Strings

Strings

Copyright © Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

Python

Python Strings

Strings are sequences of characters

Python Strings


No separate character type: just a string of length 1

Python Strings



Indexed exactly like lists

Python Strings



Indexed exactly like lists

name = 'Darwin'print name[0], name[-1]D n

Python Strings

for iterates through characters

Python Strings

for iterates through characters

name = 'Darwin'for c in name: print cDarwin

Python Strings

Use either ' or " (as long as they match)

Python Strings


print 'Alan', "Turing"Alan Turing

Python Strings



Strings are the same no matter how they're created

Python Strings



Strings are the same no matter how they're created

print 'Alan' == "Alan"True

Python Strings

Strings are compared character by character

from left to right

Python Strings


from left to right

print 'a' < 'b'True

Python Strings


from left to right

print 'a' < 'b'Trueprint 'ab' < 'abc'True

Python Strings


from left to right

print 'a' < 'b'Trueprint 'ab' < 'abc'Trueprint '1' < '9'True

Python Strings


from left to right

print 'a' < 'b'Trueprint 'ab' < 'abc'Trueprint '1' < '9'Trueprint '100' < '9'True

Python Strings


from left to right

print 'a' < 'b'Trueprint 'ab' < 'abc'Trueprint '1' < '9'Trueprint '100' < '9'Trueprint 'A' < 'a'True

Python Strings

Strings are immutable : cannot be changed in place

Python Strings


name = 'Darwin'name[0] = 'C'TypeError: 'str' object does not support item assignment

Python Strings



Immutability improves performance

Python Strings



Immutability improves performance

See later how immutability improves programmers'

performance

Python Strings

Use + to concatenate strings

Python Strings


name = 'Charles' + ' ' + 'Darwin'print nameCharles Darwin

Python Strings



Concatenation always produces a new string

Python Strings




'Charles'originaloriginal = 'Charles'

Python Strings




'Charles'

name

originaloriginal = 'Charles'name = original

Python Strings




original = 'Charles'name = originalname += ' Darwin'

'Charles'

'Charles Darwin'name

original

Python Strings

Often used to format output

Python Strings


print 'reagant: ' + str(reagant_id) + ' produced ' + \ str(percentage_yield) + '% yield'

Python Strings


print 'reagant: ' + str(reagant_id) + ' produced ' + \ str(percentage_yield) + '% yield'

There's a better way...

Python Strings

Use string % value to format output

Python Strings


output = 'reagant: %d' % 123print outputreagant: 123

Python Strings


output = 'reagant: %d' % 123print outputreagant: 123

percentage_yield = 12.3print 'yield: %6.2f' % percentage_yieldyield: 12.30

Python Strings

And string % (v1, v2, ...) for multiple values

Python Strings


reagant_id = 123percentage_yield = 12.3print 'reagant: %d produced %f%% yield' % \ (reagant_id, percentage_yield)reagant: 123 produced 12.30% yield

Python Strings


reagant_id = 123percentage_yield = 12.3print 'reagant: %d produced %f%% yield' % \ (reagant_id, percentage_yield)reagant: 123 produced 12.30% yield

% operator turns double '%%' into single '%'

Python Strings

Use \n to represent a newline character

Python Strings


Use \' for single quote, \" for double quote

Python Strings



print 'There isn\'t time\nto do it right.'There isn't timeto do it right.

Python Strings



print 'There isn\'t time\nto do it right.'There isn't timeto do it right.

print "But you said,\n\"There is time to do it over.\""But you said,"There is time to do it over."

Python Strings

Use \\ for a literal \ character

Python Strings

print 'Most mathematicians write a\\b instead of a%b.'Most mathematicians write a\b instead of a%b.


Python Strings



Common pattern with escape sequences

Python Strings




– Use a character to mean "what follows is special"

Python Strings




– Use a character to mean "what follows is special"

– Double it up to mean "that character itself"

Python Strings

Use triple quotes (either kind) for multi-line strings

Python Strings


quote = '''We can only seea short distance ahead,but we can see plenty therethat needs to be done.'''

Python Strings


d , \n b u


Python Strings


quote = 'We can only see\na short distance ahead\n' + \ 'but we can see plenty there\nthat needs to be done.'


Python Strings

Strings have methods

Python Strings


name = 'newTON'print name.capitalize(), name.upper(), name.lower(), nameNewton NEWTON newton newTON

Python Strings


name = 'newTON'print name.capitalize(), name.upper(), name.lower(), nameNewton NEWTON newton newTONdna = 'acggtggtcac'print dna.count('g'), dna.count('x')4 0

Python Strings


name = 'newTON'print name.capitalize(), name.upper(), name.lower(), nameNewton NEWTON newton newTONdna = 'acggtggtcac'print dna.count('g'), dna.count('x')4 0print dna.find('t'), dna.find('t', 5), dna.find('x')4 7 -1

Python Strings


name = 'newTON'print name.capitalize(), name.upper(), name.lower(), nameNewton NEWTON newton newTONdna = 'acggtggtcac'print dna.count('g'), dna.count('x')4 0print dna.find('t'), dna.find('t', 5), dna.find('x')4 7 -1print dna.replace('t', 'x'), dnaacggxggxcac acggtggtcac

Python Strings


name = 'newTON'print name.capitalize(), name.upper(), name.lower(), nameNewton NEWTON newton newTONdna = 'acggtggtcac'print dna.count('g'), dna.count('x')4 0print dna.find('t'), dna.find('t', 5), dna.find('x')4 7 -1print dna.replace('t', 'x')acggxggxcac acggtggtcacprint dna.replace('gt', '')acggcac

Python Strings

Can chain method calls together

Python Strings


element = 'cesium'print element.upper().center(10, '.')

Python Strings



convert to upper case

Python Strings



center in a field

10 characters wide

Python Strings


element = 'cesium'print element.upper().center(10, '.')..CESIUM..

October 2010

Copyright © Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

narrated by

Dominique Vuvan

Strings

Technology

Transcript of Strings