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Newbie group and aggegate sequences, arrays , lists ? ....
f#server

Hi there

I am trying really hard to learn F#, though I would say I am failing miserably [:(].

I have a physics back ground and I have plenty of imperative langauges under my belt, fortran, C#, C++, and C. I am also pretty good at SQL. I work designing doing fairly simple numerical algorithms (which are often done in SQL - since it is not worth the overhead of takign number out of the server to do the simple maths).

I have put the some effort into learning LINQ in C# so that I can combine SQL like calculations and iteration in a good way, for more complicated algorithms. However F# seems like a super LINQ and possibly better for my needs in certain areas.

However, and here is the question and it is really holding me back, in what I want to achieve in F#! If I have a list or array or sequence of tuples in the form

(int, float)

how do I aggregate the floats, using the int as partition. ie if I have a table (table1) like this 

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int   float
1      0.1
1      0.1
1      0.2
3      0.1

how to do I yield 

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3

int, float
1,0.4
3,0.1

in SQL this is easy 

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select int,sum(float) from table1 group by int

all other problems will flow from this type of easy aggregation.

I can do this in LINQ easily enough but can't work out the F# [:$].

help please !

Carl

.

You probably are looking for something like this:

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 > let values = [ 1, 0.1; 1, 0.2; 3, 0.9; 1, 0.8 ];;

val values : (int * float) list = [(1, 0.1); (1, 0.2); (3, 0.9); (1, 0.8)]

> let res = values 
          |> Seq.groupBy fst
          |> Seq.map (fun (key, vals) -> key, Seq.sumBy snd vals);;

val res : seq<int * float>

> res;;
val it : seq<int * float> = seq [(1, 1.1); (3, 0.9)]

To start, we take the values and group them on the first element in the tuple. This gives us a seq<int * seq<int * float>>. Then we project that sequence, pattern matching out the key (int) and the vals (seq<int * float>). We return the key unaltered, and call sumBy on the values, summing only the second element in the tuple (the floats).

By on 9/3/2009 5:15 AM ()

Thank you both for the prompt replys

I got as far as the Seq.group_by

but couldn't work out the form of the summation step.

I think I will have the wind in my heels for the next few days until the next one

Carl

By on 9/3/2009 7:22 AM ()

Hi,

You should look at groupBy, map and sumBy functions.
Here's my try (I have no F# compiler here, not sure if it compiles):

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my_table |> Seq.groupBy fst
        |> Seq.map (fun (k, li) -> k, li |> Seq.sumBy snd)

Or, using comprehensions and generating a list:

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[ for k, li in Seq.groupBy fst my_table -> k, Seq.sumBy snd li ]

Laurent

By on 9/3/2009 5:06 AM ()

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