LUA: add values in nested table

Question

I have a performance issue in my application. I would like to gather some ideas on what I can do to improve it. The application is very easy: I need to add values inside a nested table to get the total an user wants to pay out of all the pending payments. The user chooses a number of payments and I calculate how much it is they will pay.

This is what I have:

jsonstr = "{    "name": "John",
    "surname": "Doe",
    "pending_payments": [
        {
            "month": "january",
            "amount": 50,
        },
        {
            "month": "february",
            "amount": 40,
        },
        {
            "month": "march",
            "amount": 45,
        },
    ]
}"

local lunajson = require 'lunajson'
local t = lunajson.decode(jsonstr)
local limit   -- I get this from the user
local total = 0;

for i=1, limit, 1 do 
    total = total + t.pending_payments[i].amount; 
end;

It works. At the end I get what I need. However, I notice that it takes ages to do the calculation. Each JSON has only twelve pending payments (one per month). It is taking between two to three seconds to come up with a result!. I tried in different machines and LUA 5.1, 5.2., 5.3. and the result is the same.

Can anyone please suggest how I can implement this better?

Thank you!

This is not valid Lua code. Use a long string to define jsonstr. — lhf
– lhf, Commented Jun 11, 2020 at 22:12
Hi @lhf. Thanks for the comment. What's in the code is for illustration purposes. In the actual application a JSON object is decoded into a table by lunajson. — Wilmar
– Wilmar, Commented Jun 12, 2020 at 12:30

lhf · Accepted Answer · 2020-06-12 11:39:59Z

1

For this simple string, try the test code below, which extracts the amounts directly from the string, without a json parser:

jsonstr = [[{    "name": "John",
    "surname": "Doe",
    "pending_payments": [
        {
            "month": "january",
            "amount": 50,
        },
        {
            "month": "february",
            "amount": 40,
        },
        {
            "month": "march",
            "amount": 45,
        },
    ]
}]]

for limit=0,4 do
    local total=0
    local n=0
    for a in jsonstr:gmatch('"amount":%s*(%d+),') do
        n=n+1
        if n>limit then break end
        total=total+tonumber(a)
    end
    print(limit,total)
end

edited Jun 12, 2020 at 11:39

answered Jun 11, 2020 at 22:17

lhf

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3 Comments

Wilmar Over a year ago

Hi @lhf. Thanks for the suggestion. I work with Web Services that provide me with different JSON objects. The one I posted is a short version for illustration purposes. The actual JSON object is actually longer. However, more important I use the information in many differents parts in my code, it would really help me if I can keep using the JSON parser. I was looking into the Pelight library for alternatives and I think there is one but I cannot really understand how to use it (stevedonovan.github.io/Penlight/api/libraries/…) I am not the most experienced developer.

lhf Over a year ago

@Wilmar, try asking in the Lua mailing list: lua.org/lua-l.html

Wilmar Over a year ago

Thank you @lhf. I just did.

Wilmar · Accepted Answer · 2020-06-12 17:15:18Z

1

I found the delay had nothing to do with the calculation in LUA. It was related with a configurable delay in the retrieval of the limit variable.

I have nothing to share here related to the question asked since the problem was actually in an external element.

Thank @lfh for your replies.

answered Jun 12, 2020 at 17:15

Wilmar

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