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In computer programming and computer science, a standard swap is a situation in a program in which two variables need to have their values exchanged, requiring the creation of a third variable to act as an intermediary when the values are transferred. A standard swap is a programming paradigm, meaning it can be applied to any situation in which two pieces of data need to be exchanged, and is not tied to any one data type or programming language. There are many instances in programs in which values need to be swapped, but it often is a standard component in collections and arrays when used with sorting algorithms that must move items from one index in the collection to another, using a temporary variable to hold the value being swapped. From a hardware perspective, a standard swap can be a supported low-level processor function for some scalar data types such as integers, meaning the intermediary value does not need to be created in random access memory (RAM) to swap two numbers.
The concept of a standard swap can be seen with the variables A and B; at the end of a standard swap, B should equal A, and A should equal B. If a program simply assigns B to A, then the value of A will be lost and cannot later be assigned to B. This requires the use of a temporary variable that will hold the value of A while B is assigned to A. The final step is to assign the temporary variable value to B, completing the swap.
One very common application for a standard swap is when using a sorting algorithm on an array or collection. If a sorting algorithm has determined that the data at array Index 1 must be swapped with the data at Index 10, then a standard swap of the data can be performed. This can be done exceptionally fast using pointer references in languages such as C.
A complication can occur in some object oriented programming languages when two object instances need to be swapped. In some very rare cases, simply swapping the object references will not initiate an actual swap. Instead, the only way to exchange the objects is to clone the internal data, field by field, between the two objects and the temporary object variable.
There is one technique that technically can be used to perform a standard swap using the exclusive-or (XOR) logical operator. A XOR swap relies on the fact that, when two bits are calculated with XOR, they essentially will create a mask that can be reversed with one or both of the operators. In this way, two variables, A and B, can be swapped by first creating a mask with a XOR of the two variables, then using XOR to unmask first the value for B and then the value for A. Unfortunately, although this method avoids the creation of a temporary variable, it also will not make a swap in all instances, specifically when A and B are equal to each other and in cases in which compiler optimization might change the actual operation.
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