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In computer programming, orthogonality means that operations change just one thing without affecting others. The term is most-frequently used regarding assembly instruction sets, as orthogonal instruction set.
Orthogonality in a programming language means that a relatively small set of primitive constructs can be combined in a relatively small number of ways to build the control and data structures of the language. It is associated with simplicity; the more orthogonal the design, the fewer exceptions. This makes it easier to learn, read and write programs in a programming language. The meaning of an orthogonal feature is independent of context; the key parameters are symmetry and consistency (for example, a pointer is an orthogonal concept).
An example from IBM Mainframe and VAX highlights this concept. An IBM mainframe has two different instructions for adding the contents of a register to a memory cell (or another register). These statements are shown below:
A Reg1, memory_cell AR Reg1, Reg2
In the first case, the contents of
Reg1 are added to the contents of a memory cell; the result is stored in
Reg1. In the second case, the contents of
Reg1 are added to the contents of another register (
Reg2) and the result is stored in
In contrast to the above set of statements, VAX has only one statement for addition:
ADDL operand1, operand2
In this case the two operands (
operand2) can be registers, memory cells, or a combination of both; the instruction adds the contents of
operand1 to the contents of
operand2, storing the result in
VAX’s instruction for addition is more orthogonal than the instructions provided by IBM; hence, it is easier for the programmer to remember (and use) the one provided by VAX.
The design of C language may be examined from the perspective of orthogonality. The C language is somewhat inconsistent in its treatment of concepts and language structure, making it difficult for the user to learn (and use) the language. Examples of exceptions follow:
- Structures (but not arrays) may be returned from a function.
- An array can be returned if it is inside a structure.
- A member of a structure can be any data type (except void, or the structure of the same type).
- An array element can be any data type (except void). Everything is passed by value (except arrays).
- Void can be used as a type in a structure, but a variable of this type cannot be declared in a function.
Though this concept was first applied to programming language, orthogonality has since become recognized as a valuable feature in the design of APIs and even user interfaces. There, too, having a small set of composable primitive operations without surprising cross-linkages is valuable. leading to systems that are easier to explain and less frustrating to use.
- The Pragmatic Programmer: From Journeyman to Master by Andrew Hunt and David Thomas. Addison-Wesley. 2000. ISBN 978-0-201-61622-4.
- "The Art of Unix Programming", chapter about Orthogonality – Orthogonality concept well-explained