Py4Inf-06-Strings-PrintPy4Inf-06-Strings-Print

StringsChapter 6Python for Informatics: Exploring Informationww.pythonlearn.comUnless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.htp:/creativecommons.org/licenses/by/3.0/.Copyright 2010- Charles SeveranceString Data TypeA string is a sequence of charactersA string literal uses quotes Hello or “Hello”For strings, + means “concatenate”When a string contains numbers, it is stil a stringWe can convert numbers in a string into a number using int()>>> str1 = "Hello">>> str2 = 'there'>>> bob = str1 + str2>>> print bobHellothere>>> str3 = '123'>>> str3 = str3 + 1Traceback (most recent cal last):File "", line 1, in TypeError: cannot concatenate 'str' and 'int' objects>>> x = int(str3) + 1>>> print x124>>> Reading and ConvertingWe prefer to read data in using strings and then parse and convert the data as we needThis gives us more control over error situations and/or bad user inputRaw input numbers must be converted from strings>>> name = raw_input('Enter:')Enter:Chuck>>> print nameChuck>>> apple = raw_input('Enter:')Enter:100>>> x = apple - 10Traceback (most recent cal last):File "", line 1, in TypeError: unsupported operand type(s) for -: 'str' and 'int'>>> x = int(apple) - 10>>> print x90Loking Inside StringsWe can get at any single character in a string using an index specified in square bracketsThe index value must be an integer and starts at zeroThe index value can be an expression that is computed>>> fruit = 'banana'>>> letter = fruit1>>> print lettera>>> n = 3>>> w = fruitn - 1>>> print wn0b1a2n3a4n5aA Character To FarYou wil 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 zot5Traceback (most recent cal last):File "", line 1, in IndexError: string index out of range>>> Strings Have LengthThere is a built-in function len that gives us the length of a string>>> fruit = 'banana'>>> print len(fruit)60b1a2n3a4n5aLen Function>>> fruit = 'banana'>>> x = len(fruit)>>> print x6len()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 codeLen Functiondef len(inp):blahblahfor x in y:blahblahA function is some stored code that we use. A function takes some input and produces an output.>>> fruit = 'banana'>>> x = len(fruit)>>> print x6'banana' (a string)6(a number)Loping Through StringsUsing a while statement and an iteration variable, and the len function, we can construct a lop to lok at each of the letters in a string individualyfruit = 'banana'index = 0while index >> s = 'Monty Python'>>> print s0:4Mont>>> print s6:7P>>> print s6:20Python0M1o2n3t4y56P7y8t9h10o11nIf we leave off the first number or the last number of the slice, it is asumed to be the beginning or end of the string respectivelySlicing Strings>>> s = 'Monty Python'>>> print s:2Mo>>> print s8:thon>>> print s:Monty Python0M1o2n3t4y56P7y8t9h10o11nString ConcatenationWhen the + operator is applied to strings, it means "concatenation">>> a = 'Hello'>>> b = a + 'There'>>> print bHelloThere>>> c = a + ' ' + 'There'>>> print cHello There>>> Using in as an OperatorThe in keyword can also be used to check to see if one string is "in" another stringThe in expression is a logical expression and returns True or False and can be used in an if statement>>> fruit = 'banana'>>> 'n' in fruitTrue>>> 'm' in fruitFalse>>> 'nan' in fruitTrue>>> if 'a' in fruit : . print 'Found it!'. Found it!>>> String Comparisonif word = 'banana':print 'All right, bananas.'if word 'banana':print 'Your word,' + word + ', comes after banana.'else:print 'All right, bananas.'String LibraryPython has a number of string functions which are in the string libraryThese functions are already built into every string - we invoke them by appending the function to the string variableThese 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 greetHello Bob>>> print 'Hi There'.lower()hi there>>>http:/docs.python.org/lib/string-methods.html>>> stuff = 'Hello world'>>> type(stuff)>>> dir(stuff)'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'ispace', '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.htmlstr.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: