Strings

Python
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.

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 c
Darwin
Python Strings

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

print 'Alan', "Turing"
Alan Turing
Python Strings

Alan Turing
Strings are the same no matter how they're created
Python Strings

Alan Turing
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'
True
print 'ab' < 'abc'
True
Python Strings

from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
print '1' < '9'
True
Python Strings

from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
print '1' < '9'
True
print '100' < '9'
True
Python Strings

from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
print '1' < '9'
True
print '100' < '9'
True
print '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

name = 'Darwin'
name[0] = 'C'
Immutability improves performance
Python Strings

name = 'Darwin'
name[0] = 'C'
Immutability improves performance
See later how immutability improves programmers'
performance
Python Strings

Use + to concatenate strings
Python Strings

name = 'Charles' + ' ' + 'Darwin'
print name
Charles Darwin
Python Strings

print name
Charles Darwin
Concatenation always produces a new string
Python Strings

print name
Charles Darwin
original = 'Charles' original 'Charles'
Python Strings

print name
Charles Darwin
'Charles'
original = 'Charles' original
name = original
name
Python Strings

print name
Charles Darwin
original = 'Charles'
name = original
name += ' Darwin'
'Charles'
original
name 'Charles Darwin'
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' % 123
print output
reagant: 123
Python Strings

output = 'reagant: %d' % 123
print output
reagant: 123
percentage_yield = 12.3
print 'yield: %6.2f' % percentage_yield
yield: 12.30
Python Strings

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

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

reagant_id = 123
print '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 timento do it right.'
There isn't time
to do it right.
Python Strings

print 'There isn't timento do it right.'
There isn't time
to 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 ab instead of a%b.'
Most mathematicians write ab 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 see
a short distance ahead,
but we can see plenty there
that needs to be done.'''
Python Strings

d , n b u
Python Strings

quote = 'We can only seena short distance aheadn' 'but we can see plenty therenthat needs to be done.'
Python Strings

Strings have methods
Python Strings

name = 'newTON'
print name.capitalize(), name.upper(), name.lower(), Newton NEWTON newton newTON
Python Strings

name = 'newTON'
dna = 'acggtggtcac'
print dna.count('g'), dna.count('x')
4 0
Python Strings

name = 'newTON'
dna = 'acggtggtcac'
4 0
print dna.find('t'), dna.find('t', 5), dna.find('x')
4 7 -1
Python Strings

name = 'newTON'
dna = 'acggtggtcac'
4 0
4 7 -1
print dna.replace('t', 'x'), dna
acggxggxcac acggtggtcac
Python Strings

name = 'newTON'
dna = 'acggtggtcac'
4 0
4 7 -1
print dna.replace('t', 'x')
acggxggxcac acggtggtcac
print dna.replace('gt', '')
acggcac
Python Strings

Can chain method calls together
Python Strings

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

element = 'cesium'
convert to upper case
Python Strings

element = 'cesium'
center in a field
10 characters wide
Python Strings

element = 'cesium'
..CESIUM..
Python Strings

narrated by
Dominique Vuvan
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.

Strings

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