Lua : remove duplicate elements

Question

i am having a table in lua

test = {1,2,4,2,3,4,2,3,4,"A", "B", "A"}

I want to remove all duplicate elements in table.
Output should be

test = {1,2,4,3,"A","B"}

EDIT:

My try :

> items = {1,2,4,2,3,4,2,3,4,"A", "B", "A"}
> flags = {}
> for i=1,table.getn(items)  do
if not flags[items[i]] then
      io.write(' ' .. items[i])
      flags[items[i]] = true
   end
>> end
 1 2 4 3 A B>

It is working fine now. Thanks for answers and comments.

note that none of the answers thus far posted handle the case of duplicate elements which are themselves also tables, as that also requires a satisfactory answer to [how to compare tables](https://stackoverflow.com/questions/20325332/how-to-check-if-two-tablesobjects-have-the-same-value-in-lua) — TamaMcGlinn, Nov 11 '22 at 10:52

vogomatix · Accepted Answer · 2013-11-19T09:17:10.783

27

Similar to example given by @Dimitry but only one loop

local test = {1,2,4,2,3,4,2,3,4,"A", "B", "A"}
local hash = {}
local res = {}

for _,v in ipairs(test) do
   if (not hash[v]) then
       res[#res+1] = v -- you could print here instead of saving to result table if you wanted
       hash[v] = true
   end

end

edited Nov 19 '13 at 09:17

answered Nov 19 '13 at 09:06

vogomatix

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4

nice shortening. The brackets of the condition are not really necessary, though :D – Dmitry Ledentsov Nov 19 '13 at 09:09
2

@dmitry thanks - the brackets are my normal C, Perl and PHP showing through :-P – vogomatix Nov 19 '13 at 09:11

score 4 · Answer 2 · answered Nov 19 '13 at 08:49

4

local test = {1,2,4,2,3,4,2,3,4,"A", "B", "A"}

-- make unique keys
local hash = {}
for _,v in ipairs(test) do
    hash[v] = true
end

-- transform keys back into values
local res = {}
for k,_ in pairs(hash) do
    res[#res+1] = k
end

-- 1 2 3 4 A B
for _,v in ipairs(res) do
    print(v)
end

test = res

... a simple, straightforward solution just from the head, but I think, the hint is given in the PiL book

what have you tried to solve the problem?

answered Nov 19 '13 at 08:49

Dmitry Ledentsov

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please see i edited my question. thanks for your answer . will you please explain why you _ ?? – Prashant Gaur Nov 19 '13 at 08:59
I've edited the title so that it's readable as plain text. The underscores don't convey extra information. Your solution is in principle the same as mine now, just that you're not writing back to the `test` table handle. Please consider accepting the answer if you agree, it solves your problem – Dmitry Ledentsov Nov 19 '13 at 09:03
1

sure i will accept i am just doing same research to solve this problem with very less code – Prashant Gaur Nov 19 '13 at 09:06
res[#res+1] = k what # is doing here ? please explain ? – Prashant Gaur Nov 19 '13 at 09:07
1

Its finding the last index in the table for Lua 5.1 on – vogomatix Nov 19 '13 at 09:09

score 3 · Answer 3 · edited Dec 16 '18 at 18:56

3

local xx = {'a','b','c','d','a','d','f','g','a'}
table.sort(xx)
local result = {}

for key,value in ipairs(xx) do
  if value ~=xx[key+1] then
    table.insert(result,value)
  end
end

for key,value in ipairs(result) do
  print(key,value)   
end

edited Dec 16 '18 at 18:56

nonchip

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answered Jun 28 '18 at 22:47

Ilhami Mounir Aziz Girgis

157
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1

Usually, even if this code answers the question, it is recommended to briefly explain what it does and why it is the right answer. – Edson Menegatti Dec 16 '18 at 21:37

Prashant Gaur · Answer 4 · 2023-01-23T09:09:49.893

0

items = {1,2,4,2,3,4,2,3,4,"A", "B", "A"}
flags = {}
for i=1,table.getn(items)  do
if not flags[items[i]] then
      io.write(' ' .. items[i])
      flags[items[i]] = true
end

Output -

1 2 4 3 A B>

edited Jan 23 '23 at 09:09

answered Nov 19 '13 at 13:19

Prashant Gaur

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Lua : remove duplicate elements

EDIT:

4 Answers4

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