vb.net获取数组的第一行和最后一行的“值”

Question

I have an array of numbers like this:

{10 20 30 40 50 60 70 80 90}

The start and end numbers will not always be the same (in fact none of them will).

I want to get the value (not the index) of the first and last entries.

So in the above example, all I'm interested in getting are the values "10" and "90".

This array is being constructed from a comma delimited text file which looks like this:

10,3.456
20,3.588
30,3.640
40,3.790
50,3.850
60,3.680
70,3.480
80,3.280
90,3.765

This is my code:

    Dim arrName, arrValue As New List(Of String)()
    Dim sdata() As String
    Dim column1, column2 As Decimal

For Each line As String In IO.File.ReadAllLines("c:\file.txt")
    If line <> "" And Not line.StartsWith("#") Then
        sdata = line.Split(","c)
        arrName.Add(sdata(0).Trim())
        arrValue.Add(sdata(1).Trim())
        ' get data from array
        column1 = sdata(0)
        column2 = sdata(1)
End If
Next

So what I want is to be able to create two variables where:

column1Lowest=<the lowest value> (in this case 10)
column1Highest=<the highest value> (in this case 90)

I have tried several method, but all I seem to be getting back is the entire array each time, or "-1" (no match found in array I know).

Things I have tried: IndexOf(), FirstIndexOf(),LastIndexOf()

I am only interested in the lowest an highest values in column1 (but I still use the rest elsewhere).

I am now out of ideas.

Please help - with answers in the form that a novice can understand.

Thanks.

EDIT:

To add to Steve's answer below, I tried the Inumerable strings option but I believe I was running into issues with my file format - some files were fine - some were not (usually files without a line feed after the last line of data!).

This is my code:

Dim Lines As Collections.Generic.IEnumerable(Of String) = File.ReadLines("c:\file.txt").Where(Function(q) q.StartsWith("#") = False)
        Line0 = Lines.FirstOrDefault
        LineN = Lines.LastOrDefault
       lowestNum = Line0.Split(New Char() {","c})
        highestNum = LineN.Split(New Char() {","c})
        lowNum = lowestNum(0)
        highNum = highestNum(0)

etc..

So if I wanted to use this chunk of code on my already parsed file - how would I use the existing array with it?

I have just tried:

Dim Lines As Collections.Generic.IEnumerable(Of String) = {column1}
                FirstValue = Lines.FirstOrDefault
                LastValue = Lines.LastOrDefault

But this still prints the entire array?

To display the output I'm using:

    ListBox2.Items.Add(FirstValue)
    ListBox1.Items.Add(LastValue)

I've tried inserting the listbox code both in and outside of the "For Each" loop. Putting them outside gives me variable issues - so I tried to declare the variables outside of the loop to no avail.

I did say I was a novice!

Answer 1

There are these pretty IEnumerable extensions that you can use

Dim first = numbers.First()
Dim last = numbers.Last()

However from your question is not clear if you want just the first and last element of the array or the highest and lowest value in the array.

For the second case you have

Dim high = numbers.Max()
Dim low = numbers.Min()

I have also noticed that you use strings to manage these values, but if they are numbers and you want to retrieve the highest and lowest numbers then you should absolutely convert these values to integers otherwise you will have a difficult time to explain to your computer that a string "100" is higher than a string "20".

EDIT
Loop over your lines and convert every line to a decimal then get the min and max (Note, no check on the correctness of the input)

Dim names = new List(Of Decimal)()
Dim values = new List(Of Decimal)()
For Each line As String In IO.File.ReadAllLines("c:\file.txt")
    If line <> "" And Not line.StartsWith("#") Then
        sdata = line.Split(","c)
        names.Add(Convert.ToDecimal(sdata(0).Trim()))
        values.Add(Convert.ToDecimal(sdata(1).Trim()))
    End If
Next
Dim lowValue = names.Min()
Dim highValue = names.Max()

Answer 2

The first element in a list or array always has index 0 and the last element always List.Count - 1 or Array.Length - 1 depending on whether you use an array or a list. So you get the first and the last value from your array using:

arrayLowest = arrayVariable(0)
arrayHighest = arrayVariable(arrayVariable.Length - 1)
listLowest = listVariable(0)
listHighest = listVariable(listVariable.Count - 1)

The other methods you mention ( IndexOf() , FirstIndexOf() , LastIndexOf() ) are used to find the index of a given element. For example if you wanted to know at which position the element 90 is in the list/array, you'd use these.

Please note that an array or list are not sorted by default, so if you want to get the minimum and maximum values from your array or list you need to either use the extensions Steve mentioned, or sort the list/array and then get the first and last value, or loop over the list/array and perform comparisons.

Answer 3

Try this to get the first and last values:

Dim FirstValue As String = sdata(0)
Dim LastValue As String = sdata(sdata.Length - 1)

Answer 4

First make sure that you only try to access the elements if the array has a size > 0, then access the first and last element like this:

Dim lowest, highest As String;
If arrValues.Length > 0 Then
  lowest = arrValues(0);
  highest = arrValues(arrValues.Length - 1);
End If

Array.Length returns the number of elements, as elements are indexed starting from 0, the last element will be Length - 1 .