drop and trim a list in aerospike udf

问题

I try to drop and trim a list to build a pagination system

local function createMap(postId, paramDate)
 local m =  map { id = postId, date = paramDate };
 return m;
end



function get(rec, binName, from, to)

    if aerospike:exists(rec) then
        local l = rec[binName]
        if (l == nil) then
            return nil

        else
            local length = #l;
            if (length <= 10 and to <=10) then
                return l;
            elseif (to >= length) then
                local drop = list.drop(l, from); 
                return drop;
            else
                list.trim(l, to);--Remove all elements at and beyond a specified position in the List.
                list.drop(l, from); --Select all elements except the first n elements of the List
                return l;               
            end

        end
    else
    return nil;--end return empty array
    end--end else aerospike exists


end

my list has this structure :

[{"date":"2016-01-02T19:45:00.806Z", "id":"568828bc49017f16659f6978"}, {"date":"2016-01-02T19:44:56.040Z", "id":"568828b849017f16659f6977"},...]

It seems that I can't trim and then drop a list. with 21 elements for example : it first return to element 21 to element 13 , then element 21 to element 4 , then element 3 to element 1

my function in node.js is simple for change 'from' and 'to' I send the 'page' fromm the frontend to node.js and use this function :

      var skip = 9 * (page -1);
      var lastIndexToReturn = skip + 9 + 1;

so in first request from and to are '0' and '10', then '9' and '19', etc by using list.trim and list.drop I thought that I can build a pagination system

回答1:

function get(rec, binName, from, to)

    if aerospike:exists(rec) then
        local l = rec[binName]
        if (l == nil) then
            return list()--return empty array

        else
            --first index in lua is 1
            local length = list.size(l)
            local pagination = list()
            if (length < from) then
                return list()
            elseif (length >= from and length <=to) then
                for i=from,length,1 do
                    list.append(pagination, l[i])
                end--end for
                return pagination 
            else
                for i=from,to,1 do
                    list.append(pagination, l[i])
                end--end for
                return pagination
            end     
        end
    else
      return list();--end return empty array
    end--end else aerospike exists


end
--end function

I ended by coding this function : to be sure to don't get errors for list index , I get the size of the list and I compare this size to the portion I want to paginate to determine what to return: I don't iterate over the whole array.

Of course it works only if the record is a list Note : ensure that the first 'from' coming from the client is 1 because lua arrays begin to 1

来源：https://stackoverflow.com/questions/34575773/drop-and-trim-a-list-in-aerospike-udf