Lesson2: String Operations, Writing Scripts › ~naraehan › ling1901 › Lesson2.pdf · String...

Lesson2: String Operations,

Writing Scripts

Fundamentals of Text Processing for Linguists

Na-Rae Han

Objectives

Writing and executing Python scripts

Learn Python basic syntax

Variable assignments: = vs. +=

Equality comparison: ==

String operations

Boolean operators: and, or, not

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Interactive programming vs. scripts

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We have been programming INTERACTIVELY through IDLE shell.

We will now try writing and running a stand-alone

Python SCRIPT.

x = 'a'+'b' print x … …

IDLE interprets and responds to each line

of code you type in

executed Python

script file

You have to retype the code next time

Your IDLE shell

1. Open up a new window (Ctrl+n).

2. Python editor window opens up. Type in:

3. Save the script as "hello.py".

Don't forget to include ".py" extension!

4. Run the script by selecting "Run" > "Run Module" or pressing F5.

5. Success! The command executes in the IDLE shell window.

Python script in IDLE editor mode

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print "Hello, world!!"

String operations

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String: A single piece of text, composed of a sequence of letters.

Operations on string objects:

print statement. prints string

len() returns integer

+, += returns string

.endswith(), .startswith() returns True/False

in returns True/False

.upper(), .lower() returns string

.replace() returns string

.split() splits a string into a list, returns it

Concatenation and variable assignment

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>>> v = 'walk'

>>> v + 'ed'

'walked'

>>> v

'walk'

>>> vd = v + 'ed'

>>> vd

'walked'

>>> v

'walk'

>>> v = v + 'ed'

>>> v

'walked'

>>> v == vd

True

>>> v = 'walk'

>>> v + 'ed'

'walked'

>>> v

'walk'

>>> vd = v + 'ed'

>>> vd

'walked'

>>> v

'walk'

>>> v = v + 'ed'

>>> v

'walked'

>>> v == vd

True

Concatenation does not change original

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Value of v is unchanged!

Concatenation operation

>>> v = 'walk'

>>> v + 'ed'

'walked'

>>> v

'walk'

>>> vd = v + 'ed'

>>> vd

'walked'

>>> v

'walk'

>>> v = v + 'ed'

>>> v

'walked'

>>> v == vd

True

New variable

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New variable vd is assigned to the output

of concatenation

v is still unaffected

>>> v = 'walk'

>>> v + 'ed'

'walked'

>>> v

'walk'

>>> vd = v + 'ed'

>>> vd

'walked'

>>> v

'walk'

>>> v = v + 'ed'

>>> v

'walked'

>>> v == vd

True

Changing the original variable

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Here, v is assigned a new value: its former self suffixed with 'ed'

>>> v = 'walk'

>>> v + 'ed'

'walked'

>>> v

'walk'

>>> vd = v + 'ed'

>>> vd

'walked'

>>> v

'walk'

>>> v = v + 'ed'

>>> v

'walked'

>>> v == vd

True

==: Equality comparison

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Double equal sign: confirms that

v and vd have an equal value!

11

Equality comparison vs. assignment

name = value

=

value1 == value2 ==

Attaches a name to a value

Compares two values, returns True or False

>>> num = 5

>>> num = num + 2

>>> num

7

Right Hand Side (RHS) first

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num has different values in one statement. How could this be?

Answer: Right hand side is evaluated first, and then variable assignment happens.

This num has value 5

This num has value 7

>>> num = 5

>>> num = num + 2


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int 5

1. Integer 5 is created in Python memory


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2. Variable num is created, points to the memory location

int 5 num >>> num = 5

>>> num = num + 2


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3. num + 2 is evaluated; value 7 is created in memory

int 5

int 7

num >>> num = 5

>>> num = num + 2


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4. The name num is then attached to integer 7 in Python memory

int 5

int 7

num >>> num = 5

>>> num = num + 2


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4. The name num is then attached to integer 7 in Python memory

5. 5 is no longer needed; gets freed up from memory

int 5

int 7

num ✘ >>> num = 5

>>> num = num + 2

+ and +=

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>>> homer = 'doh'

>>> print homer

doh

>>> homer = homer + '!'

>>> print homer

doh!


>>> print homer

doh!!

>>> homer += '!'

>>> print homer

doh!!!

>>> homer += '!'

>>> print homer

doh!!!!

>>> homer = 'doh'

>>> print homer

doh


>>> print homer

doh!


>>> print homer

doh!!

>>> homer += '!'

>>> print homer

doh!!!

>>> homer += '!'

>>> print homer

doh!!!!

+ and +=

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Do exactly the same thing:

suffixing homer with '!'

Augmented assignments

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foo += 10

foo -= 10

foo *= 10

foo /= 10

foo **= 10

foo %= 10

foo = foo + 10

foo = foo - 10

foo = foo * 10

foo = foo / 10

foo = foo ** 10

foo = foo % 10

Subtract 10

Multiply by 10

Power to 10

Divide by 10

Modulo 10

Practice

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2 minutes

Repeated commands: Use and!

>>> chor = 'tra'

>>> chor = chor + '-la'

>>> print chor

tra-la

>>> chor += '-la'

>>> print chor

tra-la-la

>>> chor += '-la'

>>> print chor

tra-la-la-la

>>> num = 2

>>> num += 1

>>> print num

3

>>> num += 1

>>> print num

4

>>> num *= 2

>>> print num

8

>>> num *= 2

>>> print num

16

Also try: -= /= **=

.startswith(), .endswith()

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>>> len('cat')

3

>>> 'cat'.endswith('t')

True

>>> 'cat'.startswith('t')

False

>>> 'cat'.endswith('')

True

>>> c = 'cat'

>>> c.endswith('at')

True

>>> c.endswith('cat')

True

>>> c.startswith('cat')

True

>>> len('cat')

3

>>> 'cat'.endswith('t')

True

>>> 'cat'.startswith('t')

False


True

>>> c = 'cat'

>>> c.endswith('at')

True

>>> c.endswith('cat')

True

>>> c.startswith('cat')

True

.startswith(), .endswith()

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.endswith() can be called on a string

or a variable whose value is a string


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>>> 'cat'.endswith('at')

True

>>> not 'cat'.endswith('at')

False

>>> not True

False

>>> not False

True

>>> 'cat'.endswith('at') and 'dog'.endswith('g')

True

>>> 'cat'.endswith('at') and 'dog'.endswith('at')

False

>>> 'cat'.endswith('at') or 'dog'.endswith('at')

True

>>> 'cat'.endswith('at') and not 'dog'.endswith('at')

True


True


False

>>> not True

False

>>> not False

True


True


False


True


True


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not is a unary operator: not A flips the truth value of A


True


False

>>> not True

False

>>> not False

True


True


False


True


True


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and and or are binary operators: they take

two arguments A and B A or B

in as a substring operation

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>>> 'a' in 'cat'

True

>>> 'ca' in 'cat'

True

>>> 'cat' in 'scattered'

True

>>> 'et' in 'scattered'

False

>>> '' in 'scattered'

True

>>> 'a' in 'cat'

True

>>> 'ca' in 'cat'

True


True


False


True

in as a substring operator

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in as a "substring" operator A in B

True if A is a substring of B False otherwise

>>> 'a' in 'cat'

True

>>> 'ca' in 'cat'

True


True


False


True

>>> 'cat' not in 'scattered'

False

not in

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negating in with not

The empty string

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>>> ''

''

>>> len('')

0

>>> 'cat'.startswith('')

True


True

>>> ''.startswith('')

True

>>> ''.endswith('')

True

>>> ''.endswith('t')

False

>>> '' in 'cat'

True

>>> '' in ''

True

>>> ''

''

>>> len('')

0


True


True


True

>>> ''.endswith('')

True


False

>>> '' in 'cat'

True

>>> '' in ''

True

Strings can have 0 length

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'' is an empty string; the length is 0

>>> ''

''

>>> len('')

0


True


True


True

>>> ''.endswith('')

True


False

>>> '' in 'cat'

True

>>> '' in ''

True

'' starts and ends every string

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The empty string '' begins and ends

every string – even the empty string

itself!

>>> ''

''

>>> len('')

0


True


True


True

>>> ''.endswith('')

True


False

>>> '' in 'cat'

True

>>> '' in ''

True

'' is a substring of every string

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The empty string '' is a substring of every string –

even the empty string itself!

>>> not True

False

>>> not False

True


True


False


True


True

>>> 'ca' in 'cat'

True


True


False

Practice

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2 minutes

Also try with empty string ''

.upper(), .lower(), .capitalize()

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>>> sign = 'Please be quiet'

>>> sign.upper()

'PLEASE BE QUIET'

>>> sign.lower()

'please be quiet'

>>> sign.capitalize()

'Please be quiet'

>>> 'hello!'.upper()

'HELLO!'

>>> 'hello!'.upper().lower()

'hello!'

>>> 'hello!'.capitalize()

'Hello!'

.upper(), .lower()

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>>> sign.upper()

'PLEASE BE QUIET'

>>> sign.lower()

'please be quiet'


'Please be quiet'


'HELLO!'


'hello!'


'Hello!'

capitalize and lower-case every character in

a string

.capitalize()

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>>> sign.upper()

'PLEASE BE QUIET'

>>> sign.lower()

'please be quiet'


'Please be quiet'


'HELLO!'


'hello!'


'Hello!'

capitalizes the first character

in the string


'HELLO!'


'hello!'


'Hello!'


>>> sign.upper()

'PLEASE BE QUIET'

>>> sign.lower()

'please be quiet'


'Please be quiet'

Nested functions

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uppercases 'hello!' and then

lowercases it back!

.replace()

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>>> foo = 'hello, world!'

>>> foo.replace('l', 'r')

'herro, worrd!'

>>> foo.replace('l', 'r').replace('r', 'l')

'hello, wolld!'

>>> faa = 'colour rumour'

>>> faa.replace('our', 'or')

'color rumor'

>>> mary = 'Mary had a little lamb'

>>> print mary

Mary had a little lamb

>>> mary.replace(' ', '')

'Maryhadalittlelamb'

.replace()

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'herro, worrd!'

>>> foo.replace('l', 'r') .replace('r', 'l')

'hello, wolld!'



'color rumor'


>>> print mary




replaces every instance of 'l' with 'r'

Stacking .replace()

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'herro, worrd!'


'hello, wolld!'



'color rumor'


>>> print mary




replaces every instance of 'l' with 'r',

and then replaces every 'r' with 'l'

.replace()

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'herro, worrd!'


'hello, wolld!'



'color rumor'


>>> print mary




replaces a string ('our') with another string ('or')

Text has been Americanized!

.replace() for removing

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'herro, worrd!'


'hello, wolld!'



'color rumor'


>>> print mary




Removing every space: achieved by

replacing '' with the empty string ''

Splitting a string with .split()

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>>> mary

'Mary had a little lamb'

>>> print mary


>>> mary.split()

['Mary', 'had', 'a', 'little', 'lamb']

>>> mary.split(' ')


>>> mary.split('a')

['M', 'ry h', 'd ', ' little l', 'mb']

>>> len(mary)

22

>>> len(mary.split())

5


>>> mary


>>> print mary


>>> mary.split()


>>> mary.split(' ')


>>> mary.split('a')

['M', 'ry h', 'd ', ' little l', 'mb']

>>> len(mary)

22


5


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splits on

every ''

splits on every 'a'


>>> mary


>>> print mary


>>> mary.split()


>>> mary.split(' ')


>>> mary.split('a')

['M', 'ry h', 'd ', ' little l', 'mb']

>>> len(mary)

22


5


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No separator given: splits on

whitespace

splits on

every ''

Same result in this case, but

not always


>>> mary


>>> print mary


>>> mary.split()


>>> mary.split(' ')


>>> mary.split('a')

['M', 'ry h', 'd ', ' little l', 'mb']

>>> len(mary)

22


5

len() works with strings and lists

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len(string) returns the length

of string: # of characters

len(list) returns the # of

items in a list

Splitting and whitespace characters

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>>> hi = 'Hello mother,\nHello father.'

>>> print hi

Hello mother,

Hello father.

>>> hi.split()

['Hello', 'mother,', 'Hello', 'father.']

>>> hi.split('\n')

['Hello mother,', 'Hello father.']

>>> record = 'Lisa Simpson\tBorn 8/12 2002\tSpringfield'

>>> print record

Lisa Simpson Born 8/12 2002 Springfield

>>> record.split('\t')

['Lisa Simpson', 'Born 8/12 2002', 'Springfield']

Nope!

>>> la = 'la di da'

>>> la.split()

['la', 'di', 'da']

>>> la.split(' ')

['la', 'di', '', '', 'da']

>>> foo = 'colorless green ideas'

>>> foo.split('e')

['colorl', 'ss gr', '', 'n id', 'as']

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So, as long as a string contains only

spaces and no other whitespace, .split()

is the same as .split(' '), right?

three spaces ' '

.split() default behavior

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If no separator is specified:

Splits on whitespace (includes space, line break \n, tab \t).

Repeated whitespaces do NOT result in empty string tokens.

If a separator is specified, then:

Repetition DOES result in empty string tokens.

>>> la = 'la di da'

>>> la.split()

['la', 'di', 'da']

>>> la.split(' ')

['la', 'di', '', '', 'da']

>>> foo = 'colorless green ideas'

>>> foo.split('e')

['colorl', 'ss gr', '', 'n id', 'as']

Practice 1

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2 minutes

Practice 1

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2 minutes

Practice 2

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2 minutes

Practice 2

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2 minutes

Practice 3

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2 minutes

Practice 3

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2 minutes

Practice 4

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Write a Python script that:

internally stores the "Fox in Sox" text as a string variable named "fox", http://www.pitt.edu/~naraehan/ling1901/sample-texts.txt

prints out the text,

prints out how many lines are in the text,

prints out how many words are in the text,

prints out how many characters are in the text,

prints out how many times 'ree' occurs in the text.

Use .count() method. Try and figure it out!

3 minutes

http://www.pitt.edu/~naraehan/ling1901/sample-texts.txt



A note on Python syntax

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✔ ✔

✘

✘

if... : elif ... : else :

Conditional

Looking ahead

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>>> grade = 87

>>> if grade >= 90 :

print 'You got an A'

elif grade >= 80 :

print 'You got a B'

else :

print 'Try harder!'

You got a B

>>>

Wrap-up

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If you want to try more commands, visit:

A Beginner's Python Tutorial, Lesson 6

http://www.sthurlow.com/python/lesson06/

Exercise #2

http://www.pitt.edu/~naraehan/ling1991/exercise.html#ex2

Due Tuesday midnight

http://www.sthurlow.com/python/lesson06/

http://www.pitt.edu/~naraehan/ling1991/exercise.html

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