Reading Text Files in Delphi

Overview

This lecture covers the process of reading data from a text file into a Delphi program, emphasizing a seven-step algorithm to safely and efficiently handle file input.

Motivations for Using Text Files

• Inputting data via user prompts is temporary and lost when the program closes.
• Text files allow for saving and retrieving information for later use.
• Text files contain plain, unformatted text, stored line by line.

Seven-Step Algorithm for Reading Text Files in Delphi

• Declare variables: a text file variable (of type TextFile) and a string variable for each line.
• Check if the text file exists using FileExists('filename.txt').
• If the file does not exist, display an error message and exit the procedure.
• Assign the text file to the file variable using AssignFile(FileVar, 'filename.txt').
• Reset the file pointer to the beginning with Reset(FileVar).
• Use a loop (while not Eof(FileVar) do) to read each line until the end of the file.
  • Use ReadLn(FileVar, LineVar) to read a single line from the file into the string variable.
  • Process or display the current line as needed (e.g., adding to a display control).
• After completing all operations, close the file with CloseFile(FileVar).

Example: Displaying Team Names from a Text File

• A text file contains a list of NFL team names, one per line.
• The program reads each line and adds it to a display control (like a RichEdit).
• Steps include variable declaration, file existence checking, assignment, resetting, looping, reading, processing, and closing.

Key Terms & Definitions

• Text File — A file containing only plain text, usually with a .txt extension.
• TextFile Variable — A special Delphi variable type used to manage and access text files.
• FileExists — A function that checks if a specified file exists in the given location.
• AssignFile — A procedure that links a text file variable to an actual file on disk.
• Reset — Prepares the text file for reading, starting from the first line.
• ReadLn — Reads a full line from a text file into a string variable.
• Eof — A function that returns true if the end of the file is reached.
• CloseFile — Terminates the connection with the file, freeing system resources.

Action Items / Next Steps

• Practice implementing the seven-step file reading algorithm in Delphi.
• Review the example code and ensure understanding of each step in the process.
• Prepare to read and process different formats or content types from text files in future exercises.