How to extract data from iterating a sequence

问题

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.

回答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

回答2:

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

let firstOnly = mappedSeq |> Seq.map fst

来源：https://stackoverflow.com/questions/39477638/how-to-extract-data-from-iterating-a-sequence