How to print nested table using recursion in lua?

Question

I want to print this table using recursion and not by using for loop. I am trying to avoid repeating code.

local t = {x=0, y=5, other={10,10,10,11}}

function DeepPrint (t)
  local request_headers_all = ""
  for k, v in pairs(t) do
    if type(v) == "table" then
      for k1, v1 in pairs(v) do
        local rowtext = ""
        rowtext = string.format("[%s %s] ",k, v1)
        request_headers_all = request_headers_all .. rowtext
      end 
    else
      local rowtext = ""
      rowtext = string.format("[%s %s] ", k, v)
      request_headers_all = request_headers_all .. rowtext
    end
  end
  return request_headers_all
end

print(DeepPrint (t))

Expected output (order doesn't matter):

[y 5] [x 0] [other 10] [other 10] [other 10] [other 11]

Chris · Accepted Answer · 2022-07-18 22:51:00Z

1

You can simply remove the inner loop and make a call to DeepPrint again using the value of v - the table:

local t = {x=0, y=5, other={10,10,10,11}}

function DeepPrint (t)
  local request_headers_all = ""
  for k, v in pairs(t) do
    if type(v) == "table" then
      request_headers_all = request_headers_all .. "[" .. k .. " " .. DeepPrint(v) .. "] "
    else
      local rowtext = ""
      rowtext = string.format("[%s %s] ", k, v)
      request_headers_all = request_headers_all .. rowtext
    end
  end
  return request_headers_all
end

print(DeepPrint (t))

Produces:

[y 5] [x 0] [other [1 10] [2 10] [3 10] [4 11] ]

Note that you are not going to get these in order. Hashes are unordered, so you cannot guarantee that x will precede y will precede other.

Update: Strip out array indices for pure arrays:

local t = {x=0, y=5, other={10,10,10,11}}

function DeepPrint (t)
  local request_headers_all = ""
  for k, v in pairs(t) do
    if type(v) == "table" then
      request_headers_all = request_headers_all .. "[" .. k .. " " .. DeepPrint(v) .. "] "
    else
      local rowtext = ""
      if type(k) == "string" then
        rowtext = string.format("[%s %s] ", k, v)
      else
        rowtext = string.format("[%s] ", v)
      end    
      request_headers_all = request_headers_all .. rowtext
    end
  end
  return request_headers_all
end

Output:

[y 5] [x 0] [other [10] [10] [10] [11] ]

edited Jul 18, 2022 at 22:51

answered Jul 18, 2022 at 22:13

Chris

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4 Comments

Sam Over a year ago

This works, I really don't care about order. I need to print all NGINX headers in logs.

Sam Over a year ago

Anyway to exclude index e.g 0,1,2 in [other [1 10] [2 10] [3 10] [4 11] ]

Chris Over a year ago

Yeah. Pairs will give you k as a number for arrays. Therefore, if type(k) == "string" then it is a hash (x, y or other) otherwise it is a "number" and therefore an array, so you can if/else on type of k and change your print accordingly

Sam Over a year ago

Is it possible for you to update the answer? Pretty new to lua.

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