Lecture 10, Wed 11/07

File IO

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Some tools for the labs

min, max, round

print(max(15.7,2,16.2,8)) print(min(16.2,6)) print(“—”) print(round(14.9999999999)) print(round(15.5)) print(round(15.4999999999))

Files

- FILES are a valuable tool to help us solve many
types of problems.

FILES give us PERSISTENCE - So far, we’ve been running our programs in IDLE and putting our code into a file. * Data must be entered on every program run * Programs have no way to write permanent output - With PERSISTENCE, our data can be “saved” between each program execution.

FILE BASICS: - We can store files in many different forms - Examples: .xls, .docx, .pdf, .jpg, … - For this class, we’ll just deal with “plain text” files (.txt) - These CHARACTERS are represented in something called ASCII (American Standard Code for Information Interchange) - This was dominant / simple way of representing text where each character is 8 bits long - UTF-8 is the most popular format in today’s web browsers - Allows us to represent MANY characters from multiple languages

TERMS:

Directory: A folder containing files and other folders
File System: Collection of all the files and folders on the
    computer, organized in a heirarchy
For this class, we'll deal with reading and writing files
that are in the same directory as our .py file (known as
our "working directory"
    - This makes our lives much easier

The Unix File system:

• In unix the directory at the highest level of the hierarchy is called the root (denoted by /). All other directories and files are stored within the root

• Path: The path is a sequence (of directories) that specifies the location of a file or directory within the file system. For example, /Users/diba/ says that the directory diba is within the directory Users which is within the root

  • An ABSOLUTE path describes the location of a file or directory starting with the root (/)
  • A RELATIVE path describes the location of a file relative to the current directory. For example ./cs8/lab05/ (Here ‘./’ stands for “current directory” )
• You can move through the unix file system via the command line (instead of using the graphical interface)
• Few useful unix commands
  • pwd (path to your current working directory)
  • ls (list all the files and directories within the current directory)
  • mkdir (make a new directory)
  • cd (change into a directory, need to give either absolute or relative path)

FILE I/O: - I/O stands for input / output - We read data from a file into our program. - We write data from our program into a file. - Steps for File I/O 1. Open the file (creates a “connection” between your program and the file). - Choose if the connection will be for reading, writing, or appending to a file. 2. Read the data / write the data 3. Close the file (close the “connection”). This should to be done once per file.

Common ways to read data from files 1. read() method - reads the entire file into one string - Good for small data (large files may be too big to store into memory) 2. read(n): Read the next n characters from the input - Better for larger files since you only need to store n characters in memory at a time. 3. readline(): Reads everything from the current position to the next ‘\n’ (or to the end of the file, ‘EOF’). If nothing left to read, .readline() returns an empty string. 4. readlines(): Reads all the lines in the file and returns a list. 5. for a_line in infile: - a_line represents a line in the file, infile is the open file. ‘’’

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Example reading from ‘example.txt’

infile = open(‘example.txt’, ‘r’) data = infile.read() print(data) infile.close() ‘’’ ‘’’

Example writing to a file ‘example_2.txt’

outfile = open(‘example_2.txt’, ‘w’) outfile.write(“Duck\nCow\nCat”) outfile.close() ‘’’ ‘’’

Create a list of lines in the file

infile = open(‘example.txt’, ‘r’) datalines = infile.readlines() # returns a list of strings print(datalines) infile.close() ‘’’ ‘’’

Write “overwrites” the existing file

outfile = open(“example.txt”, ‘w’) outfile.write(“Something new!\n”) outfile.write(“Another line!”) outfile.close() ‘’’ ‘’’

Append to an existing file

outfile = open(“example.txt”, ‘a’) outfile.write(“Something else!\n”) outfile.write(“Yet another line.”) outfile.close() ‘’’ ‘’’

read(n)

infile = open(“example.txt”, ‘r’) data = infile.read(3) print(data) data = infile.read(6) print(data) infile.close() ‘’’ ‘’’

readline example

infile = open(“example.txt”, ‘r’) line = infile.readline() print(line) line = infile.readline() print(line) infile.close() ‘’’ ‘’’

Example of copying / writing to another file

infile = open(‘example.txt’, ‘r’) outfile = open(‘copy.txt’, ‘w’) for line in infile: outfile.write(line) infile.close() outfile.close() ‘’’