Our C interpreter will contain a compiler that translates Lox to an efficient bytecode representation (don’t worry, I’ll get into what that means soon), which it then executes. This is the same technique used by implementations of Lua, Python, Ruby, PHP, and many other successful languages.
Increment and decrement operators are unaryoperators that add or subtract one, to or from their operand, respectively. They are commonly implemented in imperativeprogramming languages. C-like languages feature two versions (pre- and post-) of each operator with slightly different semantics.
In languages syntactically derived from B (including C and its various derivatives), the increment operator is written as ++ and the decrement operator is written as --. Several other languages use inc(x) and dec(x) functions.
C-diff Symptoms And Treatment
The increment operator increases, and the decrement operator decreases, the value of its operand by 1. The operand must have an arithmetic or pointerdata type, and must refer to a modifiable data object. Pointers values are increased (or decreased) by an amount that makes them point to the next (or previous) element adjacent in memory.
In languages that support both versions of the operators:
- The pre-increment and pre-decrement operators increment (or decrement) their operand by 1, and the value of the expression is the resulting incremented (or decremented) value.
- The post-increment and post-decrement operators increase (or decrease) the value of their operand by 1, but the value of the expression is the operand's value prior to the increment (or decrement) operation.
In languages where increment/decrement is not an expression (e.g., Go), only one version is needed (in the case of Go, post operators only).
Since the increment/decrement operator modifies its operand, use of such an operand more than once within the same expression can produce undefined results. For example, in expressions such as x - ++x, it is not clear in what sequence the subtraction and increment operations should be performed. Such expressions generally invoke undefined behavior, and should be avoided.
Examples[edit]
The following C code fragment illustrates the difference between the pre and post increment and decrement operators:
In languages lacking these operators, equivalent results require an extra line of code:
The post-increment operator is commonly used with array subscripts. For example:
The post-increment operator is also commonly used with pointers:
Note that these examples also work in other C-like languages, such as C++, Java, and C#.
- Increment operator can be demonstrated by an example:
- Output:
Supporting languages[edit]
The following list, though not complete or all-inclusive, lists some of the major programming languages that support the ++/-- increment/decrement operators.
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(Apple's Swift once supported these operators,[10] but support was removed as of version 3.)
Pascal, Delphi, Modula-2, and Oberon provide the same functions, but they are called inc(x) and dec(x).
Notably Python and Rust do not support these operators.
History[edit]
The concept was introduced in the B programming language circa 1969 by Ken Thompson.[11]
Thompson went a step further by inventing the ++ and -- operators, which increment or decrement; their prefix or postfix position determines whether the alteration occurs before or after noting the value of the operand. They were not in the earliest versions of B, but appeared along the way. People often guess that they were created to use the auto-increment and auto-decrement address modes provided by the DEC PDP-11 on which C and Unix first became popular. This is historically impossible, since there was no PDP-11 when B was developed. The PDP-7, however, did have a few 'auto-increment' memory cells, with the property that an indirect memory reference through them incremented the cell. This feature probably suggested such operators to Thompson; the generalization to make them both prefix and postfix was his own. Indeed, the auto-increment cells were not used directly in implementation of the operators, and a stronger motivation for the innovation was probably his observation that the translation of ++x was smaller than that of x=x+1.
See also[edit]
- Augmented assignment – for
+=and-=operators
References[edit]
- ^'GNU Awk's User Guide'. Free Software Foundation.
- ^'8.3. The Double-Parentheses Construct'. The Linux Documentation Project.
- ^Ritchie, Brian W. Kernighan; Dennis M.; Ritchie, Dennis (1988). The C programming language (2. ed., [Nachdr.] ed.). Englewood Cliffs, N.J.: Prentice Hall. p. 18. ISBN0-13-110362-8.
- ^'Increment/decrement operators'. cppreference.com.
- ^'++ Operator (C# Reference)'. Microsoft Developer Network.
- ^'Operator Overloading'. dlang.org.
- ^'GP Operators and their Priorities'.
- ^'About Assignment Operators'.
- ^'Increment Wolfram Language Symbol'. Wolfram Language Documentation Center.
- ^'Basic Operators'. developer.apple.com.
- ^Ritchie, Dennis M. (March 1993). 'The Development of the C Language'. ACM SIGPLAN Notices. 28 (3): 5. doi:10.1145/155360.155580.
> FSMs. They cannot be implemented more naturally and efficiently with anything else.It depends a bit on what your motivation for using a Lua interpreter embedded into the .NET environment is. Instinctively, it wouldn't be my choice for things that need to be exceedingly fast. But let's say you do want to implement an FSM in this context where performance isn't critical. States and state transitions could very well be expressed by functions instead of code blocks, and there are implementations in the wild that do this.
C# Lua Interpreter Download
> Huge switch can be costly.
Goto doesn't really help alleviate the cost of chaining if statements either (which is what Lua forces you to do in the absence of a switch statement). But if it's critical, you could build dispatch tables with pure Lua tables to mirror a static switch statement.
If you're concerned about processor cache though, I don't think interpreted Lua on top of .NET is the right choice to base an FSM of to begin with.
> DSLs may need goto to represent features not directly available in the host language structural elements
True, but it depends a bit on the paradigms chosen to implement the necessary transformations. It's correct though that goto statements provide you with a very intuitive way to shape control flow in the absence of corresponding features in the host language. If that facility is missing, you lose the option of translating your canonical C-based algorithms faithfully into Lua code. Whether that's a bad thing is a matter of debate.
C Compiler
For me it comes down to the consideration of using the right tool for the appropriate job, which was the context of my original comment. If you're building a language that needs to satisfy every possible use case efficiently (both in respect to execution and programmer effort), you're just screwed. This is especially true for interpreted languages, and extraordinarily so if they're supposed to host higher level interpreted languages on top.