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CS 177 Week 9 Recitation Slides

Creating and Modifying Text

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ANY QUESTIONS?

Text and String

Like sound, text is usually processed in an array—a long line of characters

Strings are defined with quote marks: Single quotes ‘ my string ‘ Double quotes “ my string “ Triple quotes “““ my string “““

Question: When triple quotes are used?

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Encoding for Strings Strings are just arrays of characters Characters are either encoded in ASCII (single byte) or

unicode (2 bytes).

ord(char): returns the ASCI value of char. Example:

>>> str = “JES"

>>> for char in str:

... print ord(char)

74

69

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Backslash escape

“\b”: backspace “\n”: newline “\t” : tab “\uXXXX”: unicode of XXXX

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What is the output?

>>> print "Good\nMorning“ >>>print "Bo\bok“ >>> print "Good\tMorning“

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Getting Parts of a String

string[n] gives you the nth character in the string string[n:m] gives you the nth up to (but not including) the

mth character. string[0:len(string)] is the whole string

Example: >>> str = "mathematics“ >>> print str[4] e >>> print str[6] a

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Example continued

>>> print str[:4] math >>> print str[4:] ematics >>> print str[:] mathematics

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String Methods find(findstring) and find(findstring,start) and

find(findstring,start,end) finds the findstring in the object string and returns the index number where the string starts. You can tell it what index number to start from, and even where to stop looking. It returns -1 if it fails.

There is also rfind(findstring) (and variations) that searches from the end of the string toward the front.

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Example

>>> str = "one and one“

>>> str.find("ne")

1

>>> str.find("ne", 5)

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>>> str.find("on", 7)

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>>> str.rfind("o")

8

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More String Methods swapcase() makes all upper->lower and vice versa isalpha() returns true if the string is not empty and all

letters isdigit() returns true if the string is not empty and all

numbers replace(char1, char 2) replace every char1 with char 2

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Examples>>> str= "Purdue University"

>>> str.swapcase()

'pURDUE uNIVERSITY‘

>>> str = "computer science"

>>> str.upper()

'COMPUTER SCIENCE'

>>> str = "computer AND books"

>>> str.upper()

'COMPUTER AND BOOKS'

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Examples continued>>> str.lower()

'computer and books'

>>> str.title()

'Computer And Books‘

>>> str="123"

>>> str.isdigit()

1

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Examples continued

>>> str = "computer"

>>> str.isalpha()

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>>> str = "data"

>>> str.replace("a","z")

'dztz'

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Lists Lists can contain strings, numbers, even other lists.

Example:

>>> myList = ["X", "B", 3, "A", 1]

>>> print myList

['X', 'B', 3, 'A', 1]

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List methods

append(something) puts something in the list at the end. remove(something) removes something from the list … if

it’s there. sort() puts the list in alphabetical order reverse() reverses the list count(something) tells you the number of times that

something is in the list. max() and min() are methods (we’ve seen them before)

that take a list as input and give you the maximum and minimum value in the list.

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Examples

>>> myList = ["X", "B", 3, "A", 1]

>>> print myList

['X', 'B', 3, 'A', 1]

>>> myList.append("F")

>>> print myList

['X', 'B', 3, 'A', 1, 'F']

>>> myList.remove("B")

>>> print myList

['X', 3, 'A', 1, 'F']

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Examples continued>>> myList.sort()

>>> print myList

[1, 3, 'A', 'F', 'X']

>>> myList.reverse()

>>> print myList

['X', 'F', 'A', 3, 1]

>>> myList.count("A")

1

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Examples continued

>>> max(myList)

'X'

How?

Max returns the element with the maximum ASCII value. Looking up the ASCII values on the list, X value is 88 which is greater than all of the elements.

>>> min(myList)

1

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Element X F A 3 1

ASCII 88 70 65 51 49

Using lists to represent trees

>>> tree = [["Leaf1","Leaf2"],[["Leaf3"],["Leaf4"],"Leaf5"]]

>>> print tree[['Leaf1', 'Leaf2'], [['Leaf3'], ['Leaf4'], 'Leaf5']]>>> print tree[0]['Leaf1', 'Leaf2']>>> print tree[1][['Leaf3'], ['Leaf4'], 'Leaf5']>>> print tree[1][0]['Leaf3']>>> print tree[1][1]['Leaf4']>>> print tree[1][2]Leaf5

Leaf1

Leaf2

Leaf3Leaf4

Leaf5

The Point: Lists allow us to represent complex relationships, like trees

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Open File

Files can be opened for: Read Text (rt) Write Text (wt) Read or write bytes (rb)

In the command you specify the reason how you open your file:

open(filename,how)

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File Methods

open() returns a file object that you use to manipulate the file Example: file=open(“myfile”,”wt”)

file.read() reads the whole file as a single string. file.readlines() reads the whole file into a list where

each element is one line. read() and readlines() can only be used once without closing and

reopening the file. file.write(something) writes something to the file file.close() closes the file—writes it out to the disk, and

won’t let you do any more to it without re-opening it.

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Searching Phonebook

Create Phonebook

def phonebook():

return """

Mary:893-0234:Realtor:

Fred:897-2033:Boulder crusher:

Barney:234-2342:Professional bowler:"""

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Split the Entries in Phonebook

def phones():

phones = phonebook()

phonelist = phones.split('\n')

newphonelist = []

for list in phonelist:

newphonelist = newphonelist + [list.split(":")]

return newphonelist

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split by line

split elements in each line

Find People in Phonebook

def findPhone(person):

for people in phones():

if people[0] == person:

print "Phone number for",person,"is",people[1]

>>>findPhone(“Mary”)

Phone number for Mary is 893-0234

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Final QUESTIONS???