Basic string types and how read&convert it. Indexing, looping and slicing a string.

Week 6 Strings

1.String 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
>>> 

2.Read/Convert

• 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

3.Indexing 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

• 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
>>> 

3.1 length of strings

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

>>> fruit = 'banana'
>>> print len(fruit)
6

>>> fruit = 'banana'
>>> x = len(fruit)
>>> print x
6

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

4.Slicing strings [2:4]

• 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

• 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

5.Looping through strings with for and while

• 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

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

fruit = 'banana'
for letter in fruit : 
   print letter

b
a
n
a
n
a

• 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

• The iteration variable “iterates” through 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

6.Concatenating strings with +

• 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
>>> 

7.String operations

• 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

>>> 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']

7.1 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.'

7.2 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

7.3 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
>>> 

7.4 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
>>> 

7.5 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'
>>> 

7.6 Prefixes

>>> line = 'Please have a nice day’
>>> line.startswith('Please')
True
>>> line.startswith('p')
False

7.7 Parsing and Extracting

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

>>> 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