Procedura: calcolare i valori di colonna in un file di testo CSV (LINQ)
In questo esempio viene illustrato come eseguire i calcoli di aggregazione come Somma, Media, Min e Max nelle colonne di un file con estensione CSV. I principi di esempio illustrati in questo argomento possono essere applicati ad altri tipi di testo strutturato.
Per creare il file di origine
Copiare le righe riportate di seguito in un file denominato scores.csv e salvarlo nella cartella Soluzione. Si supponga che la prima colonna rappresenti un ID studente e che le colonne successive rappresentino i punteggi di quattro esami.
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
Esempio
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 runtime.
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
class SumColumns
{
static void Main(string[] args)
{
string[] lines = System.IO.File.ReadAllLines(@"../../../scores.csv");
// Specifies the column to compute
int 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.
SingleColumn(lines, exam + 1);
Console.WriteLine();
MultiColumns(lines);
Console.WriteLine("Press any key to exit");
Console.ReadKey();
}
static void SingleColumn(IEnumerable<string> strs, int examNum)
{
Console.WriteLine("Single Column Query:");
// examNum specifies the column to run the
// calculations on. This could also be
// passed in dynamically at runtime.
// columnQuery is a IEnumerable<int>
// This query performs two steps:
// 1) split the string into a string[]
// 2) convert the specified element to
// int and select it.
var columnQuery =
from line in strs
let x = line.Split(',')
select Convert.ToInt32(x[examNum]);
// Execute and cache the results for performance.
// Only needed with very large files.
var results = columnQuery.ToList();
// Perform aggregate calculations
// on the column specified by examNum.
double average = results.Average();
int max = results.Max();
int min = results.Min();
Console.WriteLine("Exam #{0}: Average:{1:##.##} High Score:{2} Low Score:{3}",
examNum, average, max, min);
}
static void MultiColumns(IEnumerable<string> strs)
{
Console.WriteLine("Multi Column Query:");
// Create the columnQuery. Explicit typing is used
// to make clear that the columnQuery will produce
// nested sequences. You can also just use 'var'.
// The columnQuery performs these steps:
// 1) convert the string to a string[]
// 2) skip over the "Student ID" column and take the rest
// 3) convert each string to an int and select that
// entire sequence as one row in the results.
IEnumerable<IEnumerable<int>> query =
from line in strs
let x = line.Split(',')
let y = x.Skip(1)
select (from str in y
select Convert.ToInt32(str));
// Execute and cache the results for performance.
// ToArray could also be used here.
var results = query.ToList();
// Find out how many columns we have.
int 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 columnQuery in the call to ToList.
for (int column = 0; column < columnCount; column++)
{
var res2 = from row in results
select row.ElementAt(column);
double average = res2.Average();
int max = res2.Max();
int min = res2.Min();
// 1 is added to column because Exam numbers
// begin with 1
Console.WriteLine("Exam #{0} Average: {1:##.##} High Score: {2} Low Score: {3}",
column + 1, average, max, min);
}
}
}
/* 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 query funziona utilizzando il metodo Split per convertire ogni riga di testo in una matrice. Ogni elemento della matrice rappresenta una colonna. Il testo in ogni colonna viene infine convertito nella relativa rappresentazione numerica. Se si tratta di un file separato da tabulazioni, aggiornare solo l'argomento nel metodo Split con \t.
Compilazione del codice
Creare un progetto di Visual Studio destinato a .NET Framework versione 3.5. Per impostazione predefinita, il progetto include un riferimento a System.Core.dll e una direttiva using (C#) o un'istruzione Imports (Visual Basic) per lo spazio dei nomi System.Linq.
Copiare questo codice nel progetto.
Premere F5 per compilare ed eseguire il programma.
Premere un tasto per chiudere la finestra della console.