I have stumbled upon this line of code and I am not sure what the [ ? ] part represents (my guess is it's a sort of a wildcard but I searched it for a while and couldn't find anything):
['?'] = function() return is_canadian and "eh" or "" end
I understand that RHS is a functional ternary operator. I am curious about the LHS and what it actually is.
Edit: reference (2nd example):
http://lua-users.org/wiki/SwitchStatement
Actually, it is quite simple.
local t = {
a = "aah",
b = "bee",
c = "see",
It maps each letter to a sound pronunciation. Here, a need to be pronounced aah and b need to be pronounced bee and so on. Some letters have a different pronunciation if in american english or canadian english. So not every letter can be mapped to a single sound.
z = function() return is_canadian and "zed" or "zee" end,
['?'] = function() return is_canadian and "eh" or "" end
In the mapping, the letter z and the letter ? have a different prononciation in american english or canadian english. When the program will try to get the prononciation of '?', it will calls a function to check whether the user want to use canadian english or another english and the function will returns either zed or zee.
Finally, the 2 following notations have the same meaning:
local t1 = {
a = "aah",
b = "bee",
["?"] = "bee"
}
local t2 = {
["a"] = "aah",
["b"] = "bee",
["?"] = "bee"
}
If you look closely at the code linked in the question, you'll see that this line is part of a table constructor (the part inside {}). It is not a full statement on its own. As mentioned in the comments, it would be a syntax error outside of a table constructor. ['?'] is simply a string key.
The other posts alreay explained what that code does, so let me explain why it needs to be written that way.
['?'] = function() return is_canadian and "eh" or "" end is embedded in {}
It is part of a table constructor and assigns a function value to the string key '?'
local tbl = {a = 1} is syntactic sugar for local tbl = {['a'] = 1} or
local tbl = {}
tbl['a'] = 1
String keys that allow that convenient syntax must follow Lua's lexical conventions and hence may only contain letters, digits and underscore. They must not start with a digit.
So local a = {? = 1} is not possible. It will cause a syntax error unexpected symbol near '?' Therefor you have to explicitly provide a string value in square brackets as in local a = {['?'] = 1}
they gave each table element its own line
local a = {
1,
2,
3
}
This greatly improves readability for long table elements or very long tables and allows you maintain a maximum line length.
You'll agree that
local tbl = {
z = function() return is_canadian and "zed" or "zee" end,
['?'] = function() return is_canadian and "eh" or "" end
}
looks a lot cleaner than
local tbl = {z = function() return is_canadian and "zed" or "zee" end,['?'] = function() return is_canadian and "eh" or "" end}
Related
I am trying following simple code to get a decimal value from user:
require( "iuplua" )
t1 = iup.text{expand = "YES", padding = "10x10", alignment="ARIGHT", size="40x"}
btn = iup.button {title = "Print:", padding = "10x10", alignment="ACENTER", size="40x"}
qbtn = iup.button{title="Quit", expand = "YES", padding = "10x10", alignment="ACENTER", size="40x"}
function btn:action()
strval1 = string.match (t1.value, "%d+")
print (strval1)
num = tonumber(strval1)
print (num)
end
function qbtn:action()
os.exit()
end
dlg = iup.dialog {
iup.vbox{
iup.hbox{
iup.label{title="Decimal:", padding = "10x10", alignment="ALEFT", size="40x"},
t1 },
iup.hbox{
btn,
qbtn }}}
dlg:show()
iup.MainLoop()
However, it prints out only an integer number (part before decimal- even 25.9999 prints as 25).
How can I get float or decimal value entered by user? Thanks for your help.
The problem with the code is that the pattern "%d+" in btn:action only gets the first run of digits.
You can change the pattern to handle decimal points, but it is hard to write a pattern that works in all cases, including optional sign and decimal point and scientific format.
It is best to let tonumber do its work: it will return nil if the string cannot be converted to a number.
Another option is to use the MASK attribute of the IupText to let the user only be able to enter numbers.
I have a program that grabs a list of peripheral types, matches them to see if they're a valid type, and then executes type-specific code if they are valid.
However, some of the types can share parts of their name, with the only difference being their tier, I want to match these to the base type listed in a table of valid peripherals, but I can't figure out how to use a pattern to match them without the pattern returning nil for everything that doesn't match.
Here is the code to demonstrate my problem:
connectedPeripherals = {
[1] = "tile_thermalexpansion_cell_basic_name",
[2] = "modem",
[3] = "BigReactors-Turbine",
[4] = "tile_thermalexpansion_cell_resonant_name",
[5] = "monitor",
[6] = "tile_thermalexpansion_cell_hardened_name",
[7] = "tile_thermalexpansion_cell_reinforced_name",
[8] = "tile_blockcapacitorbank_name"
}
validPeripherals = {
["tile_thermalexpansion_cell"]=true,
["tile_blockcapacitorbank_name"]=true,
["monitor"]=true,
["BigReactors-Turbine"]=true,
["BigReactors-Reactor"]=true
}
for i = 1, #connectedPeripherals do
local periFunctions = {
["tile_thermalexpansion_cell"] = function()
--content
end,
["tile_blockcapacitorbank_name"] = function()
--content
end,
["monitor"] = function()
--content
end,
["BigReactors-Turbine"] = function()
--content
end,
["BigReactors-Reactor"] = function()
--content
end
}
if validPeripherals[connectedPeripherals[i]] then periFunctions[connectedPeripherals[i]]() end
end
If I try to run it like that, all of the thermalexpansioncells aren't recognized as valid peripherals, and if I add a pattern matching statement, it works for the thermalexpansioncells, but returns nil for everything else and causes an exception.
How do I do a match statement that only returns a shortened string for things that match and returns the original string for things that don't?
Is this possible?
If the short version doesn't contain any of the special characters from Lua patterns you can use the following:
long = "tile_thermalexpansion_cell_basic_name"
result = long:match("tile_thermalexpansion_cell") or long
print(result) -- prints the shorter version
result = long:match("foo") or long
print(result) -- prints the long version
Based on the comment, you can also use string.find to see the types match your peripheral names:
for i,v in ipairs(connectedPeripherals) do
local Valid = CheckValidity(v)
if Valid then Valid() end
end
where, CheckValidity will return the key from validPeripherals:
function CheckValidity( name )
for n, b in pairs(validPeripherals) do
if name:find( n ) then return n end
end
return false
end
I've googled and I'm just not getting it. Seems like such a simple function, but of course Lua doesn't have it.
In Python I would do
string = "cat,dog"
one, two = string.split(",")
and then I would have two variables, one = cat. two = dog
How do I do this in Lua!?
Try this
str = 'cat,dog'
for word in string.gmatch(str, '([^,]+)') do
print(word)
end
'[^,]' means "everything but the comma, the + sign means "one or more characters". The parenthesis create a capture (not really needed in this case).
If you can use libraries, the answer is (as often in Lua) to use Penlight.
If Penlight is too heavy for you and you just want to split a string with a single comma like in your example, you can do something like this:
string = "cat,dog"
one, two = string:match("([^,]+),([^,]+)")
Add this split function on the top of your page:
function string:split( inSplitPattern, outResults )
if not outResults then
outResults = { }
end
local theStart = 1
local theSplitStart, theSplitEnd = string.find( self, inSplitPattern, theStart )
while theSplitStart do
table.insert( outResults, string.sub( self, theStart, theSplitStart-1 ) )
theStart = theSplitEnd + 1
theSplitStart, theSplitEnd = string.find( self, inSplitPattern, theStart )
end
table.insert( outResults, string.sub( self, theStart ) )
return outResults
end
Then do as follows:
local myString = "Flintstone, Fred, 101 Rockledge, Bedrock, 98775, 555-555-1212"
local myTable = myString:split(", ")
for i = 1, #myTable do
print( myTable[i] ) -- This will give your needed output
end
For more information, visit : Tutorial: Lua String Magic
Keep Coding...............:)
-- like C strtok, splits on one more delimiter characters (finds every string not containing any of the delimiters)
function split(source, delimiters)
local elements = {}
local pattern = '([^'..delimiters..']+)'
string.gsub(source, pattern, function(value) elements[#elements + 1] = value; end);
return elements
end
-- example: var elements = split("bye# bye, miss$ american# pie", ",#$# ")
-- returns "bye" "bye" "miss" "american" "pie"
To also handle optional white space you can do:
str = "cat,dog,mouse, horse"
for word in str:gmatch('[^,%s]+') do
print(word)
end
Output will be:
cat
dog
mouse
horse
This is how I do that on mediawiki:
str = "cat,dog"
local result = mw.text.split(str,"%s*,%s*")
-- result[0] will give "cat", result[1] will give "dog"
actually, if you don't care spaces, you can use:
str = "cat,dog"
local result = mw.text.split(str,",")
-- result[0] will give "cat", result[1] will give "dog"
The API used here is implemented in Scribunto MediaWiki extension. Here is the split() method reference documentation and here is the source code for that. It relies on a lot of other capabilities in Scribunto's Lua common libraries, so it will only work for you if you are actually using MediaWiki or plan to import most of the Scribunto common library.
Functions like string.split() are largely unnecessary in Lua since you can
express string operations in LPEG.
If you still need a dedicated function a convenient approach is
to define a splitter factory (mk_splitter() in below snippet)
from which you can then derive custom splitters.
local lpeg = require "lpeg"
local lpegmatch = lpeg.match
local P, C = lpeg.P, lpeg.C
local mk_splitter = function (pat)
if not pat then
return
end
pat = P (pat)
local nopat = 1 - pat
local splitter = (pat + C (nopat^1))^0
return function (str)
return lpegmatch (splitter, str)
end
end
The advantage of using LPEG is that the function accepts
both valid Lua strings and patterns as argument.
Here is how you would use it to create a function that
splits strings at the , character:
commasplitter = mk_splitter ","
print (commasplitter [[foo, bar, baz, xyzzy,]])
print (commasplitter [[a,b,c,d,e,f,g,h]])
Hi I've got this function in JavaScript:
function blur(data) {
var trimdata = trim(data);
var dataSplit = trimdata.split(" ");
var lastWord = dataSplit.pop();
var toBlur = dataSplit.join(" ");
}
What this does is it take's a string such as "Hello my name is bob" and will return
toBlur = "Hello my name is" and lastWord = "bob"
Is there a way i can re-write this in Lua?
You could use Lua's pattern matching facilities:
function blur(data) do
return string.match(data, "^(.*)[ ][^ ]*$")
end
How does the pattern work?
^ # start matching at the beginning of the string
( # open a capturing group ... what is matched inside will be returned
.* # as many arbitrary characters as possible
) # end of capturing group
[ ] # a single literal space (you could omit the square brackets, but I think
# they increase readability
[^ ] # match anything BUT literal spaces... as many as possible
$ # marks the end of the input string
So [ ][^ ]*$ has to match the last word and the preceding space. Therefore, (.*) will return everything in front of it.
For a more direct translation of your JavaScript, first note that there is no split function in Lua. There is table.concat though, which works like join. Since you have to do the splitting manually, you'll probably use a pattern again:
function blur(data) do
local words = {}
for m in string.gmatch("[^ ]+") do
words[#words+1] = m
end
words[#words] = nil -- pops the last word
return table.concat(words, " ")
end
gmatch does not give you a table right away, but an iterator over all matches instead. So you add them to your own temporary table, and call concat on that. words[#words+1] = ... is a Lua idiom to append an element to the end of an array.
I am starting to learn Lua from Programming in Lua (2nd edition)
I didn't understand the following in the book. Its very vaguely explained.
a.) w={x=0,y=0,label="console"}
b.) x={math.sin(0),math.sin(1),math.sin(2)}
c.) w[1]="another field"
d.) x.f=w
e.) print (w["x"])
f.) print (w[1])
g.) print x.f[1]
When I do print(w[1]) after a.), why doesn't it print x=0
What does c.) do?
What is the difference between e.) and print (w.x)?
What is the role of b.) and g.)?
You have to realize that this:
t = {3, 4, "eggplant"}
is the same as this:
t = {}
t[1] = 3
t[2] = 4
t[3] = "eggplant"
And that this:
t = {x = 0, y = 2}
is the same as this:
t = {}
t["x"] = 0
t["y"] = 2
Or this:
t = {}
t.x = 0
t.y = 2
In Lua, tables are not just lists, they are associative arrays.
When you print w[1], then what really matters is line c.) In fact, w[1] is not defined at all until line c.).
There is no difference between e.) and print (w.x).
b.) creates a new table named x which is separate from w.
d.) places a reference to w inside of x. (NOTE: It does not actually make a copy of w, just a reference. If you've ever worked with pointers, it's similar.)
g.) Can be broken up in two parts. First we get x.f which is just another way to refer to w because of line d.). Then we look up the first element of that table, which is "another field" because of line c.)
There's another way of creating keys in in-line table declarations.
x = {["1st key has spaces!"] = 1}
The advantage here is that you can have keys with spaces and any extended ASCII character.
In fact, a key can be literally anything, even an instanced object.
function Example()
--example function
end
x = {[Example] = "A function."}
Any variable or value or data can go into the square brackets to work as a key. The same goes with the value.
Practically, this can replace features like the in keyword in python, as you can index the table by values to check if they are there.
Getting a value at an undefined part of the table will not cause an error. It will just give you nil. The same goes for using undefined variables.
local w = {
--[1] = "another field"; -- will be set this value
--["1"] = nil; -- not save to this place, different with some other language
x = 0;
y = 0;
label = "console";
}
local x = {
math.sin(0);
math.sin(1);
math.sin(2);
}
w[1] = "another field" --
x.f = w
print (w["x"])
-- because x.f = w
-- x.f and w point one talbe address
-- so value of (x.f)[1] and w[1] and x.f[1] is equal
print (w[1])
print ((x.f)[1])
print (x.f[1])
-- print (x.f)[1] this not follows lua syntax
-- only a function's has one param and type of is a string
-- you can use print "xxxx"
-- so you print x.f[1] will occuur error
-- in table you can use any lua internal type 's value to be a key
-- just like
local t_key = {v=123}
local f_key = function () print("f123") end
local t = {}
t[t_key] = 1
t[f_key] = 2
-- then t' key actualy like use t_key/f_key 's handle
-- when you user t[{}] = 123,
-- value 123 related to this no name table {} 's handle