# [erlang-questions] Question on building a variable with multiple math operations

Sverker Eriksson <>
Fri Jul 10 17:32:00 CEST 2009

```1> lists:foldl(fun({A,B}, Acc) -> Acc + A*B end, 0, [{1,5},{2,7},{4,8}]).
51

/Sverker

Greg Perry wrote:
> I have a unique challenge that I can't seem to find a right way to do based on the immutability of Erlang.
>
> I have built a unique factoring application with Erlang, which first analyzes any given N and depending on the properties of that subprime number a list of coefficients are assigned to a list (the list contains tuples actually).
>
> http://blog.liveammo.com/2009/06/factoring-fun/
>
> For example:
>
> \$ erl
> Erlang R13B01 (erts-5.7.2) [source] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]
>
> Eshell V5.7.2  (abort with ^G)
> 1> c("reductionsieve.erl").
> {ok,reductionsieve}
> 2> c("lin.erl").
> {ok,lin}
> 3> N1 = reductionsieve:make_prime(5) * reductionsieve:make_prime(5).
> Generating a 5 digit prime ......
> Generating a 5 digit prime ..................
> 823221601
> 4> reductionsieve:analyze(N1).
> [{1,7},{2,8},{4,4},{5,5}]
> 5> N2 = reductionsieve:make_prime(5) * reductionsieve:make_prime(5).
> Generating a 5 digit prime .
> Generating a 5 digit prime ....
> 3193425841
> 6> reductionsieve:analyze(N2).
> [{1,4},{2,2},{5,8},{7,7}]
> 7> N3 = reductionsieve:make_prime(5) * reductionsieve:make_prime(5).
> Generating a 5 digit prime ....
> Generating a 5 digit prime .
> 5575777817
> 8> reductionsieve:analyze(N3).
> [{1,5},{2,7},{4,8}]
>
> The list filled with tuples that returns is variable in length, depending on the properties of any given N.  I would like to be able to build a variable that takes each tuple from a list and multiplies them together, then adds them to the other products.  For example, if N yields properties of [{1,5},{2,7},{4,8}], I need to be able to build a variable which is: (1 * 5) + (2 * 7) + (4 * 8); some N will return a list as small as three tuples, other N will return a list that may have as many as nine tuples that need to be multiplied in that fashion.
>
> I have been experimenting with lists:fold1 but it seems that is expecting a simple list without the use of tuples.
>
> Any ideas on this one?
>