Reading and Writing Files

Using Files for Data Storage Allows data to be retained between program runs It’s lost when a local variable goes out of scope or when the program terminates Computers store files on secondary storage devices hard disks, optical disks, flash drives and magnetic tapes. Data maintained in files is persistent data because it exists beyond the duration of program execution. Used in many applications: Word processing, Image Editors, Databases, Spreadsheets, Compilers, Games and Web Browsers File: a set of data stored on a computer, often on a disk drive Programs can read from and/or write to files Steps: Open the file Use the file (read from, write to, or both) Close the file

Data Hierarchy The smallest data item in a computer can assume the value 0 or 1. Such data items are called bits Python uses Unicode characters that are composed of two bytes, each composed of eight bits(lang) Fields are composed of characters or bytes. Data items processed by computers form a data hierarchy that becomes larger and more complex in structure as we progress from bits to characters to fields, and so on. Typically, several fields compose a record (implemented as a class in Java). A file is a group of related records.  Database

Opening a File Stream The first part of read/write a file is to create a stream to the file. A stream is a one way path for information to travel Code: varName = open(fileName, mode) Ex. fileWrite = open(“test.txt”,”w”) input=open(“c:\\py\\tst.txt”,”r”)

Writing Files To write to a file use the file stream name like a variable with the write function. Code: write(str) Ex. output = open(“city.txt”) output.write(“blah blah blah\n”) Note: Loops can be used to write numerous values to a file

Reading Files Reading a file is similar to writing a file in that you use the file stream name followed by one of the read functions. Code: read() – reads all characters from a file read(#) – read a number of characters readline() – reads all characters up to and including \n readlines() – reads all lines

Reading Data From a File Two main ways to read data from a file: 1. While loop that iterates to the end of the file Code: line = infile.readline() while line != ‘’: #process line here line = infile.readline() 2. A enhanced for loop Code: for varname in fileName #process the line here

Closing a File Stream Clean up is a necessary and essential step. Just like when you make a mess, life will go on but there are implications of not cleaning up. Ensure you close a stream after you have processed all of your data. Code: fileStream.close()

Reading and Writing Files Example def main(): # Prompt the user to enter filenames f1 = input("Enter a source file: ").strip() f2 = input("Enter a target file: ").strip() # Open files for input and output infile = open(f1, "r") outfile = open(f2, "w") # Copy from input file to output file countLines = countChars = 0 for line in infile: countLines += 1 countChars += len(line) outfile.write(line) print(countLines, "lines and", countChars, "chars copied") infile.close() # Close the input file outfile.close() # Close the output file main() # Call the main function

Checkpoint 1. What are the 3 phases to a file stream? 2. What are some different ways to setup your file stream? 3. What are the key differences between a read and write file stream?