How to: Compute Column Values in a CSV Text File (LINQ)
This example shows how to perform aggregate computations such as Sum, Average, Min, and Max on the columns of a .csv file. The example principles that are shown here can be applied to other types of structured text.
To create the source file
Copy the following lines into a file that is named scores.csv and save it in your solution folder. Assume that the first column represents a student ID, and subsequent columns represent scores from four exams.
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
Example
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(","c) _
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(","c) _
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
*/
The query works by using the Split method to convert each line of text into an array. Each array element represents a column. Finally, the text in each column is converted to its numeric representation. If your file is a tab-separated file, just update the argument in the Split method to \t.
Compiling the Code
Create a Visual Studio project that targets the .NET Framework version 3.5. By default, the project has a reference to System.Core.dll and a using directive (C#) or Imports statement (Visual Basic) for the System.Linq namespace.
Copy this code into your project.
Press F5 to compile and run the program.
Press any key to exit the console window.