6.4.2 CSV file handling

What Is a CSV File?

A CSV (Comma-Separated Values) file is a plain text file where each line represents one record and the fields within each record are separated by commas. CSV is a universal format for storing tabular data — it can be opened in spreadsheet applications and read by programs in any language.

Name,Age,Score
Alice,16,85
Bob,17,72
Carol,16,91

The first line is typically a header row naming the columns. Each subsequent line is a data record.

Reading a CSV File

# Reading a CSV file in Python
file = open("students.csv", "r")
header = file.readline()        # read and discard the header row

for line in file:
    line = line.strip()         # remove trailing newline character
    fields = line.split(",")    # split on comma into a list
    name  = fields[0]
    age   = int(fields[1])      # convert to correct type
    score = int(fields[2])
    print(name, "| Age:", age, "| Score:", score)

file.close()                    # always close the file when done

.strip() removes the invisible newline at the end of each line. Without it, the last field of every record contains a trailing newline character.

// Reading a CSV file in C#
using System.IO;
string[] lines = File.ReadAllLines("students.csv");
// lines[0] is the header — start from index 1
for (int i = 1; i < lines.Length; i++) {
    string[] fields = lines[i].Split(',');
    string name  = fields[0];
    int    age   = int.Parse(fields[1]);
    int    score = int.Parse(fields[2]);
    Console.WriteLine(name + " | Age: " + age + " | Score: " + score);
}

' Reading a CSV file in VB.NET
Imports System.IO
Dim lines() As String = File.ReadAllLines("students.csv")
' lines(0) is the header — start from index 1
For i As Integer = 1 To lines.Length - 1
    Dim fields() As String = lines(i).Split(","c)
    Dim name  As String  = fields(0)
    Dim age   As Integer = Integer.Parse(fields(1))
    Dim score As Integer = Integer.Parse(fields(2))
    Console.WriteLine(name & " | Age: " & age & " | Score: " & score)
Next

Writing a CSV File

# Writing a CSV file in Python
file = open("results.csv", "w")
file.write("Name,Score,Grade
")   # write header row

students = [["Alice", 85, "A"],
            ["Bob",   72, "B"],
            ["Carol", 91, "A"]]

for student in students:
    line = student[0] + "," + str(student[1]) + "," + student[2] + "
"
    file.write(line)

file.close()
print("File written successfully.")

Opening with "w" creates a new file (or overwrites an existing one). To add to an existing file without overwriting, use "a" (append mode).

using System.IO;
string[][] students = {
    new[] {"Alice", "85", "A"},
    new[] {"Bob",   "72", "B"},
    new[] {"Carol", "91", "A"}
};
using (StreamWriter sw = new StreamWriter("results.csv")) {
    sw.WriteLine("Name,Score,Grade");
    foreach (string[] s in students)
        sw.WriteLine(string.Join(",", s));
}
Console.WriteLine("File written.");

Imports System.IO
Dim students()() As String = {
    New String() {"Alice", "85", "A"},
    New String() {"Bob",   "72", "B"},
    New String() {"Carol", "91", "A"}
}
Using sw As New StreamWriter("results.csv")
    sw.WriteLine("Name,Score,Grade")
    For Each s() As String In students
        sw.WriteLine(String.Join(",", s))
    Next
End Using
Console.WriteLine("File written.")

Key Takeaways

CSV files store tabular data as plain text: one record per line, fields separated by commas.
Reading: open file → read lines → .strip() newlines → .split(",") to get fields → convert types.
Writing: open file with "w" → write header → write each record as a comma-separated string followed by → close.
Always call file.close() — or use a with statement — to ensure the file is properly saved and resources are released.
.strip() is essential when reading — it removes the invisible newline character from the end of each line.