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Lua pattern matching vs. regular expressions

I'm currently learning lua. regarding pattern-matching in lua I found the following sentence in the lua documentation on lua.org:

Nevertheless, pattern matching in Lua is a powerful tool and includes some features that are difficult to match with standard POSIX implementations.

As I'm familiar with posix regular expressions I would like to know if there are any common samples where lua pattern matching is "better" compared to regular expression -- or did I misinterpret the sentence? and if there are any common examples: why is any of pattern-matching vs. regular expressions better suited?

How do I get the number of keys in a hash table in Lua?

myTable = {}
myTable["foo"] = 12
myTable["bar"] = "blah"
print(#myTable) -- this prints 0

Do I actually have to iterate through the items in the table to get the number of keys?

numItems = 0
for k,v in pairs(myTable) do
    numItems = numItems + 1
end
print(numItems) -- this prints 2

Lua string to int

How can I convert a string to an integer in Lua? Thank you.

I have a string like this:

a = "10"

I would like it to be converted to 10, the number.

Why use Mongrel2?

I'm confused what purpose Mongrel2 serves/provides that nginx doesn't already do.

(Yes, I've read the manual but I must to be too much of a noob to understand how it's fundamentally different than nginx)

My current web application stack is:
- nginx: webserver
- Lua: programming language
- FastCGI + LuaJIT: to connect nginx to Lua
- Postgres: database

Why does Lua have no "continue" statement?

I have been dealing a lot with Lua in the past few months, and I really like most of the features but I'm still missing something among those:

• Why is there no continue?
• What workarounds are there for it?

Lua - Current time in milliseconds

another Lua question: Is there a common way to get the current time in or with milliseconds? There is os.time(), but it only provides full seconds.

how can I embed lua in java?

is LuaJava a must for this? or can I embed lua into java without it?

subtle differences between JavaScript and Lua

I simply love JavaScript. It's so elegant (imagine the quiet sound of lovestruck fanboy sighing in the background).

So, recently I have played with Lua via the löve2d framework (nice!) - and I think Lua is also great. They way I see it, those two languages are very similar.

There are obvious differences, like

• syntax
• problem domain
• libraries
• types (a bit)

but which are the more subtle ones? Is there anything a JavaScript coder would take for granted that works in Lua just slightly different? Are there any pitfalls that may not be obvious to the experienced coder of one language trying the other one?

For example: in Lua, arrays and hashes are not separate (there are only tables) - in JavaScript, they are numerical Arrays and hashed Objects. Well, this is one of the more obvious differences.

But are there differences in variable scope, immutability or something like this?

What multithreading package for Lua "just works" as shipped?

Coding in Lua, I have a triply nested loop that goes through 6000 iterations. All 6000 iterations are independent and can easily be parallelized. What threads package for Lua compiles out of the box and gets decent parallel speedups on four or more cores?

Here's what I know so far:

• luaproc comes from the core Lua team, but the software bundle on luaforge is old, and the mailing list has reports of it segfaulting. Also, it's not obvious to me how to use the scalar message-passing model to get results ultimately into a parent thread.

• Lua Lanes makes interesting claims but seems to be a heavyweight, complex solution. Many messages on the mailing list report trouble getting Lua Lanes to build or work for them. I myself have had trouble getting the underlying "Lua rocks" distribution mechanism to work for me.

• LuaThread requires explicit locking and requires that communication between threads be mediated by global variables that are protected by locks. I could imagine worse, but I'd be happier with a higher level of abstraction.

• Concurrent Lua provides an attractive message-passing model similar to Erlang, but it says that processes do not share memory. It is not clear whether spawn actually works with any Lua function or whether there are restrictions.

• Russ Cox proposed an occasional threading model that works only for C threads. Not useful for me.

I will upvote all answers that report on actual experience with these or any other multithreading package, or any answer that provides new information.

For reference, here is the loop I would like to parallelize:

for tid, tests in pairs(tests) do
  local results = { }
  matrix[tid] = results
  for i, test in pairs(tests) do
    if test.valid then
      results[i] = { }
      local results = results[i]
      for sid, bin in pairs(binaries) do
        local outcome, witness = run_test(test, bin)
        results[sid] = { outcome = outcome, witness = witness }
      end
    end
  end
end

The run_test function is passed in as an argument, so a package can be useful to me only if it can run arbitrary functions in parallel. My goal is enough parallelism to get 100% CPU utilization on 6 to 8 cores.

Solving random crashes

I am getting random crashes on my C++ application, it may not crash for a month, and then crash 10 times in a hour, and sometimes it may crash on launch, while sometimes it may crash after several hours of operation (or not crash at all).

I use GCC on GNU/Linux and MingW on Windows, thus I can't use the Visual Studio JIT Debug...

I have no idea on how to proceed, looking randomly on the code would not work, the code is HUGE (and good part was not my work, also it has some good amount of legacy stuff on it), and I also don't have a clue on how to reproduce the crash.

EDIT: Lots of people mentioned that... how I make a core dump, minidump or whateverdump? This is the first time I need postmortem debugging.

EDIT2: Actually, DrMingw captured a call stack, no memory info... Unfortunately, the call stack don't helped me much, because near the end suddenly it go into some library (or something) that I don't have debug info, resulting only into some hexadecimal numbers... So I still need some decent dump that give more information (specially about what was in the memory... specifically, what was in the place that gave the "access violation" error)

Also, my application use Lua and Luabind, maybe the error is being caused by a .lua script, but I have no idea on how to debug that.

Lua as a general-purpose scripting language?

When I see Lua, the only thing I ever read is "great for embedding", "fast", "lightweight" and more often than anything else: "World of Warcraft" or in short "WoW".

Why is it limited to embedding the whole thing into another application? Why not write general-purpose scripts like you do with Python or Perl?

Lua seems to be doing great in aspects like speed and memory-usage (The fastest scripting language afaik) so why is it that I never see Lua being used as a "Desktop scripting-language" to automate tasks? For example:

• Renaming a bunch of files
• Download some files from the web
• Webscraping

Is it the lack of the standard library?

Lua - merge tables?

I need to merge two tables, with the contents of the second overwriting contents in the first if a given item is in both. I looked but the standard libraries don't seem to offer this. Where can i get such a function?

What web server to use for Lua web development [closed]

What web server (and why) should I use for Lua web development?

Is LuaJIT really faster than every other JIT-ed dynamic languages? [closed]

According to the computer language benchmark game, the LuaJIT implementation seems to beat every other JIT-ed dynamic language (V8, Tracemonkey, PLT Scheme, Erlang HIPE) by an order of magnitude.

I know that these benchmarks are not representative (as they say: "Which programming language implementations have the fastest benchmark programs?"), but this is still really impressive.

In practice, is it really the case? Someone have tested that Lua implementation?

Alternatives to Lua as an embedded language?

I am working on an embedded system running Linux on a DSP. Now we want to make some parts of it scriptable and we are looking for a nice embeddable scripting language. These scripts should integrate nicely with our existing C++ code base, be small and fast.

I understand that Lua is the industry choice for problems like this. We will probably go with Lua because it is tried-and-true and proven to be stable and so on. However, as a programming language it has some rather quirky corners.

So, what alternatives are out there for embeddable languages?

EDIT:

This is about a year later.

We actually used Lua on our embedded system and it performs marvelously well. Over time, we added more and more scripting support to more and more parts of the project and that really helped to bring it along.

Performance is outstanding, really. Even rather complex operations that involve searching through long arrays or fancy string operations perform surprisingly well. We basically never ran into Lua related performance problems at all.

Interfacing with C functions is very straightforward and works really well. This allowed us to grow the scripting system painlessly.

Finally, we were astounded at how flexible Lua proved to be. Our Lua interpreter has to run on a system with a nonstandard memory allocator and without support for the double data type. There are two well-documented places in one header file we had to modify to make Lua work on that system. It is really well suited for embedding!