Chapter 13: Programing and Data Representation

Overview of the chapter:

• 13.9 Text files

13.9 Handling text files

Some data needs to be stored permanently. If stored in variables data will be lost when program stops / powered off. If stored in a file the data is no longer volatile.

You need to be able to perform the following tasks:

• Open the file, in read / write / append mode
• Operate the file, read / write
• Close the file

In the following languages:

• Python
• Pseudocode

13.9.1 Opening files

There are three methods of opening files: Read, Write, Append.

Mode	Explanation	Pseudocode syntax	Python Syntax
Read	Open the file for reading its contents.	OPENFILE <filename> FOR READ	file = open(“<filename>”, “r”)
Write	Open the file for writing new contents. By writing all original data will be overridden.	OPENFILE <filename> FOR WRITE	file = open(“<filename>”, “w”)
Append	Add more data to the end of the file.	OPENFILE <filename> FOR APPEND	file = open(“<filename>”, “a”)

Note that:

• In pseudocode there is no need to covert filename to string. But in python the filename must be in a string.
• In pseudocode the modes are key words, while in python the mode are strings.

It is not recommended to use the method of “WITH OPEN”. The method doesn’t need to close the file explicitly, but there is a mark given for the command. Teachers will often miss that mark if you use “WITH OPEN”.

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13.9.2 File operations

Command	Pseudocode	Python
Read the next line from file	READFILE <filename>, <variableToAssign>	<variableToAssign> = file.readline()
Write a line to file	WRITEFILE <filename>, <stringValue>	file.write(<stringValue>)

The READLINE command reads the next line of the file.

If the file is opened in “READ” mode, the WRITEFILE command will first erase everything in the file, then write the stringValue in.

If the file is opened in “APPEND” mode, the WRITEFILE command will only add the stringValue to the end.

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13.9.3 Closing the file

Always close the file after editing. This command will update all the changes to the file.

Command	Pseudocode	Python
Close the file	CLOSEFILE <filename>	file.close()

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13.9.4 End-of-file marker

The marker is the only way to identify the whole file had been finished reading.

Command	Pseudocode	Python
Find if the file have been read completely	EOF( ) // The function returns TRUE //if the end of file had been reached.	data = file.readline( ) if len(data) == 0: //End of file if the line read is empty.

Sample code:

//Pseudocode

DECLARE total, grade: INTEGER <- 0

OPENFILE Grades.txt FOR READ
WHILE NOT EOF("Grades.txt") DO     //Yes, the parameter is a STRING!
    READFILE Grades.txt, grade
    total <- total + grade
ENDWHILE

CLOSEFILE Grades.txt

#Python
total, grade = 0, 0

file = open("grades.txt", "r")
grade = "-"

while len(grade) != 0:
    grade = file.readline()
    if len(grade) != 0:      #Yes, I want to use break too...
        total += int(grade)

file.close()