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Programming in Lua: 7.2 This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences. The fourth edition targets Lua 5.3 and is available at and other bookstores.

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By buying the book, you also help to. 7.2 – The Semantics of the Generic for One drawback of those previous iterators is that we need to create a new closure for each new loop. For most situations, this is not a real problem.

For instance, in the allwords iterator, the cost of creating one single closure is negligible compared to the cost of reading a whole file. However, in a few situations this overhead can be undesirable. In such cases, we can use the generic for itself to keep the iteration state. We saw that the generic for keeps the iterator function internally, during the loop. Actually, it keeps three values: The iterator function, an invariant state, and a control variable.

Let us see the details now. The syntax for the generic for is as follows: for in do end where is a list of one or more variable names, separated by commas, and is a list of one or more expressions, also separated by commas.

More often than not, the expression list has only one element, a call to an iterator factory. For instance, in the code for k, v in pairs(t) do print(k, v) end the list of variables is k, v; the list of expressions has the single element pairs(t). Often the list of variables has only one variable too, as in for line in io.lines() do io.write(line, ' n') end We call the first variable in the list the control variable.

Its value is never nil during the loop, because when it becomes nil the loop ends. The first thing the for does is to evaluate the expressions after the in. These expressions should result in the three values kept by the for: the iterator function, the invariant state, and the initial value for the control variable. Like in a multiple assignment, only the last (or the only) element of the list can result in more than one value; and the number of values is adjusted to three, extra values being discarded or nils added as needed. Gruzinskij filjm gocha i hvicha. (When we use simple iterators, the factory returns only the iterator function, so the invariant state and the control variable get nil.) After this initialization step, the for calls the iterator function with two arguments: the invariant state and the control variable. (Notice that, for the for structure, the invariant state has no meaning at all. It only gets this value from the initialization step and passes it when it calls the iterator function.) Then the for assigns the values returned by the iterator function to variables declared by its variable list.

If the first value returned (the one assigned to the control variable) is nil, the loop terminates. Otherwise, the for executes its body and calls the iteration function again, repeating the process. More precisely, a construction like for var_1., var_n in explist do block end is equivalent to the following code: do local _f, _s, _var = explist while true do local var_1., var_n = _f(_s, _var) _var = var_1 if _var == nil then break end block end end So, if our iterator function is f, the invariant state is s, and the initial value for the control variable is a 0, the control variable will loop over the values a 1 = f(s, a 0), a 2 = f(s, a 1), and so on, until a i is nil. If the for has other variables, they simply get the extra values returned by each call to f. Copyright © 2003–2004 Roberto Ierusalimschy. All rights reserved.

The problem is that the stock Lua distribution (nearly) only includes features that are specified in standard C. Standard C makes no presumptions about there actually being a file system of any specific sort out there (or even an operating system, for that matter), so the os and io modules don't provide access information not available from the standard C library.

If you were attempting to code in pure standard C, you would have the same issue. There is a chance that you can learn whether the folder exists implicitly from an attempt to use it. If you expect it to exist and be writable to you, then create a temporary file there and if the that succeeds, the folder exists. If it fails, you might not be able to distinguish a non-existent folder from insufficient permissions, of course.

By far the lightest-weight answer to getting a specific answer would be a thin binding to just those OS-specific function calls that provide the information you need. If you can accept the module, then you can like do the binding in otherwise pure Lua. Simpler, but slightly heavier, is to accept Lua File System. It provides a portable module that supports most things one might want to learn about files and the file system.