Select which array to write to based on variables given to a function, Lua

Question

I am trying to run this code:

function calcs.grps(Number,ion_color)
    grp .. ion_color .. Y[Number] = (ion_py_mm)
    grp .. ion_color .. Z[Number] = (ion_pz_mm)
end

in a Lua script, the arrays already exist (eg grp2Y,grp5Z etc) and I want to use this function to populate them based upon the two variables fed in. I keep getting the error ' '=' expected near '..' '. What am I doing wrong?

To flesh it out a bit:

I am 'flying' 120 ions in my simulation. This is actually 12 groups of 10 ions. The individual groups of 10 are distinguished by ion_color, which is an integer value from 1 to 12. The variable 'Number' just cycles through 1 to 10 each time before moving on to the next color. Once I have populated these arrays I want to get the standard deviation for each group.

Thank you!

Lua is a proper name, not an acronym. Don't use ALL CAPS for it. — Nicol Bolas
– Nicol Bolas, Commented Dec 10, 2015 at 17:55

Oleg V. Volkov · Accepted Answer · 2015-12-10 17:12:59Z

3

You can't "construct" name of variable, but you can construct an index. Use two levels of nested tables.

function calcs.grps(Number,ion_color)
    ion['grp' .. ion_color .. 'Y'][Number] = (ion_py_mm)
    ion['grp' .. ion_color .. 'Z'][Number] = (ion_pz_mm)
end

Well, actually you can, since all global variables are just entries in _G table, but don't do that since it is bad - it is unreadable, makes stuff spill to other functions you didn't intend to, etc.

answered Dec 10, 2015 at 17:12

Oleg V. Volkov

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2 Comments

Nick Ramsbottom Over a year ago

ah that makes so much sense now. I'm quite new to it all so I'm picking it all up as I go. this also means I only need to make one empty array called 'ion', thank you!

Oleg V. Volkov Over a year ago

You'll also need to manually construct empty sub-tables on first level - there's no autovivification as in som other languages. You can use something like

ion['grp' .. ion_color .. 'Y'] = ion['grp' .. ion_color .. 'Y'] or {}; ion['grp' .. ion_color .. 'Y'][Number] = (ion_py_mm)

warspyking · Accepted Answer · 2015-12-10 20:21:22Z

1

The technical answer to your question is to simply index _G, _G is a table which holds all global variables:

function calcs.grps(Number,ion_color)
    _G['grp' .. ion_color .. Y'][Number] = (ion_py_mm)
    _G['grp' .. ion_color .. 'Z'][Number] = (ion_pz_mm)
end

But I think the better question, is why aren't you organizing it like this...

local ions = {
    Red = {
        {
            Y = 0, --Y property
            Z = 0 --Z property
        },
        --Continue your red ions
    },
    NewColor = {
            Y = 0, --Y property
            Z = 0 --Z property
        },
        --Continue this color's ions
    },
    --You get the idea
}

function calcs.grps(color, number)
    ions[color][number].Y = (ion_py_mm)
    ions[color][number].Z = (ion_pz_mm)
end

Then you would pass a color, and a number indicating which ion of this color

It looks a lot cleaner, IMO.

edited Dec 10, 2015 at 20:21

answered Dec 10, 2015 at 19:39

warspyking

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11 Comments

warspyking Over a year ago

@Nick You mean the period? It's syntactic sugar from keys that are strings. Just like instead of {['hi']=1,['hello']=2} you can just use {hi=1,hello=2} (I hope you make great use of this)

warspyking Over a year ago

@Nick Give me like 2 min to rewrite my answer a bit

warspyking Over a year ago

@Nick Let's nuke our comments, and look at the new code

warspyking Over a year ago

Exactly what would you want to iterate? The ions of a specific color? for i,v in pairs(ions.Red) do

warspyking Over a year ago

for i,v in ipairs(ions.Red) do print(v.Y, v.Z) end

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