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Concept
Augmented assignment
Summary
Augmented assignment (or compound assignment) is the name given to certain
assignment operators in certain programming languages (especially those derived from C). An augmented assignment is generally used to replace a statement where an operator takes a variable as one of its arguments and then assigns the result back to the same variable. A simple example is x += 1 which is expanded to x = x + 1. Similar constructions are often available for various binary operators.
In general, in languages offering this feature, most operators that can take a variable as one of their arguments and return a result of the same type have an augmented assignment equivalent that assigns the result back to the variable in place, including arithmetic operators, bitshift operators, and bitwise operators.
For example, the following statement or some variation of it can be found in many programs:
x = x + 1
This means "find the number stored in the variable , add 1 to it, and store the result of the addition in the variable ." As simple as this seems, it may have an inefficiency, in that the location of variable has to be looked up twice if the compiler does not recognize that two parts of the expression are identical: might be a reference to some array element or other complexity. In comparison, here is the augmented assignment version:
x += 1
With this version, there is no excuse for a compiler failing to generate code that looks up the location of variable just once, and modifies it in place, if of course the machine code supports such a sequence. For instance, if x is a simple variable, the machine code sequence might be something like
Load x
Add 1
Store x
and the same code would be generated for both forms. But if there is a special op code, it might be
MDM x,1
meaning "Modify Memory" by adding 1 to x, and an optimizing compiler would generate the same code for both forms. Some machine codes offer INC and DEC operations (to add or subtract one), others might allow constants other than one.
Official source
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