save strings in lua table

Question

Does someone know a solution to save the key and the values to an table? My idea does not work because the length of the table is 0 and it should be 3.

local newstr = "3 = Hello, 67 = Hi, 2 = Bye"

a = {}
for k,v in newstr:gmatch "(%d+)%s*=%s*(%a+)" do 
    --print(k,v)
    a[k] = v
end

print(#a)

Piglet · Accepted Answer · 2021-10-26 13:57:28Z

4

The output is correct.

run for k,v in pairs(a) do print(k,v) end to check the contents of your table.

The problem is the length operator which by default cannot be used to get the number of elements of any table but a sequence.

Please refer to the Lua manual: https://www.lua.org/manual/5.4/manual.html#3.4.7

When t is a sequence, #t returns its only border, which corresponds to the intuitive notion of the length of the sequence. When t is not a sequence, #t can return any of its borders. (The exact one depends on details of the internal representation of the table, which in turn can depend on how the table was populated and the memory addresses of its non-numeric keys.)

Only use the length operator if you know t is a sequence. That's a Lua table with integer indexes 1,..n without any gap.

You don't have a sequence as you're using non-numeric keys only. That's why #a is 0

The only safe way to get the number of elements of any table is to count them.

local count = 0
for i,v in pairs(a) do
  count = count + 1
end

edited Oct 26, 2021 at 13:57

answered Oct 26, 2021 at 11:24

Piglet

29.3k3 gold badges31 silver badges51 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Glupschi Over a year ago

yeah i see. thanks

koyaanisqatsi · Accepted Answer · 2021-10-26 13:16:44Z

1

You can put @Piglet' code in the metatable of a as method __len that is used for table key counting with length operator #.

local newstr = "3 = Hello, 67 = Hi, 2 = Bye"

local a = setmetatable({},{__len = function(tab)
local count = 0
for i, v in pairs(tab) do
 count = count + 1
end
return count
end})

for k,v in newstr:gmatch "(%d+)%s*=%s*(%a+)" do 
    --print(k,v)
    a[k] = v
end

print(#a) -- puts out: 3

The output of #a with method __len even is correct if the table holds only a sequence.

You can check this online in the Lua Sandbox...
...with copy and paste.
Like i do.

edited Oct 26, 2021 at 13:16

answered Oct 26, 2021 at 12:11

koyaanisqatsi

2,9052 gold badges11 silver badges18 bronze badges

2 Comments

Piglet Over a year ago

imho it is not a good idea to suggest something like this as a general solution. you don't want to make it a beginner's habit to give every table a __len metamethod. I'm not even sure if you should confront any beginner who doesn't know the default behaviour of Lua's operators with metatables. for the few cases you need the number of table fields you can just call a function. I'd probably add it to the table library. also it is safe to assume that the OP already knows how to copy&paste and run Lua code as he complains about getting unexpected results. ;-) no offense

koyaanisqatsi Over a year ago

You put such a function in the table library. I put the table library functions one by one in the __index method of a table designed as table and after that i can change them without affecting the original table library. - It is so easy:

newtable=setmetatable({},{__index={}}) for k,v in pairs(table) do getmetatable(newtable).__index[k]=v end newtable:insert('Hello World') print(newtable:concat())

Collectives™ on Stack Overflow

save strings in lua table

2 Answers 2

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related