Count frequency of elements into an array in Lua

Question

I have a table in Lua:

p = {'sachin', 'sachin', 'dravid', 'Dhoni', 'yuvraj', 'kohli'}

I want to count frequency of each name in table .

test1 = {sachin=2, dravid=1, Dhoni=1, yuvraj=1, kohli=1}

I tried this program with lot of for loops .Please see my code

> function exec(ele,p)
count = 0
for k,v in pairs(p) do
if ele == p[k] then
count = count +1 
end
end
return count
end


> new_table = {}
> for k,v in pairs(p) do
new_table[v] = exec(v,p)
end
> 
> for k,v in pairs(new_table) do
print(k,v)
end
dhone   1
yuvraj  1
kohli   1
sachin  2
dravid  1

I want to do this more efficient way. How can I achieve this?

greatwolf · Accepted Answer · 2013-11-14 11:45:29Z

5

You can count the frequency like this:

function tally(t)
  local freq = {}
  for _, v in ipairs(t) do
    freq[v] = (freq[v] or 0) + 1
  end
  return freq
end

And here's another demo example.

answered Nov 14, 2013 at 11:45

greatwolf

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Comments

Yu Hao · Accepted Answer · 2013-11-15 07:30:13Z

1

Using metatable may be a little unnecessary for this simple case, just showing another option:

local mt = {__index = function() return 0 end}
local newtable = {}
setmetatable(newtable, mt)

for _, v in pairs(p) do
    newtable[v] = newtable[v] + 1
end

The metamethod __index above gives the table 0 as the default value.

edited Nov 15, 2013 at 7:30

answered Nov 14, 2013 at 12:19

Yu Hao

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1 Comment

Yu Hao Over a year ago

@ms2008vip Ah yes, I tested with the name t, but when posted here, I noticed OP used the name newtable, so I changed it accordingly, apparently I missed one. Fixed, thanks.

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