File Handling

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Why Do We Store Data in Files?

There are several important reasons why programs need to store data in files:

1. Permanent storage
When your program finishes running, any variables you created disappear from memory. Files let you save data permanently on your hard drive, so it's still there the next time you run the program.

Example: A game that saves your high score, or a to-do list app that remembers your tasks even after you close it.

2. Large amounts of data
Files can hold much more information than you could type into a program every time you run it. Instead of asking the user to type in 1,000 customer names every time, you can read them from a file.

3. Sharing data between programs
Different programs can read and write to the same file. For example, one program might collect data and save it to a file, and another program can read that file and analyze the data.

4. Creating records
Files provide a permanent record of what happened. For example, a program might write a log file that records every action it took, which can be useful for finding errors later.

Opening Files

Before you can read from or write to a file, you must open it first. When you open a file, you need to specify what you want to do with it — this is called the file mode.

There are three main file modes:

Python example:

file = open("students.txt", "r")    # Open for reading
file = open("students.txt", "w")    # Open for writing (deletes old content!)
file = open("students.txt", "a")    # Open for appending (adds to the end)

Pseudocode example:

OPENFILE "students.txt" FOR READ
OPENFILE "students.txt" FOR WRITE
OPENFILE "students.txt" FOR APPEND

Important: Always make a backup of important files before writing to them. One mistake with "w" mode and you could lose everything!

Reading from Files

Once a file is open in read mode, you can get information out of it.

Reading a single line of text:

The readline() command (in Python) or READFILE command (in pseudocode) reads one line of text from the file. Think of it like reading one sentence at a time from a page.

Python example:

file = open("fruit.txt", "r")
line = file.readline()  # Reads the first line
print(line)
file.close()

Pseudocode example:

OPENFILE "fruit.txt" FOR READ
READFILE "fruit.txt", LineOfText
OUTPUT LineOfText
CLOSEFILE "fruit.txt"

Reading multiple lines:

To read all the data in a file, you need to keep reading lines until you reach the end. You can use a loop (a section of code that repeats) to do this.

Python example:

file = open("employees.txt", "r")

endOfFile = False

while not endOfFile:
    name = file.readline().strip()
    
    if name == "":  # Empty string means we've reached the end
        endOfFile = True
    else:
        print("Name:", name)

file.close()

What is .strip()?
When you read a line from a file, it often includes extra spaces or a "newline" character (the invisible character that creates a new line). The strip() method removes these extra characters, giving you just the actual text.

In pseudocode, you use TRIM() to do the same thing:

name ← TRIM(name)

Complete Example: Reading Employee Data

Let's look at a complete example that reads employee information from a file.

The file (employees.txt) contains:

Greg
Sales
39000
43
Lucy
Human resources
26750
28
Jordan
Payroll
45000
31

Each employee has four lines of data: name, department, salary, and age.

Python program to read this data:

# Open file in read mode
file = open("employees.txt", "r")

endOfFile = False

while not endOfFile:
    name = file.readline().strip()          # Read line 1
    department = file.readline().strip()    # Read line 2
    salary = file.readline().strip()        # Read line 3
    age = file.readline().strip()           # Read line 4
    
    if name == "":  # Check if we've reached the end
        endOfFile = True
    else:
        print("Name:", name)
        print("Department:", department)
        print("Salary:", salary)
        print("Age:", age)
        print()  # Blank line for readability

# Close the file
file.close()

How this works:

1. Open the file in read mode
2. Set a flag (endOfFile) to False to start
3. Loop while we haven't reached the end:
   • Read four lines (one for each piece of information)
   • Remove extra spaces with strip()
   • Check if the first line is empty (means end of file)
   • If not empty, print all the information
4. Close the file when done