CS 105 Lecture 9: Fun with Files Craig Zilles (Computer Science) - - PowerPoint PPT Presentation

Lecture 9: Fun with Files

Craig Zilles (Computer Science) October 25, 2019 https://go.illinois.edu/cs105fa19

CS 105

Are you sitti ting next t to someone to talk to for th the clicker questi tions? To Today I'm using: a text editor

and the command line Also, I'll post today's code

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To Today

• 1. Files
• Extensions,
• Writing, flushing, closing
• 2. Comma-separated values (CSV) files
• Reading, splitting, filter & find best patterns
• csv library
• 3. More on Lists
• 4. List Comprehesions

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Fi Files es

• Files are how data is stored on external storage (disk)
• Managed by O/S
• Disk is slow
• opening files moves them to memory for the program
• Data is buffered (temporarily stored) in memory

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Vi Viewing ng the he filesystem

• Mac Finder / Windows explorer
• Command line:
• Mac:

ls ls -l

• Windows:

dir

• Shows all of the files in the current directory
• Can show file size
• Looking at files (Mac): more
• ne page at a time

cat whole file at once

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Write programs a few lines at t a ti time! Test t every couple lines to make sure th they do what t you want! t!

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Wr Writing to files

• file_object = open('filename', 'w')
• file_object.write('thing to write')
• file_object.close() automatic at program end
• file_object.flush()
• ptional

Example program 1: Diary that records user input into a file. with open('filename', 'w') as outf: closes file when code block ends

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Di Diar ary that records user input into a a file.

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Co Continue e writing g to exi xisting g file? e? (i.e., new ew writes go to end of file)

A) open('filename', 'r') B) open('filename', 'x') C) open('filename', 'i') D) open('filename', 'a') E) open('filename', 'e')

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Com Comma-sepa separated ed value ue (CSV) files es

• Commonly-used data file format
• Can be generated from Excel and other spreadsheets

Processing a CSV manually:

• Each row is its own line of the file
• Can use split(',') to separate the columns
• Use indexing to read columns of interest

Example program 2: Create a list of Illinois U.S. representatives

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Cr Create a lis list t of

• f Illin

llinois

• is U.S

.S. . representativ tives

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Re Reading from files

• file_object = open('filename')
• lines = file_object.readlines()
• for line in lines:

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Sk Skipping first t line

A) for line in lines[:1]: B) for line in lines[1:]: C) for line in lines[:2]: D) for line in lines[2]: E) for line in lines[2:]:

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Fi Filter ering a a collec ection (p (patter ern)

newlist = [] for thing in collection: if thing meets criteria: newlist.append(thing)

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Sor Sorti ting data

• a_list.sort()
• a_list.sort(key = a_function)
• a_list.sort(reverse = True)

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An Annou

• unce

cements ts

• Exam 2 is ongoing
• Please don't start early; listen to the proctors
• Informal Early Feedback (IEF)
• Have results of scantron
• Still processing written responses
• Will discuss next week

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Fi Find da nd date o

• f hi

f highe hest une unempl ploym yment r rate

• Example 3

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Pa Pattern: Finding best in a collection

current_best = a value you know is worse than best for thing in collection: if thing is better than current_best: current_best = thing return / do something with current_best

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Fi Finding info asso associated ed with bes est

current_best = a value you know is worse than best best_info = None for thing in collection: if thing is better than current_best: current_best = thing best_info = info about thing return / do something with current_best, best_info

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Com Command line arguments ts

• Avoid hard coding things like file names

import sys sys.argv is a list of arguments

• First argument is the name of the Python script
• Give 'Usage' message to help user use your program

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