How to extract data from iterating a sequence

Question

csvData1 contains the data in a .csv file. I've created a sequence out of just two of the columns in the spreadsheet ("GIC-ID", "COVERAGE DESCRIPTION")

let mappedSeq1 = 
   seq { for csvRow in csvData1 do yield (csvRow.[2], csvRow.[5]) }

Looking in the Visual Studio debugger x winds up being a System.Tuple<string,string>.

for x in mappedSeq1 do
    printfn "%A" x
    printfn "%A" x.ToString

Here is the result of executing

for x in mappedSeq1

("GIC-ID", "COVERAGE DESCRIPTION")
<fun:main@28>

I am having difficulty figuring out how to access x, so I can extract the first element.

You should be able to do `for (gicId, desc) in mappedSeq1 do...` — Rob Lyndon, Sep 13 '16 at 19:34
I think you're getting the because x.ToString means "the ToString function of x", probably you wanted to do x.ToString() with parentheses which means "execute the ToString function of x". — Édgar Sánchez Gordón, Sep 13 '16 at 22:21

score 1 · Accepted Answer · answered Sep 13 '16 at 19:35

1

You can use pattern matching to deconstruct the tuple

for (a, b) in mappedSeq1 do
  // ...

or

for x in mappedSeq1 do
  let a, b = x

Alternatively for a 2-tuple you can use the built-in function fst and snd

answered Sep 13 '16 at 19:35

Sehnsucht

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@octopusgrabbus `x` shouldn't be a `seq` anyway but a 2-tuple (from the sample you gave) and so should work (or there are missing information in the question) – Sehnsucht Sep 13 '16 at 19:45
When I print out x, it's a function. I cannot use .fst, because it's not a method of the object. – octopusgrabbus Sep 13 '16 at 19:55
1

@octopusgrabbus `x` isn't a function ; you think it is because of `printfn "%A" x.ToString`. `ToString` is a function and `ToString ()` (note the parens) is the result of calling that function. Also you don't "dot into" to use `fst` it's a "free function" and have to be used this way `let firstPart = fst x` – Sehnsucht Sep 13 '16 at 20:23

PiotrWolkowski · Answer 2 · 2016-09-13T19:47:55.147

1

Use Seq.map and fst to get a sequence of only the first component of a tuple:

let firstOnly = mappedSeq |> Seq.map fst

edited Sep 13 '16 at 19:47

answered Sep 13 '16 at 19:42

PiotrWolkowski

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`Seq.map fst` is enough ;) – Sehnsucht Sep 13 '16 at 19:44
@Sehnsucht Thanks. Linq habits. – PiotrWolkowski Sep 13 '16 at 19:48
Cannot get this to work on the x in for x in mappedSeq1. It's a function. – octopusgrabbus Sep 13 '16 at 19:54
If x is a function then this part of your question: `x` winds up being a System.Tuple. is not correct. Please adjust the question. – PiotrWolkowski Sep 13 '16 at 19:59
Also, this syntax: `seq { for csvRow in csvData1 do yield (csvRow.[2], csvRow.[5]) }` will result in a sequence of tuples. – PiotrWolkowski Sep 13 '16 at 20:01
@PiotrWolkowski just nitpicking but in Linq it would be the same, you can use a method group when you have the pattern for example `Select (arg => fct (arg))` you can replace it by `Select (fct)` – Sehnsucht Sep 13 '16 at 20:25

How to extract data from iterating a sequence

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