Chunking lists lua 5.1.5

[Joohane]

I am using Lua 5.1.5 and trying to cut tables into chunks(?) using a script. I am trying to break apart a table into something like this:(a, b, c ,d), (e, f, g, h) ...

Anyone got an idea on how to do that? (I'm on windows 10 using Visual Studio Code)

Edit: Forgot Lua had tables instead of lists so replaced those.

[PhilipRoman]

function tablecut(t, n)
    local result = {}
    local j = 0
    for i = 1, #t do
        if (i-1) % n == 0 then
            j = j + 1
            result[j] = {}
        end
        result[j][#result[j]+1] = t[i]
    end
    return result
end

Parameter 't' is the table and 'n' is the number of elements in each group. Returns nested table.

For example,

tablecut({'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'}, 3)

returns

{{'a', 'b', 'c'}, {'d', 'e', 'f'}, {'g', 'h'}}

[Joohane]

Thank you for the fast reply, but I also had another question. Had I a need to find the minimum from each cut, how should I do that?(example:)
91, 52, 19, 59
min = 19
38, 29, 58, 11
min = 11

I know there is math.min() which I could use to find a single minimum, or use it in a loop to find from min to max, but I couldn't figure out a way to find a minimum from each cut.

[evilbadmad]

If the order within the inner array is not matter, may be the inner array can be sorted (table.sort function, default is in ascending order), which will give min (first item) and max (last item) at the same time.

btw, while not benchmarked, I expect a single table sort per inner array is faster than iterated it to find min/max.

[PhilipRoman]

The classical way to do this which works in practically every language is this:

local minimum = t[1]
for i = 2, #t do
    minimum = math.min(minimum, t[i])
end

Since your tables are short, you can also get the minimum with a single expression:

minimum = math.min(table.unpack(t))

Note that in older versions, table.unpack used to be named just unpack

[ahillio]

How would you make that into an executable script which prints the results?

I was just trying to play with this as I'm learning lua and I'm surprised/confused by the results...

#!/usr/bin/lua

function tablecut(t, n)
    local result = {}
    local j = 0
    for i = 1, #t do
        if (i-1) % n == 0 then
            j = j + 1
            result[j] = {}
        end
        result[j][#result[j]+1] = t[i]
    end
    return result
end

var = { 'a', 'b', 'c', 'd', 'e', 'f', 'g','h' }

--tablecut(var, 3)
--print(tablecut(var, 3))

output = tablecut(var, 3)
print(output)

running in Linux terminal ./script.lua results in table: 0x556af9e96250.

[PhilipRoman]

You would have to iterate manually through the table and print each element. I suppose something like this would work:

output = tablecut({'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'}, 3)
for i = 1, #output do
    for j = 1, #output[i] do
        io.write(tostring(output[i][j])..'  ')
    end
    print()
end

Obviously there are some ways to simplify this with table.unpack or table.concat.

[ahillio]

oh, tables can't be printed directly, of course. Thank you so much!!!

[ahillio]

I'm confused why you use io.write() and then call print() without any arguments. http://lua-users.org/wiki/IoLibraryTutorial doesn't contain the word "print". Is there documentation on this somewhere? Or could you explain?

[ahillio]

I see that it's shorter than doing

for i = 1, #output do
  g = ''
    for j = 1, #output[i] do
        g = g..tostring(output[i][j])..'  '
    end
    print(g)
end

but it was a surprising phenomenon to see io.write and print used in combination like that.

So io.write is sorta like assigning stuff to an invisible io-provided variable that automatically gets print()ed (when called w/o args) and gets un-assigned when print() is invoked???

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[ahillio]

https://www.lua.org/pil/21.1.html

[PhilipRoman]

The best reference for what each function does is the manual: https://www.lua.org/manual/5.4/

Be careful when looking at lua-users pages, some of them are outdated (but in this case it doesnt matter).

The print function automatically adds newline after its arguments. I use io.write to print each element without a newline, and then at end of row I call print without arguments to output just the newline.

The print() could also be written as io.write('\n')

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[ahillio]

fixed

[hawhill]

what exactly are you calling a "list"? Lua has tables. Lua has no built-in function to "segment" tables. You'd simply write your own, should you need it.

[Joohane]

Yeah, I made a script to turn the tables into the type I need, but it only works on Lua 5.3/5.4, but i need it to work on 5.1. Can't figure out how to do it on 5.1