the iterator, if called correctly, always starts at the beginning of the vector and ends at exactly the end. Explicit specification of an incorrect index is still possible in certain contexts, but the built-in bounds checking of vectors, which translates to their iterators, prevents out of bounds overflow issues.

In C++, vectors can be modified while being traversed by iterators, but Java explicitly forbids this from happening. Java's actions in this circumstance are much safer because changing a vector while an iterator is moving on it can cause the iterator to inadvertently read outside of the vector. Attempting to add or remove items from a vector while iterating is especially dangerous, particularly if the start and end indices the vector must cover are hard coded. A vector iterator is not equipped to handle sudden changes in vectors, and vectors are meant to be relatively static while being traversed.

A vector iterator can be implicit or explicit, and either syntactic form runs the traversal just as smoothly. Despite being coded for efficiency, a vector iterator can be slowed by a very large vector. In this case, hard coding the end index of the vector speeds up traversals. This problem doesn't occur with small vectors, so hard coding the end index doesn't cause any appreciable speedup. Hard coding the end index can raise the risk of overflow, so in general it should be done sparingly.

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