How to Dump a Table to Console?
I'm Having Trouble Displaying the Contents of a Table Which Contains Nested Tables (N-Deep). I'd Like to Just Dump It to Std out or the Console via a Print...
I'm having trouble displaying the contents of a table which contains nested tables (n-deep). I'd like to just dump it to std out or the console via a print statement or something quick and dirty but I can't figure out how. I'm looking for the rough equivalent that I'd get when printing an NSDictionary using gdb.
19 Answers
If the requirement is "quick and dirty"
I've found this one useful. Because if the recursion it can print nested tables too. It doesn't give the prettiest formatting in the output but for such a simple function it's hard to beat for debugging.
function dump(o)
if type(o) == 'table' then
local s = '{ '
for k,v in pairs(o) do
if type(k) ~= 'number' then k = '"'..k..'"' end
s = s .. '['..k..'] = ' .. dump(v) .. ','
end
return s .. '} '
else
return tostring(o)
end
end
e.g.
local people = {
{
name = "Fred",
address = "16 Long Street",
phone = "123456"
},
{
name = "Wilma",
address = "16 Long Street",
phone = "123456"
},
{
name = "Barney",
address = "17 Long Street",
phone = "123457"
}
}
print("People:", dump(people))
Produces the following output:
People: { [1] = { ["address"] = 16 Long Street,["phone"] = 123456,["name"] = Fred,} ,[2] = { ["address"] = 16 Long Street,["phone"] = 123456,["name"] = Wilma,} ,[3] = { ["address"] = 17 Long Street,["phone"] = 123457,["name"] = Barney,} ,}
I know this question has already been marked as answered, but let me plug my own library here. It's called inspect.lua, and you can find it here:
It's just a single file that you can require from any other file. It returns a function that transforms any Lua value into a human-readable string:
local inspect = require('inspect')
print(inspect({1,2,3})) -- {1, 2, 3}
print(inspect({a=1,b=2})
-- {
-- a = 1
-- b = 2
-- }
It indents subtables properly, and handles "recursive tables" (tables that contain references to themselves) correctly, so it doesn't get into infinite loops. It sorts values in a sensible way. It also prints metatable information.
Regards!
Feel free to browse the Lua Wiki on table serialization. It lists several ways on how to dump a table to the console.
You just have to choose which one suits you best. There are many ways to do it, but I usually end up using the one from Penlight:
> t = { a = { b = { c = "Hello world!", 1 }, 2, d = { 3 } } }
> require 'pl.pretty'.dump(t)
{
a = {
d = {
3
},
b = {
c = "Hello world!",
1
},
2
}
}
found this:
-- Print contents of `tbl`, with indentation.
-- `indent` sets the initial level of indentation.
function tprint (tbl, indent)
if not indent then indent = 0 end
for k, v in pairs(tbl) do
formatting = string.rep(" ", indent) .. k .. ": "
if type(v) == "table" then
print(formatting)
tprint(v, indent+1)
elseif type(v) == 'boolean' then
print(formatting .. tostring(v))
else
print(formatting .. v)
end
end
end
from here
works pretty good for me...
Most pure lua print table functions I've seen have a problem with deep recursion and tend to cause a stack overflow when going too deep. This print table function that I've written does not have this problem. It should also be capable of handling really large tables due to the way it handles concatenation. In my personal usage of this function, it outputted 63k lines to file in about a second.
The output also keeps lua syntax and the script can easily be modified for simple persistent storage by writing the output to file if modified to allow only number, boolean, string and table data types to be formatted.
function print_table(node)
local cache, stack, output = {},{},{}
local depth = 1
local output_str = "{\n"
while true do
local size = 0
for k,v in pairs(node) do
size = size + 1
end
local cur_index = 1
for k,v in pairs(node) do
if (cache[node] == nil) or (cur_index >= cache[node]) then
if (string.find(output_str,"}",output_str:len())) then
output_str = output_str .. ",\n"
elseif not (string.find(output_str,"\n",output_str:len())) then
output_str = output_str .. "\n"
end
-- This is necessary for working with HUGE tables otherwise we run out of memory using concat on huge strings
table.insert(output,output_str)
output_str = ""
local key
if (type(k) == "number" or type(k) == "boolean") then
key = "["..tostring(k).."]"
else
key = "['"..tostring(k).."']"
end
if (type(v) == "number" or type(v) == "boolean") then
output_str = output_str .. string.rep('\t',depth) .. key .. " = "..tostring(v)
elseif (type(v) == "table") then
output_str = output_str .. string.rep('\t',depth) .. key .. " = {\n"
table.insert(stack,node)
table.insert(stack,v)
cache[node] = cur_index+1
break
else
output_str = output_str .. string.rep('\t',depth) .. key .. " = '"..tostring(v).."'"
end
if (cur_index == size) then
output_str = output_str .. "\n" .. string.rep('\t',depth-1) .. "}"
else
output_str = output_str .. ","
end
else
-- close the table
if (cur_index == size) then
output_str = output_str .. "\n" .. string.rep('\t',depth-1) .. "}"
end
end
cur_index = cur_index + 1
end
if (size == 0) then
output_str = output_str .. "\n" .. string.rep('\t',depth-1) .. "}"
end
if (#stack > 0) then
node = stack[#stack]
stack[#stack] = nil
depth = cache[node] == nil and depth + 1 or depth - 1
else
break
end
end
-- This is necessary for working with HUGE tables otherwise we run out of memory using concat on huge strings
table.insert(output,output_str)
output_str = table.concat(output)
print(output_str)
end
Here is an example:
local t = {
["abe"] = {1,2,3,4,5},
"string1",
50,
["depth1"] = { ["depth2"] = { ["depth3"] = { ["depth4"] = { ["depth5"] = { ["depth6"] = { ["depth7"]= { ["depth8"] = { ["depth9"] = { ["depth10"] = {1000}, 900}, 800},700},600},500}, 400 }, 300}, 200}, 100},
["ted"] = {true,false,"some text"},
"string2",
[function() return end] = function() return end,
75
}
print_table(t)
Output:
{
[1] = 'string1',
[2] = 50,
[3] = 'string2',
[4] = 75,
['abe'] = {
[1] = 1,
[2] = 2,
[3] = 3,
[4] = 4,
[5] = 5
},
['function: 06472B70'] = 'function: 06472A98',
['depth1'] = {
[1] = 100,
['depth2'] = {
[1] = 200,
['depth3'] = {
[1] = 300,
['depth4'] = {
[1] = 400,
['depth5'] = {
[1] = 500,
['depth6'] = {
[1] = 600,
['depth7'] = {
[1] = 700,
['depth8'] = {
[1] = 800,
['depth9'] = {
[1] = 900,
['depth10'] = {
[1] = 1000
}
}
}
}
}
}
}
}
}
},
['ted'] = {
[1] = true,
[2] = false,
[3] = 'some text'
}
}
As previously mentioned, you have to write it. Here is my humble version: (super basic one)
function tprint (t, s)
for k, v in pairs(t) do
local kfmt = '["' .. tostring(k) ..'"]'
if type(k) ~= 'string' then
kfmt = '[' .. k .. ']'
end
local vfmt = '"'.. tostring(v) ..'"'
if type(v) == 'table' then
tprint(v, (s or '')..kfmt)
else
if type(v) ~= 'string' then
vfmt = tostring(v)
end
print(type(t)..(s or '')..kfmt..' = '..vfmt)
end
end
end
example:
local mytbl = { ['1']="a", 2, 3, b="c", t={d=1} }
tprint(mytbl)
output (Lua 5.0):
table[1] = 2
table[2] = 3
table["1"] = "a"
table["t"]["d"] = 1
table["b"] = "c"
The simplest way, with circular reference handling and all:
function dump(t, indent, done)
done = done or {}
indent = indent or 0
done[t] = true
for key, value in pairs(t) do
print(string.rep("\t", indent))
if type(value) == "table" and not done[value] then
done[value] = true
print(key, ":\n")
dump(value, indent + 2, done)
done[value] = nil
else
print(key, "\t=\t", value, "\n")
end
end
end
I use my own function to print the contents of a table but not sure how well it translates to your environment:
---A helper function to print a table's contents.
---@param tbl table @The table to print.
---@param depth number @The depth of sub-tables to traverse through and print.
---@param n number @Do NOT manually set this. This controls formatting through recursion.
function PrintTable(tbl, depth, n)
n = n or 0;
depth = depth or 5;
if (depth == 0) then
print(string.rep(' ', n).."...");
return;
end
if (n == 0) then
print(" ");
end
for key, value in pairs(tbl) do
if (key and type(key) == "number" or type(key) == "string") then
key = string.format("[\"%s\"]", key);
if (type(value) == "table") then
if (next(value)) then
print(string.rep(' ', n)..key.." = {");
PrintTable(value, depth - 1, n + 4);
print(string.rep(' ', n).."},");
else
print(string.rep(' ', n)..key.." = {},");
end
else
if (type(value) == "string") then
value = string.format("\"%s\"", value);
else
value = tostring(value);
end
print(string.rep(' ', n)..key.." = "..value..",");
end
end
end
if (n == 0) then
print(" ");
end
end
There are 2 solutions that I want to mention: a quick&dirty one, and another which properly escapes all keys and values but is bigger
Simple & fast solution (use only on "safe" inputs):
local function format_any_value(obj, buffer)
local _type = type(obj)
if _type == "table" then
buffer[#buffer + 1] = '{"'
for key, value in next, obj, nil do
buffer[#buffer + 1] = tostring(key) .. '":'
format_any_value(value, buffer)
buffer[#buffer + 1] = ',"'
end
buffer[#buffer] = '}' -- note the overwrite
elseif _type == "string" then
buffer[#buffer + 1] = '"' .. obj .. '"'
elseif _type == "boolean" or _type == "number" then
buffer[#buffer + 1] = tostring(obj)
else
buffer[#buffer + 1] = '"???' .. _type .. '???"'
end
end
Usage:
local function format_as_json(obj)
if obj == nil then return "null" else
local buffer = {}
format_any_value(obj, buffer)
return table.concat(buffer)
end
end
local function print_as_json(obj)
print(_format_as_json(obj))
end
print_as_json {1, 2, 3}
print_as_json(nil)
print_as_json("string")
print_as_json {[1] = 1, [2] = 2, three = { { true } }, four = "four"}
David Miller brings 15 years of experience in global economics, personal finance strategy, and market dynamics. He specializes in turning complex economic trends into actionable insights for everyday readers.
