Preloading variables with one array

Hello friends.
Can you help me. In my forms I have one question of type unique selection to know the currency in which people bought or spent on something; for example: dollar, euro, lempira, etc.
Then I have an expression to sum all the expenses or income, but some of these are in a different currency. So I would like to have a conversion factor to convert all expenses or income into my national currency.
I’m thinking about creating a matrix in a variable, like this one:

new [] {conversion factor to currency 1, conversion factor to currency 2, conversion factor to currency 3, …, conversion factor to currency n} [(int) Question of currency]

The problem is that the conversion factor every day changes for this reason, I would like to preload this variable that contains the value of the conversion factor for each currency.

I consider using a lookup table, but first I would like to know more about the use of a matrix defined in a variable.

Also I would like know how to get the value of each item of the array.

For example if my variable with the array is named vArray and I want to know what is the value of the first item in the array. How I can do it?

Note that you can not preload the values that would be current, since you preload data before the interview happens and it will occur later than preloading, and you don’t know the future exchange rates.

There is no variable type ‘matrix’. So even you can create an array and use it in the same statement, you can’t store an array in a variable.

Also variables are calculated and can’t be preloaded - that’s hidden questions then!

A look up table is the same for all interviewers. This doesn’t solve the dynamics problem.

Thanks for explain me it.
Can you suggest me what could be are the best way to do it?