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Guide pratique : calculer des valeurs de colonnes dans un fichier texte CSV (LINQ) (Visual Basic)

  • Article

Cet exemple montre comment effectuer des calculs d’agrégation tels que Sum, Average, Min et Max, avec les colonnes d’un fichier .csv. Les exemples de principes montrés ici sont applicables à d’autres types de texte structuré.

Pour créer le fichier source

  1. Copiez les lignes suivantes dans un fichier nommé scores.csv, puis enregistrez-le dans votre dossier de projet. Supposons que la première colonne contienne un ID d’étudiant et que les colonnes suivantes contiennent les résultats de quatre examens.

    111, 97, 92, 81, 60
112, 75, 84, 91, 39
113, 88, 94, 65, 91
114, 97, 89, 85, 82
115, 35, 72, 91, 70
116, 99, 86, 90, 94
117, 93, 92, 80, 87
118, 92, 90, 83, 78
119, 68, 79, 88, 92
120, 99, 82, 81, 79
121, 96, 85, 91, 60
122, 94, 92, 91, 91

Exemple

Class SumColumns

    Public Shared Sub Main()

        Dim lines As String() = System.IO.File.ReadAllLines("../../../scores.csv")

        ' Specifies the column to compute
        ' This value could be passed in at run time.
        Dim exam = 3

        ' Spreadsheet format:
        ' Student ID    Exam#1  Exam#2  Exam#3  Exam#4
        ' 111,          97,     92,     81,     60
        ' one is added to skip over the first column
        ' which holds the student ID.
        SumColumn(lines, exam + 1)
        Console.WriteLine()
        MultiColumns(lines)

        ' Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit...")
        Console.ReadKey()

    End Sub

    Shared Sub SumColumn(ByVal lines As IEnumerable(Of String), ByVal col As Integer)

        ' This query performs two steps:
        ' split the string into a string array
        ' convert the specified element to
        ' integer and select it.
        Dim columnQuery = From line In lines
                           Let x = line.Split(",")
                           Select Convert.ToInt32(x(col))

        ' Execute and cache the results for performance.
        ' Only needed with very large files.
        Dim results = columnQuery.ToList()

        ' Perform aggregate calculations
        ' on the column specified by col.
        Dim avgScore = Aggregate score In results Into Average(score)
        Dim minScore = Aggregate score In results Into Min(score)
        Dim maxScore = Aggregate score In results Into Max(score)

        Console.WriteLine("Single Column Query:")
        Console.WriteLine("Exam #{0}: Average:{1:##.##} High Score:{2} Low Score:{3}",
                     col, avgScore, maxScore, minScore)

    End Sub

    Shared Sub MultiColumns(ByVal lines As IEnumerable(Of String))

        Console.WriteLine("Multi Column Query:")

        ' Create the query. It will produce nested sequences.
        ' multiColQuery performs these steps:
        ' 1) convert the string to a string array
        ' 2) skip over the "Student ID" column and take the rest
        ' 3) convert each field to an int and select that
        '    entire sequence as one row in the results.
        Dim multiColQuery = From line In lines
                            Let fields = line.Split(",")
                            Select From str In fields Skip 1
                                        Select Convert.ToInt32(str)

        Dim results = multiColQuery.ToList()

        ' Find out how many columns we have.
        Dim columnCount = results(0).Count()

        ' Perform aggregate calculations on each column.
        ' One loop for each score column in scores.
        ' We can use a for loop because we have already
        ' executed the multiColQuery in the call to ToList.

        For j As Integer = 0 To columnCount - 1
            Dim column = j
            Dim res2 = From row In results
                       Select row.ElementAt(column)

            ' Perform aggregate calculations
            ' on the column specified by col.
            Dim avgScore = Aggregate score In res2 Into Average(score)
            Dim minScore = Aggregate score In res2 Into Min(score)
            Dim maxScore = Aggregate score In res2 Into Max(score)

            ' Add 1 to column numbers because exams in this course start with #1
            Console.WriteLine("Exam #{0} Average: {1:##.##} High Score: {2} Low Score: {3}",
                              column + 1, avgScore, maxScore, minScore)

        Next
    End Sub

End Class
' Output:
' Single Column Query:
' Exam #4: Average:76.92 High Score:94 Low Score:39

' Multi Column Query:
' Exam #1 Average: 86.08 High Score: 99 Low Score: 35
' Exam #2 Average: 86.42 High Score: 94 Low Score: 72
' Exam #3 Average: 84.75 High Score: 91 Low Score: 65
' Exam #4 Average: 76.92 High Score: 94 Low Score: 39

La requête fonctionne à l’aide de la méthode Split pour convertir chaque ligne de texte en un tableau. Chaque élément du tableau représente une colonne. Enfin, le texte de chaque colonne est converti en sa représentation numérique. Si votre fichier est un fichier séparé par des tabulations, remplacez l’argument de la méthode Split par \t.

Compiler le code

Créez un projet d’application console Visual Basic avec une instruction Imports pour l’espace de noms System.Linq.

Voir aussi