1. Strings, Lists, and Files
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Strings, Lists, and Files

2. Reading Assignment Chapter 5:– Sequences
I.e., Strings, Lists, and Files
CS-2301, D-Term 2014
Introduction

3. What is a String? String literal:–
Any piece of text enclosed in matching quotes
May be single or double quotes
“This sentence, of course, is written in English.”
‘Esta frase se escrita en español.’
‘Wir können auch in Deutsch und Französisch zu schreiben‘
“En fait, nous pouvons écrire les chaînes en Python en japonais, aussi.”
‘Pythonはあっても、私たちは日本語などのアジアの言語で書くことができます。’
Anything that you can type in ANY language can be represented and stored as a string in Python!
Unicode standard
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Introduction

4. String literal (continued)
A string literal is a constant, …
… just like a numerical value
Example:–
S = 'This is a string!'
The name S refers to the value ‘This is a string!’
Without the quotes
Quotes must be paired
Either single or double
CS-2301, D-Term 2014
Introduction

5. Operations on Strings + — concatenation * — repetition [] — indexing
S = "This is a string"
T = "and this is a another string"
U = S + ', ' + T + '.'
What is the value of U?
* — repetition
Y = "yuck, yuckity, "
Y * 5
[] — indexing
Like lists
S[0] T[1] U[2]
U[-1] U[-2]
CS-2301, D-Term 2014
Introduction

6. Operations on Strings (continued)
Slicing
I.e., taking a subset of a string
S = "This is a string"
T = S[5:7]
Special cases
U1 = S[-7:-1]
U2 = S[-7:]
U3 = S[:-7]
...
Starting position of substring
End of substring — This character is not included
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Introduction

7. Operations on Strings (continued again)
len(S) — length of string in characters
for c in S:
Iterate thru the characters of the string S
ord('c')
Get the Unicode character number of c
'c' must be a string of length 1
chr(int)
Return a string containing a single character, the ord of which is int
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Introduction

8. Useful string methods s.capitalize() s.count(sub) s.find(sub)
Count occurrences of a substring
s.find(sub)
Find a substring
s.strip([chars])
Removes designated characters from string
s.replace(old, new)
Replaces instances of old substring with new substring …
All return a (modified) copy of string
Do not update original string
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Introduction

9. Lists have methods, too! l.append() l.sort() l.reverse()
Our friend
l.sort()
Sorts elements in place. May be high-to-low or low- to-high
l.reverse()
Reverses the list
l.index(x) l.count(x)
Returns index of first occurrence of x in list or count of x’s in list
l.pop(i) l.remove(x)
Removes ith element (pop) or first occurrence of x
See p. 345, also Python documentation
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10. Questions?
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11. Definition — File
A (potentially) large amount of information that lives a (potentially) very long time
May be (much) larger than the amount of RAM in your computer
(Usually) expected to outlive the running of your program
(May be) expected to outlive the computer itself!
Stored on
Hard drive
Flash drive
Spread out across multiple disks
Somewhere in the “cloud”
On some other medium …
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Introduction

12. Files (continued) (Usually) stored as a sequence of bytes
Byte: an 8-bit character
The standard unit of storage since 1964
Other data types built up from sequences of bytes
Organization of data within file defined by application
Text
Numerical data
Big databases
Program code …
Directory (a.k.a. folder) — a list names and locations of other files
Maintained by operating system
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Introduction

13. Remember to close your files before exiting your program!
Using Files
Must be Opened before use
Tells OS to make file ready for access
Must be Closed when finished
Tells OS to “put the file away”
Make it safe for long term storage
Note: Most operating systems automatically close files that are still open when program exits
Don’t depend on this!
Stale data may live in volatile memory for long time
Where it can become corrupted …
… or forgotten …
… or lost
… before OS gets around to writing to disk!
Remember to close your files before exiting your program!
CS-2301, D-Term 2014
Introduction

14. Three other modes:–. 't' – text mode (default). 'b' – binary
Three other modes:– 't' – text mode (default) 'b' – binary '+' – update in place
Open
Gets a file read for use
OS sets up internal tables
May fetch copy off remote disk
Validates protection,
Etc.
f = open(filename, mode)
Built-in function
Filename
String of text
Name of the file (as seen in directory), with extension
Possibly including directory “path”
Mode
'r' – read (default)
'w' – write (truncate to zero length)
'a' – append
… (other modes — see Python documentation)
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Introduction

15. Close f.close() = open(filename, mode) File method Closes the file
Clears internal buffers
I.e., puts it away safely
Don’t forget to do this in your program!
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Introduction

16. Reading from files f = open(filename, mode) f.read()
Reads entire remaining contents of file into one (potentially humungous) string
Line endings represented by '\n' characters
“Remaining” means from where we left off reading most recently to end of file
f.readline()
Reads one line
Including '\n' character (a.k.a. newline character)
Returns line as a string
f.readlines()
Reads all of the lines
Returns a list of strings
Each representing one line — as from readline()
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Introduction

17. Reading from files (continued)
Homework #4 requires you to read multiple files
Process each one
More useful information about file reading:–
§5.9.2 of textbook
Chapter 7 of online Python tutorial
In IDLE, select Help > Python Docs
Click Contents tab, select The Python Tutorial
Click Input and Output
About 7 lines down!
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18. Homework #4
Once you have opened a file, need to extract all of the individual words
Add to a list
How to do that?
Questions?
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19. Writing to a file Also required in Homework #4
f = open(outputfilename, 'w')
f.write(string)
Writes the string at the end of the file
Returns number of bytes written
You need to supply trailing '\n' character
To denote end of line
You may write partial lines
i.e., with no trailing '\n'
You may write multiple lines at one time
Until you are more skilled, concentrate on writing one full line at a time!
CS-2301, D-Term 2014
Introduction

20. What next? Close the file!
Note:– both Python and OS keep contents of file buffered in memory
I.e., volatile memory!
Closing flushes the buffers to disk
Where it is stored safe from (most) failures
CS-2301, D-Term 2014
Introduction

21. Questions?
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