File I/O (continued)

Winter 2025

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Adrian Salguero

Announcements

• Midterm information now available on the website
• Midterm practice problems also available on the website
• Homework 3 due tonight at 11:59pm
• Homework 4, Part 1 will be released on Monday
• Coding Practice 4 now available, due Wednesday February 5th at 11:59pm

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Files and Filenames

• A filename states where your program should look for the given file
• Paths are how we specify where to search for these files
• Without the correct path, our program does not know how to search for a file
• Step by step instructions on how to reach a file
• Relative/Absolute directories
• Relative: give instructions starting from where you are currently at
• Absolute: give instructions starting from the first drive

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Navigating File Directories

• Which directory am I in? Where am I in my computer?
• Can also use pwd to print the current working directory or current folder or current location in your computer you are in

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The path to a file you can use in your program

Navigating File Directories

• How can I tell which folders exist in my current directory/location?
• Type ls into your terminal → this will list all the folders and files held in your current directory

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Navigating File Directories

• How do I change directories? How do I move into a folder? Or out of a folder?
• cd <folder_name/directory_name> to enter the folder
• cd .. to go “up” on folder or one directory

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Opening/Reading a File

myfile = open(filepath)

for line in myfile:

# Read file contents

myfile.close()

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myfile = open(filepath, “r”)

for line in myfile:

# Read file contents

myfile.close()

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with open(filepath) as f:

# Read file contents

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By default, file is opened for reading

Adding "r" makes read-only explicit.

Writing/Appending to a File

myfile = open(filepath, “w”)

# Write to your file

myfile.close()

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myfile = open(filepath, “a”)

# Append (add) to your file

myfile.close()

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Does the file exist?

• “w” will overwrite it
• “a” will add to it

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Does the file not exist?

• Both “w” and “a” will create it

Adding "a" opens for appending.

Adding "w" opens for writing.

How does reading look like?

myfile = open(filename)

for line in myfile:

print(line)

myfile.close()

This is a line

More text

80, 50, 100

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Line

“This is a line”

“More text”

“80, 50, 100”

Python reads these numbers as a string!

split() and strip() methods

• split([delimiter]) - returns a list of strings that are separated by a given delimiter. If no delimiter is specified, defaults to blank space

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• strip([character]) - removes any leading or trailing characters. If no characters are specified, defaults to white space

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Reading a file Example

# Count the number of words in a text file

in_file = "song.txt"

myfile = open(in_file)

num_words = 0

for line_of_text in myfile:

word_list = line_of_text.split()

num_words += len(word_list)

myfile.close()

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print("Total words in file: ", num_words)

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Writing to a file Example

# Replaces any existing file of this name

myfile = open("output.dat", "w")

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# Similar to printing output

myfile.write("a bunch of data")

# but you must add newline if desired, unlike print

myfile.write("a line of text\n")

# and the argument must be a string

myfile.write(4)

myfile.write(str(4))

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myfile.close()

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open for Writing�(give no argument, or

"r", for Reading)

“\n” means end of line (Newline)

Incorrect; results in:

TypeError: expected a character buffer object

Similar to if you tried to do print(4)

Correct. Argument must be a string

close when done with all writing

Next thing written will be on this same line.

Generate 100 rows of random “data”

import random

myfile = open("sample_data.dat", "w")

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for i in range(100):

for j in range(9):

myfile.write(str(random.randint(0, 100)))

myfile.write(",")

myfile.write(str(random.randint(0, 100)))

myfile.write('\n')

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myfile.close()

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Practice Problem

Suppose we have many, many rows of data separated by commas (csv files!). Write a Python program that reads the data line by line, calculates the average value of each row, and prints out a list of all the average values.

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Bonus: instead of printing out the list, write the averages, line by line, to a new file

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Solution: Practice Problem

Will be posted after class

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