Lua Arrays Help

[FawKa]

I am new to Lua and trying to grasp arrays but sooooo confused.

Why do i get error with the follow code?

local aliens={
{posX, posY, speedX, speedY}
}
for i=1, 5 do
aliens[i].posX = i*100
aliens[i].posY = i*200
aliens[i].speedX = math.random(2,4)
aliens[i].speedY = math.random(-4,4)
end

The error i get is:
main.lua:5: attempt to index field '?' (a nil value)
stack traceback:
main.lua:5: in main chunk

[Banknorris]

Your definition and use of arrays should match.

In this line:

local aliens={
{posX, posY, speedX, speedY}
}

You say aliens has 4 members:
aliens.posX (which is the same as aliens["posX"])
aliens.posY (which is the same as aliens["posY"])
aliens.SpeedX (which is the same as aliens["speedX"])
aliens.SpeedY (which is the same as aliens["speedY"])

By doing so it will imply that aliens[i] will be nill (because it is not one of the four members. So if aliens[i] is nil you can't use aliens[i].posX. posX is a member of aliens (which is a table) but posX is not a member of aliens[i] because aliens[i] is just nil.
The correct would be:

local aliens = {}
for i=1,5 do
     aliens[i] = {}
     aliens[i].posX = i*100
     aliens[i].posY = i*200
     aliens[i].speedX = math.random(2,4)
     aliens[i].speedY = math.random(-4,4)
end

[Seerah]

Banknorris: No, he's saying that aliens has 1 member, a table containing those other values.


FawKa: When doing this

local aliens={
{posX, posY, speedX, speedY}
}

You are doing this:

local aliens = {
     [1] = {
          [1] = posX, 
          [2] = posY, 
          [3] = speedX, 
          [4] = speedY
     }
}

If you are actually using posX, posY, etc. and nothing has been assigned to those values yet (ie, they are nil) then you end up with this instead (an empty table):

local aliens = {
     [1] = {}
}

If I assume what you want to do correctly, however, then what Banknorris provided will do it for you.

However, I greatly encourage you to go over this if you're still having trouble: http://lua-users.org/wiki/TablesTutorial

[SDPhantom]

Originally Posted by FawKa I am new to Lua and trying to grasp arrays but sooooo confused. Why do i get error with the follow code? local aliens={ {posX, posY, speedX, speedY} } for i=1, 5 do aliens[i].posX = i*100 aliens[i].posY = i*200 aliens[i].speedX = math.random(2,4) aliens[i].speedY = math.random(-4,4) end The error i get is: main.lua:5: attempt to index field '?' (a nil value) stack traceback: main.lua:5: in main chunk

The problem is you make a table at index 1 of aliens, then loop through entries 1 thru 5. You don't have any tables created for indices 2 and up. In addition to this, the way you're creating the aliens table isn't doing what you think it is. The first pair of braces creates the main table that has only one entry, which is the nested table defined by the second pair of braces. The nested table takes the values stored in variables with the names you specified and stores them in consecutive numerical indices (1 thru 4). To facilitate using a loop to populate the aliens table, you need to split this process as noted previously.

Originally Posted by Banknorris local aliens = {} for i=1,5 do      aliens[i] = {}      aliens[i].posX = i*100      aliens[i].posY = i*200      aliens[i].speedX = math.random(2,4)      aliens[i].speedY = math.random(-4,4) end

However, this can be done much more efficiently if you used the table constructor correctly.

local aliens={};--  Main table
for i=1,5 do
    aliens[i]={--   Nested table
        posX=i*100,
        posY=i*200,
        speedX=math.random(2,4),
        speedY=math.random(-4,4)
    };
end

[Banknorris]

Originally Posted by SDPhantom However, this can be done much more efficiently if you used the table constructor correctly.

Funny that you mention that because coincidently I just read this http://www.lua.org/gems/sample.pdf and yes you are right it avoids an unnecessary re-hash.

Just noticed I misinterpreted his code. So my explanation is totally wrong.

[FawKa]

To all who have reply to my question..

Thank you very much...

Learned from it. But i need to learn more as i am really new to Lua...