I got a simple code like:
table = {}
print(table.."hello")
then got a error like the title. I know i need to use tostring(table) to fix it . Why table or other types can't convert to string to concatenate a String automatically except number type ?
print(table) is available But print(table.."hello") is not .
Does lua have some rules?
Thanks you.
Why table or other types can't convert to string to concatenate a String automatically except number type?
This is a deliberate choice made by the Lua language designers. Strings and numbers are coerced: Every operation that expects a string will also accept a number and tostring it; every operation that expects a number will also accept a string and tonumber it.
Coercion is an operation applied to strings. Numbers will be tostringed. Any other type won't. For other primitive types like bools and nils this is somewhat questionable, since they can be converted to string without issue. For tables it's reasonable though since they are a reference type.
Unlike other languages which make such decisions for you, Lua is highly metaprogrammable: You can simply override the decision! In this case, metatables are the solution, specifically the __concat metamethod which gets called if concatenation (..) is applied to two values of which one has the metamethod (and is neither a string or number):
table = setmetatable({}, {
__concat = function(left, right)
if type(left) == "string" then
return left .. tostring(right)
end
return tostring(left) .. tostring(right)
end
})
print(table .. "hello") -- hellotable: 0x563eb139bea0
You could even extend this to primitive types (nils, booleans), some other reference types (functions, coroutines) using debug.setmetatable, but I'd advise against this.
The declaration of table = {} destroying the table library
The datatype table is not a string or number so concat with .. must fail
Try this instead...
mytab = {}
table.insert(mytab, "hello")
print(table.concat(mytab))
For the table library functions look: https://www.lua.org/manual/5.4/manual.html#6.6
Related
I am confused by the following output:
local a = "string"
print(a.len) -- function: 0xc8a8f0
print(a.len(a)) -- 6
print(len(a))
--[[
/home/pi/test/wxlua/wxLua/ZeroBraneStudio/bin/linux/armhf/lua: /home/pi/Desktop/untitled.lua:4: attempt to call global 'len' (a nil value)
stack traceback:
/home/pi/Desktop/untitled.lua:4: in main chunk
[C]: ?
]]
What is the proper way to calculate a string length in Lua?
Thank you in advance,
You can use:
a = "string"
string.len(a)
Or:
a = "string"
a:len()
Or:
a = "string"
#a
EDIT: your original code is not idiomatic but is also working
> a = "string"
> a.len
function: 0000000065ba16e0
> a.len(a)
6
The string a is linked to a table (named metatable) containing all the methods, including len.
A method is just a function, taking the string as the first parameter.
function a.len (string) .... end
You can call this function, a.len("test") just like a normal function. Lua has a special syntax to make it easier to write. You can use this special syntax and write a:len(), it will be equivalent to a.len(a).
print(a.len) -- function: 0xc8a8f0
This prints a string representation of a.len which is a function value. All strings share a common metatable.
From Lua 5.4 Reference Manual: 6.4 String Manipulation:
The string library provides all its functions inside the table string.
It also sets a metatable for strings where the __index field points to
the string table. Therefore, you can use the string functions in
object-oriented style. For instance, string.byte(s,i) can be written
as s:byte(i).
So given that a is a string value, a.len actually refers to string.len
For the same reason
print(a.len(a))
is equivalent to print(string.len(a)) or print(a:len()). This time you called the function with argument a instead of printing its string representation so you print its return value which is the length of string a.
print(len(a))
on the other hand causes an error because you attempt to call a global nil value. len does not exist in your script. It has never been defined and is hence nil. Calling nil values doesn't make sense so Lua raises an error.
According to Lua 5.4 Reference Manual: 3.4.7 Length Operator
The length of a string is its number of bytes. (That is the usual
meaning of string length when each character is one byte.)
You can also call print(#a) to print a's length.
The length operator was introduced in Lua 5.1,
I am a newcomer to coding in general and I want to learn basic Lua scripting for my own hobby.
After working on a Lua script, the syntax all seems to be without error but I have come across one issue that I don't understand, I broke it down to this basic function:
{$lua}
ID1 = "10"
ID2 = "0"
if (ID1 ~= nil and ID1 == "10") then
writeInteger(ID2,"25")
end
print(ID2)
The issue is that the writeInteger does not seem to work at all, ID2 remains at value "0" while it should become "25".
Any help would be greatly appreciated.
This is not valid Lua, or at least it isn't valid vanilla (out-of-the-box) Lua, and since you have not specified anything else, there is not much we can do to help. I will assume writeInteger is a valid function (since your interpreter isn't complaining about a call to a nil value), but I don't think it works as you expect.
If you want to set the ID2 variable to 25, simply write:
ID2 = 25
Lua will convert the string type to an integer type automatically. You can run print(type(ID2)) to confirm this
If you are using cheat engine (as a quick google search suggests) the writeInteger function requires an address and a value.
function writeInteger(Address, Value): Boolean - Returns true on success.
I am not sure if ID2, a Lua variable, is a valid address, but I am sure that "25" is not an integer. You should remove the quotation marks to start, and since the function returns a boolean you can see if the function was successful by doing:
print(writeInteger(ID2, 25))
Lua uses pass by value for primitive types like numbers, booleans and strings. So if you pass a number to a function like writeInteger, it creates a local copy within the function. The function can alter this copy but it will have no effect on caller (of writeInteger in this case). I don't know how writeInteger is supposed to work but if you want to call a function which alters its argument you can create a table and pass that. Tables are still passed by value but the "value" of a table is its memory address (so in effect tables are passed by reference and you can alter the contents of a table by passing it to a function).
See more here
Function/variable scope (pass by value or reference?)
Hello im a newbie in Lua i just want to know if there is a way to get key and value of table not using pairs,ipairs,next or other iterators? thanks in advance.!
I don't believe this is possible, as you've phrased your question in such a way that implies that the key is unknown. The only way to check for a certain value and its corresponding key would be to iterate through the whole table.
However, maybe I misunderstood and you want to get a certain value from a key without iterating through the whole table.
Say you have a table named morse as follows:
morse = { a = ".-"; b = "-..."; } -- And so on
If you wanted to convert a single character to morse you could do as follows:
morse["a"] --Which will return the string ".-"
You can do the opposite, and define a table with all the morse values and their corresponding letters like below. Note the use of square brackets to 'escape' the characters.
morse = { [".-"] = "a"; ["-..."] = "b" }
morse[".-"] -- This will return "a"
Based on your comment, I think you are looking for a string substitution using a mapping table. I think you can use string.gsub here (if your teacher still insists that .gsub is an iterator; you can ask them politely that you are unaware of the method they claim and would be delighted to actually learn about the same):
local str = "sos sos sos"
local morse = {s = "...", o = "---"}
print( str:gsub("%a", morse) )
Luas string.format is pretty straight forward, if you know what to format.
However, I stuck at writing a function which takes a wildcard-string to format, and a variable number of arguments to put into that blank string.
Example:
str = " %5s %3s %6s %6s",
val = {"ttyS1", "232", "9600", "230400"}
Formatting that by hand is pretty easy:
string.format( str, val[1], val[2], val[3], val[4] )
Which is the same as:
string.format(" %5s %3s %6s %6s", "ttyS1, "232", "9600","230400")
But what if I wan't to have a fifth or sixth argument?
For example:
string.format(" %1s %2s %3s %4s %5s %6s %7s %", ... )
How can I implement a string.format with an variable number of arguments?
I want to avoid appending the values one by one because of performance issues.
The application runs on embedded MCUs.
Generate arbitrary number of repeats of whatever format you want with string.rep if format is the same for all arguments. Or fill table with all formats and use table.concat. Remember that you don't need to specify index of argument in format if you don't want to reorder them.
If you just need to concatenate strings together separated by space, use more suitable tool: table.concat(table_of_strings, ' ').
You can create a table using varargs:
function foo(fmt, ...)
local t = {...}
return t[6] -- might be nil
end
Ps, don't use # on the table if you expect the argument list might contain nil. Instead use select("#", ...).
I'm trying to understand what identifiers represent and what they don't represent.
As I understand it, an identifier is a name for a method, a constant, a variable, a class, a package/module. It covers a lot. But what can you not use it for?
Every language differs in terms of what entities/abstractions can or cannot be named and reused in that language.
In most languages, you can't use an identifier for infix arithmetic operations.
For example, plus is an identifier and you can make a function named plus. But write you can write a = b + c;, there's no way to define an operator named plus to make a = b plus c; work because the language grammar simply does not allow an identifier there.
An identifier allows you to assign a name to some data, so that you can reference it later. That is the limit of what identifiers do; you cannot "use" it for anything other than a reference to some data.
That said, there are a lot of implications that come from this, some subtle. For example, in most languages functions are, to some degree or another, considered to be data, and so a function name is an identifier. In languages where functions are values, but not "first-class" values, you can't use an identifier for a function in an place you could use an identifier for something else. In some languages, there will even be separate namespaces for functions and other data, and so what is textually the same identifier might refer to two different things, and they would be distinguished by the context in which they are used.
An example of what you usually (i.e., in most languages) cannot use an identifier for is as a reference to a language keyword. For example, this sort of thing generally can't be done:
let during = while;
during (true) { print("Hello, world."); }
You could say it's used for everything that you'll want to refer to multiple times, or maybe even once (but use it to clarify the referent's purpose).
What can/can't be named differs per language, it's often quite intuitive, IMHO.
An "Anonymous" entity is something which is not named, although referred to somehow.
#!/usr/bin/perl
$subroutine = sub { return "Anonymous subroutine returning this text"; }
In Perl-speak, this is anonymous - the subroutine is not named, but it is referred to by the reference variable $subroutine.
PS: In Perl, the subroutine would be named like this:
sub NAME_HERE {
# some code...
}
Say, in Java your cannot write something like:
Object myIf = if;
myIf (a == b) {
System.out.println("True!");
}
So, you cannot name some code statement, giving it an alias. While in REBOL it is perfectly possible:
myIf: if
myIf a = b [print "True!"]
What can and what can't be named depends on language, as you see.
as its name implifies, an identifier is used to identify something. so for everything that can be identified uniquely, you can use an identifier. But for example a literal (e.g. string literal) is not unique so you can't use an identifier for it. However you can create a variable and assign a string literal to it.
Making soup out them is rather foul.
In languages such as Lisp, an identifier exists in its own right as an symbol, whereas in languages which are not introspective identifiers don't exist in the runtime.
You write a literal identifier/symbol by putting a single quote in front of it:
[1]> 'a
A
You can create a variable and assign a symbol literal to it:
[2]> (setf a 'Hello)
HELLO
[3]> a
HELLO
[4]> (print a)
HELLO
HELLO
You can set two variables to the same symbol
[10]> (setf b a)
HELLO
[11]> b
HELLO
[12]> a
HELLO
[13]> (eq b a)
T
[14]> (eq b 'Hello)
T
Note that the values bound to b and a are the same, and the value is the literal symbol 'Hello
You can bind a function to the symbol
[15]> (defun hello () (print 'hello))
HELLO
and call it:
[16]> (hello)
HELLO
HELLO
In common lisp, the variable binding and the function binding are distinct
[19]> (setf hello 'goodbye)
GOODBYE
[20]> hello
GOODBYE
[21]> (hello)
HELLO
HELLO
but in Scheme or JavaScript the bindings are in the same namespace.
There are many other things you can do with identifiers, if they are reified as symbols. I suspect that someone more knowledgable than me in Lisp will be able to demonstrate any of the things that you 'can't do with identifiers' exist.
But even Lisp can not make identifier soup.
Sort of a left-field thought, but JSON has all those quotations in it to eliminate the danger of a JavaScript keyword messing up the parsing.