14-Strings-In-Python strings with oops .pdf

Strings in Python
Jerry Cain
CS 106AX
October 23, 2024
slides leveraged from those constructed by Eric Roberts

Selecting Characters from a String
• A string is (still) an ordered collection of characters. The
character positions in a Python string are, as in most computer
languages, identified by an index beginning at 0.
• You can select an individual character using the syntax str[k],
where k is the index of the desired character. The expression
returns the one-character string "w" that appears at index 7.
s[7]
• For example, if s is initialized as
s = "hello, world"
the characters in s are arranged like this:
0 1 2 3 4 5 6 7 8 9 10 11
h e l l o , w o r l d

Negative Indexing
• You can select the "w" toward the end of this string using the
expression
which is shorthand for the positive indexing expression
s[-5]
s[len(s) - 5]
• Unlike JavaScript, Python allows you to specify a character
position in a string by using negative index numbers, which
count backwards from the end of the string. The characters in
the "hello, world" string on the previous slide can therefore
be numbered using the following indices:
–12 –11 –10 –9 –8 –7 –6 –5 –4 –3 –2 –1
h e l l o , w o r l d

Concatenation
• One of the more familiar operations available to Python
strings is concatenation, which consists of combining two
strings end to end with no intervening characters.
• Concatenation is built into Python in the form of the +
operator. This is consistent with how JavaScript and most
other languages support concatenation.
• Noteworthy difference between Python and JavaScript:
Python interprets the + operator as concatenation only if both
operands are strings. If one of the operands is something other
than a string, then string concatenation isn’t applied.
Restated, Python doesn’t automatically convert numbers to
strings as JavaScript does.

Repetition
• In much the same way that Python redefines the + operator to
indicate string concatenation, it also redefines the * operator
for strings to indicate repetition, so that the expression s * n
indicates n copies of the string s concatenated together.
• The expression "la" * 3 therefore returns "lalala", which is
three copies of the string "la" concatenated together.
• Note that this interpretation is consistent with the idea that
multiplication is repeated addition:
"la" * 3 ® "la" + "la" + "la"
• You can use this feature, for example, to print a line of 80
hyphens like this:
print("-" * 80)

Exercise: String Repetition
• Use string repetition to encode the following songs in as short
a Python program as possible:
Let it be
Let it be
Let it be
Let it be
Whisper words of wisdom
Let it be
—Paul McCartney, "Let it Be", 1970
I can feel your halo halo halo
I can see your halo halo halo
I can feel your halo halo halo
I can see your halo halo halo
—Beyoncé, "Halo", 2008
• In 1984, computer Don Knuth published a paper on "The
Complexity of Songs" in which he concludes that Casey and
the Sunshine Band holds the record for longest song lyric.
u = " uh huh" * 2
s = ("That's the way" + u + " I like it" + u + "n") * ∞

Exercise: removeDoubledLetters
• In the early part of the 20th century, there was considerable
interest in both England and the United States in simplifying
the rules used for spelling English words, which has always
been a difficult proposition. One suggestion advanced as part
of this movement was to eliminate all doubled letters, so that
bookkeeper would be written as bokeper and committee
would become comite. Write a function
removeDoubledLetters(s)
that returns a new string in which any neighboring duplicated
characters in s have been replaced by a single copy.

Slicing
• Python allows you to extract a substring by specifying a range
of index positions inside the square brackets. This operation
is known as slicing.
• The simplest specification of a slice is [start:stop], where start
is the index at which the slice begins, and stop is the past-the-
end index where the slice ends.
• The start and stop components of a slice are optional, but the
colon must be present. If start is missing, it defaults to 0, and
if stop is missing, it defaults to the length of the string.
• A slice specification may also contain a third component
called a stride, as with [start:stop:stride]. Strides indicate
how many positions are omitted between selected characters.
• The stride component can be negative, in which case the
selection occurs backwards from the end of the string.

Exercise: Slicing
• Suppose that you have initialized ALPHABET as
ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
so that the index numbers (in both directions) run like this:
–12 –11 –10 –9 –8 –7 –6 –5 –4 –3 –2 –1
B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A
–24 –23 –22 –21 –20 –19 –18 –17 –16 –15 –14 –13
–26 –25
14 15 16 17 18 19 20 21 22 23 24 25
2 3 4 5 6 7 8 9 10 11 12 13
0 1
• What are the values of the following slice expressions?
(a) ALPHABET[7:9]
(b) ALPHABET[-3:-1]
(c) ALPHABET[:3]
(d) ALPHABET[-1:]
(e) ALPHABET[14:-12]
(f) ALPHABET[1:-1]
(g) ALPHABET[0:5:2]
(h) ALPHABET[::-1]
(i) ALPHABET[5:2:-1]
(j) ALPHABET[14:2:-3]

Methods for Finding Patterns
str.find(pattern)
Returns the first index of pattern in str, or -1 if it does not appear.
str.find(pattern, k)
Same as the one-argument version but starts searching from index k.
str.startswith(prefix)
Returns True if this string starts with prefix.
str.endswith(suffix)
Returns True if this string ends with suffix.
str.rfind(pattern)
Returns the last index of pattern in str, or -1 if it does not appear.
str.rfind(pattern, k)
Same as the one-argument version but searches backward from index k.

Methods for Transforming Strings
str.lower()
Returns a copy of str with all letters converted to lowercase.
str.upper()
Returns a copy of str with all letters converted to uppercase.
str.replace(old, new)
Returns a copy of str with all instances of old replaced by new.
str.capitalize()
Capitalizes the first character in str and converts the rest to lowercase.
str.strip()
Removes whitespace characters from both ends of str.

Methods for Classifying Characters
ch.isalpha()
Returns True if ch is a letter.
ch.isdigit()
Returns True if ch is a digit.
ch.isupper()
Returns True if ch is an uppercase letter.
ch.isspace()
Returns True if ch is a whitespace character (space, tab, or newline).
ch.isalnum()
Returns True if ch is a letter or a digit.
ch.islower()
Returns True if ch is a lowercase letter.
str.isidentifier()
Returns True if this string is a legal Python identifier.

Revisiting Pig Latin in Python
>>> toPigLatin("this is pig latin")
line result start i ch
"this is pig latin" "" -1 0 "t"
0 1 "h"
2 "i"
3 "s"
4 " "
word vp head tail
"this"
word i
"this" 0
ch
"t"
1
2
2 "th" "is"
"isthay"
"isthay" -1
"isthay " 5 "i"
5 6 "s"
7 " "
word vp head tail
"is" 0
"isway"
"isthay isway" -1
"isthay isway " 8 "p"
8 9 "i"
10 "g"
11 " "
word vp head tail
"pig" 1 "p" "ig"
"igpay"
"isthay isway igpay" -1
"isthay isway igpay " 12 "l"
12 13 "a"
14 "t"
15 "i"
16 "n"
17
word vp head tail
"latin" 1 "l" "atin"
"atinlay"
"isthay isway igpay atinlay"
isthay isway igpay atinlay

Exercise: Implement translate
Pig Latin is just one example of a language game designed to
render spoken words to be incomprehensible to the untrained
ear. B-Language is another such language game—known
primarily in Germany—where words are transformed such that
every vowel cluster is reduplicated with a leading "b".
Implement of translate, which accepts a lowercase (English)
word and returns its B-Language translation.
Examples:
translate("quick") Þ "quibuick"
translate("spaghetti") Þ "spabaghebettibi"
translate("adieu") Þ "abadieubieu"
translate("audiophile") Þ "aubaudiobiophibilebe"
translate("queueing") Þ "queueibueueing"

Exercise: Implement translate
def translate(word, sep='b'):
"""
Accepts the provided word, assumed to be comprised of
lowercase letters, and returns its B-Language translation.
An optional second argument can be supplied to override
the 'b' with any other string.
"""
start = -1
translation = ''
for i in range(len(word)):
ch = word[i]
if isEnglishVowel(ch):
if start == -1: start = i
else:
if start >= 0:
translation += word[start:i] + sep + word[start:i]
start = -1
translation += ch
if start >= 0:
translation += word[start:] + sep + word[start:]
return translation

Exercise: Implement extract
Uniform Resource Locators, or URLs, are string expressions
used to identify where documents can be found on the Internet.
Many URLs include query strings, which are substrings
appearing after a "?" that further refine what portion of a
document should be retrieved. For example:
https://www.google.edu/search?q=hello&tbm=vid
lists public videos associated with "hello" , which
unsurprisingly includes a whole lot of Adele.

Query strings are really just string serializations of maps in the
sense you learned about JavaScript aggregates, where keys are
separated from values by equal signs, and key-value pairs are
separated from one another by ampersands.
Implement extract, which lists all key value pairs, one per line,
embedded within the query string of a URL. For example:
extract("https://explorecourses.stanford.edu/search?view=catalog&"
"academicYear=&page=0&q=CS&filter-departmentcode-CS=on&"
"filter-coursestatus-Active=on&filter-term-Winter=on")
would print the following in the Terminal
Key: "view", Value: "catalog"
Key: "academicYear", Value: ""
Key: "page", Value: "0"
Key: "q", Value: "CS"
Key: "filter-departmentcode-CS", Value: "on"
Key: "filter-coursestatus-Active", Value: "on"
Key: "filter-term-Winter", Value: "on"

def extract(url):
"""
Parses the supplied URL and prints out all
of the key value pairs, one per line.
"""
qmpos = url.find('?')
if qmpos == -1: return
query = url[qmpos + 1:]
hpos = query.find('#')
if hpos != -1: query = query[:hpos]
start = 0
while start < len(query):
epos = query.find('=', start)
key = query[start:epos]
apos = query.find('&', epos + 1)
if apos == -1: apos = len(query)
value = query[epos + 1: apos]
print('Key: "{}", Value: "{}"'.format(key, value))
start = apos + 1

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