Strings

Chapter 6

Python for Informatics: Exploring Information

www.pythonlearn.com

Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.

http://creativecommons.org/licenses/by/3.0/.

​

Copyright 2010- Charles Severance

String Data Type

• A string is a sequence of characters
• A string literal uses quotes 'Hello' or “Hello”
• For strings, + means “concatenate”
• When a string contains numbers, it is still a string
• We can convert numbers in a string into a number using int()

>>> str1 = "Hello”

>>> str2 = 'there'

>>> bob = str1 + str2

>>> print bob

Hellothere

>>> str3 = '123'

>>> str3 = str3 + 1

Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: cannot concatenate 'str' and 'int' objects

>>> x = int(str3) + 1

>>> print x

124

>>>

Reading and Converting

• We prefer to read data in using strings and then parse and convert the data as we need
• This gives us more control over error situations and/or bad user input
• Raw input numbers must be converted from strings

>>> name = raw_input('Enter:')

Enter:Chuck

>>> print name

Chuck

>>> apple = raw_input('Enter:')

Enter:100

>>> x = apple – 10

Traceback (most recent call last): File "<stdin>", line 1, in <module>TypeError: unsupported operand type(s) for -: 'str' and 'int'

>>> x = int(apple) – 10

>>> print x

90

Looking Inside Strings

• We can get at any single character in a string using an index specified in square brackets
• The index value must be an integer and starts at zero
• The index value can be an expression that is computed

>>> fruit = 'banana'

>>> letter = fruit[1]

>>> print letter

a

>>> n = 3

>>> w = fruit[n - 1]

>>> print w

n

0

b

1

a

2

n

3

a

4

n

5

a

A Character Too Far

• You will get a python error if you attempt to index beyond the end of a string.
• So be careful when constructing index values and slices

>>> zot = 'abc'

>>> print zot[5]

Traceback (most recent call last): File "<stdin>", line 1, in <module>IndexError: string index out of range

>>>

Strings Have Length

• There is a built-in function len that gives us the length of a string

>>> fruit = 'banana'

>>> print len(fruit)

6

0

b

1

a

2

n

3

a

4

n

5

a

Len Function

>>> fruit = 'banana'

>>> x = len(fruit)

>>> print x

6

len()

function

'banana'

(a string)

6

(a number)

A function is some stored code that we use. A function takes some input and produces an output.

Guido wrote this code

Len Function

def len(inp):

blah

blah

for x in y:

blah

blah

A function is some stored code that we use. A function takes some input and produces an output.

>>> fruit = 'banana'

>>> x = len(fruit)

>>> print x

6

'banana'

(a string)

6

(a number)

Looping Through Strings

• Using a while statement and an iteration variable, and the len function, we can construct a loop to look at each of the letters in a string individually

fruit = 'banana'

index = 0

while index < len(fruit) :

letter = fruit[index]

print index, letter

index = index + 1

0 b

1 a

2 n

3 a

4 n

5 a

Looping Through Strings

• A definite loop using a for statement is much more elegant
• The iteration variable is completely taken care of by the for loop

b

a

n

a

n

a

fruit = 'banana'

for letter in fruit :

print letter

Looping Through Strings

• A definite loop using a for statement is much more elegant
• The iteration variable is completely taken care of by the for loop

index = 0

while index < len(fruit) :

letter = fruit[index]

print letter

index = index + 1

fruit = 'banana'

for letter in fruit :

print letter

b

a

n

a

n

a

Looping and Counting

• This is a simple loop that loops through each letter in a string and counts the number of times the loop encounters the 'a' character.

word = 'banana'

count = 0

for letter in word :

if letter == 'a' :

count = count + 1

print count

Looking deeper into in

• The iteration variable “iterates” though the sequence (ordered set)
• The block (body) of code is executed once for each value in the sequence
• The iteration variable moves through all of the values in the sequence

for letter in 'banana' :

print letter

Done?

Yes

print letter

Advance letter

for letter in 'banana' :

print letter

b

a

n

a

n

a

The iteration variable “iterates” though the string and the block (body) of code is executed once for each value in the sequence

Slicing Strings

• We can also look at any continuous section of a string using a colon operator
• The second number is one beyond the end of the slice - “up to but not including”
• If the second number is beyond the end of the string, it stops at the end

>>> s = 'Monty Python'

>>> print s[0:4]

Mont

>>> print s[6:7]

P

>>> print s[6:20]

Python

0

M

1

o

2

n

3

t

4

y

5

6

P

7

y

8

t

9

h

10

o

11

n

Slicing Strings

• If we leave off the first number or the last number of the slice, it is assumed to be the beginning or end of the string respectively

>>> s = 'Monty Python'

>>> print s[:2]

Mo

>>> print s[8:]

Thon

>>> print s[:]

Monty Python

0

M

1

o

2

n

3

t

4

y

5

6

P

7

y

8

t

9

h

10

o

11

n

String Concatenation

• When the + operator is applied to strings, it means "concatenation"

>>> a = 'Hello'

>>> b = a + 'There'

>>> print b

HelloThere

>>> c = a + ' ' + 'There'

>>> print c

Hello There

>>>

Using in as an Operator

• The in keyword can also be used to check to see if one string is "in" another string
• The in expression is a logical expression and returns True or False and can be used in an if statement

>>> fruit = 'banana’

>>> 'n' in fruit

True

>>> 'm' in fruit

False

>>> 'nan' in fruit

True

>>> if 'a' in fruit :

... print 'Found it!’

...

Found it!

>>>

String Comparison

if word == 'banana':

print 'All right, bananas.'

​

if word < 'banana':

print 'Your word,' + word + ', comes before banana.’

elif word > 'banana':

print 'Your word,' + word + ', comes after banana.’

else:

print 'All right, bananas.'

String Library

• Python has a number of string functions which are in the string library
• These functions are already built into every string - we invoke them by appending the function to the string variable
• These functions do not modify the original string, instead they return a new string that has been altered

>>> greet = 'Hello Bob'>>> zap = greet.lower()>>> print zaphello bob

>>> print greet

Hello Bob>>> print 'Hi There'.lower()

hi there

>>>

http://docs.python.org/lib/string-methods.html

>>> stuff = 'Hello world’

>>> type(stuff)<type 'str'>

>>> dir(stuff)

['capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

http://docs.python.org/lib/string-methods.html

String Library

str.capitalize()

str.center(width[, fillchar])

str.endswith(suffix[, start[, end]])

str.find(sub[, start[, end]])

str.lstrip([chars])

str.replace(old, new[, count])

str.lower()

str.rstrip([chars])

str.strip([chars])

str.upper()

http://docs.python.org/lib/string-methods.html

Searching a String

• We use the find() function to search for a substring within another string
• find() finds the first occurance of the substring
• If the substring is not found, find() returns -1
• Remember that string position starts at zero

>>> fruit = 'banana'

>>> pos = fruit.find('na')

>>> print pos

2

>>> aa = fruit.find('z')

>>> print aa

-1

0

b

1

a

2

n

3

a

4

n

5

a

Making everything UPPER CASE

• You can make a copy of a string in lower case or upper case
• Often when we are searching for a string using find() - we first convert the string to lower case so we can search a string regardless of case

>>> greet = 'Hello Bob'

>>> nnn = greet.upper()

>>> print nnn

HELLO BOB

>>> www = greet.lower()

>>> print www

hello bob

>>>

Search and Replace

• The replace() function is like a “search and replace” operation in a word processor
• It replaces all occurrences of the search string with the replacement string

>>> greet = 'Hello Bob'

>>> nstr = greet.replace('Bob','Jane')

>>> print nstr

Hello Jane

>>> nstr = greet.replace('o','X')

>>> print nstrHellX BXb

>>>

Stripping Whitespace

• Sometimes we want to take a string and remove whitespace at the beginning and/or end
• lstrip() and rstrip() to the left and right only
• strip() Removes both begin and ending whitespace

>>> greet = ' Hello Bob '

>>> greet.lstrip()

'Hello Bob '

>>> greet.rstrip()

' Hello Bob'

>>> greet.strip()

'Hello Bob'

>>>

>>> line = 'Please have a nice day’

>>> line.startswith('Please')

True

>>> line.startswith('p')

False

Prefixes

>>> data = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008’

>>> atpos = data.find('@')

>>> print atpos

21

>>> sppos = data.find(' ',atpos)

>>> print sppos

31

>>> host = data[atpos+1 : sppos]

>>> print host

uct.ac.za

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

21

31

Parsing and

Extracting

Summary

• String type
• Read/Convert
• Indexing strings []
• Slicing strings [2:4]
• Looping through strings with for and while
• Concatenating strings with +
• String operations