Strings. Chapter 6. Python for Informatics: Exploring Information

Transcription

1 Strings Chpter 6 Python for Informtics: Exploring Informtion

2 String Dt Type A string is sequence of chrcters A string literl uses quotes 'Hello' or "Hello" For strings, + mens conctente When string contins numbers, it is still string We cn convert numbers in string into number using int() >>> str1 = "Hello" >>> str2 = 'there' >>> bob = str1 + str2 >>> print bob Hellothere >>> str3 = '123' >>> str3 = str3 + 1 Trcebck (most recent cll lst): File "<stdin>", line 1, in <module>typeerror: cnnot conctente 'str' nd 'int' objects >>> x = int(str3) + 1 >>> print x 124 >>>

3 Reding nd Converting We prefer to red dt in using strings nd then prse nd convert the dt s we need This gives us more control over error situtions nd/or bd user input Rw input numbers must be converted from strings >>> nme = rw_input('enter:') Enter:Chuck >>> print nme Chuck >>> pple = rw_input('enter:') Enter:100 >>> x = pple 10 Trcebck (most recent cll lst): File "<stdin>", line 1, in <module>typeerror: unsupported opernd type(s) for -: 'str' nd 'int' >>> x = int(pple) 10 >>> print x 90

4 Looking Inside Strings We cn get t ny single chrcter in string using n index specified in squre brckets The index vlue must be n integer nd strts t zero The index vlue cn be n expression tht is computed b 0 1 n 2 3 n 4 5 >>> fruit = 'bnn' >>> letter = fruit[1] >>> print letter >>> x = 3 >>> w = fruit[x - 1] >>> print w n

5 A Chrcter Too Fr You will get python error if you ttempt to index beyond the end of string. So be creful when constructing index vlues nd slices >>> zot = 'bc' >>> print zot[5] Trcebck (most recent cll lst): File "<stdin>", line 1, in <module>indexerror: string index out of rnge >>>

6 Strings Hve Length There is built-in function len tht gives us the length of string b 0 1 n 2 3 n 4 5 >>> fruit = 'bnn' >>> print len(fruit) 6

7 Len Function >>> fruit = 'bnn' >>> x = len(fruit) >>> print x 6 A function is some stored code tht we use. A function tkes some input nd produces n output. 'bnn' ( string) len() function 6 ( number) Guido wrote this code

8 Len Function >>> fruit = 'bnn' >>> x = len(fruit) >>> print x 6 A function is some stored code tht we use. A function tkes some input nd produces n output. 'bnn' ( string) def len(inp): blh blh for x in y: blh blh 6 ( number)

9 Looping Through Strings Using while sttement nd n itertion vrible, nd the len function, we cn construct loop to look t ech of the letters in string individully fruit = 'bnn' index = 0 while index < len(fruit): letter = fruit[index] print index, letter index = index b 1 2 n 3 4 n 5

10 Looping Through Strings A definite loop using for sttement is much more elegnt The itertion vrible is completely tken cre of by the for loop fruit = 'bnn' for letter in fruit: print letter b n n

11 Looping Through Strings A definite loop using for sttement is much more elegnt The itertion vrible is completely tken cre of by the for loop fruit = 'bnn' for letter in fruit : print letter index = 0 while index < len(fruit) : letter = fruit[index] print letter index = index + 1 b n n

12 Looping nd Counting This is simple loop tht loops through ech letter in string nd counts the number of times the loop encounters the '' chrcter word = 'bnn' count = 0 for letter in word : if letter == '' : count = count + 1 print count

13 Looking deeper into in The itertion vrible itertes through the sequence (ordered set) The block (body) of code is executed once for ech vlue in the sequence Itertion vrible Six-chrcter string for letter in 'bnn' : print letter The itertion vrible moves through ll of the vlues in the sequence

14 Yes Done? No Advnce letter b n n print letter for letter in 'bnn' : print letter The itertion vrible itertes through the string nd the block (body) of code is executed once for ech vlue in the sequence

15 M o n t y P y t h o n We cn lso look t ny continuous section of string using colon opertor 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 t the end >>> s = 'Monty Python' >>> print s[0:4] Mont >>> print s[6:7] P >>> print s[6:20] Python Slicing Strings

16 M o n t y P y t h o n If we leve off the first number or the lst number of the slice, it is ssumed 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 Slicing Strings

17 String Conctention When the + opertor is pplied to strings, it mens conctention >>> = 'Hello' >>> b = + 'There' >>> print b HelloThere >>> c = + ' ' + 'There' >>> print c Hello There >>>

18 Using in s logicl Opertor The in keyword cn lso be used to check to see if one string is in nother string The in expression is logicl expression tht returns True or Flse nd cn be used in n if sttement >>> fruit = 'bnn' >>> 'n' in fruit True >>> 'm' in fruit Flse >>> 'nn' in fruit True >>> if '' in fruit :... print 'Found it!'... Found it! >>>

19 String Comprison if word == 'bnn': print 'All right, bnns.' if word < 'bnn': print 'Your word,' + word + ', comes before bnn.' elif word > 'bnn': print 'Your word,' + word + ', comes fter bnn.' else: print 'All right, bnns.'

20 String Librry Python hs number of string functions which re in the string librry These functions re lredy built into every string - we invoke them by ppending the function to the string vrible These functions do not modify the originl string, insted they return new string tht hs been ltered >>> greet = 'Hello Bob' >>> zp = greet.lower() >>> print zp hello bob >>> print greet Hello Bob >>> print 'Hi There'.lower() hi there >>>

21 >>> stuff = 'Hello world' >>> type(stuff) <type 'str'> >>> dir(stuff) ['cpitlize', 'center', 'count', 'decode', 'encode', 'endswith', 'expndtbs', 'find', 'formt', 'index', 'islnum', 'islph', 'isdigit', 'islower', 'isspce', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'prtition', 'replce', 'rfind', 'rindex', 'rjust', 'rprtition', 'rsplit', 'rstrip', 'split', 'splitlines', 'strtswith', 'strip', 'swpcse', 'title', 'trnslte', 'upper', 'zfill']

22

23 String Librry str.cpitlize() str.center(width[, fillchr]) str.endswith(suffix[, strt[, end]]) str.find(sub[, strt[, end]]) str.lstrip([chrs]) str.replce(old, new[, count]) str.lower() str.rstrip([chrs]) str.strip([chrs]) str.upper()

24 Serching String We use the find() function to serch for substring within nother string b 0 1 n 2 3 n 4 5 find() finds the first occurrence of the substring If the substring is not found, find() returns -1 Remember tht string position strts t zero >>> fruit = 'bnn' >>> pos = fruit.find('n') >>> print pos 2 >>> = fruit.find('z') >>> print -1

25 Mking everything UPPER CASE You cn mke copy of string in lower cse or upper cse Often when we re serching for string using find() - we first convert the string to lower cse so we cn serch string regrdless of cse >>> greet = 'Hello Bob' >>> nnn = greet.upper() >>> print nnn HELLO BOB >>> www = greet.lower() >>> print www hello bob >>>

26 Serch nd Replce The replce() function is like serch nd replce opertion in word processor It replces ll occurrences of the serch string with the replcement string >>> greet = 'Hello Bob' >>> nstr = greet.replce('bob','jne') >>> print nstr Hello Jne >>> nstr = greet.replce('o','x') >>> print nstr HellX BXb >>>

27 Stripping Whitespce Sometimes we wnt to tke string nd remove whitespce t the beginning nd/or end lstrip() nd rstrip() remove whitespce t the left or right strip() removes both beginning nd ending whitespce >>> greet = ' Hello Bob ' >>> greet.lstrip() 'Hello Bob ' >>> greet.rstrip() ' Hello Bob' >>> greet.strip() 'Hello Bob' >>>

28 Prefixes >>> line = 'Plese hve nice dy' >>> line.strtswith('plese') True >>> line.strtswith('p') Flse

29 Prsing nd Extrcting From St Jn 5 09:14: >>> dt = 'From stephen.mrqurd@uct.c.z St Jn 5 09:14: ' >>> tpos = dt.find('@') >>> print tpos 21 >>> sppos = dt.find(' ',tpos) >>> print sppos 31 >>> host = dt[tpos+1 : sppos] >>> print host uct.c.z

30 Summry String type Red/Convert Indexing strings [] Slicing strings [2:4] Looping through strings with for nd while Conctenting strings with + String opertions String librry String Comprisons Serching in strings Replcing text Stripping white spce

31 Acknowledgements / Contributions These slides re Copyright Chrles R. Severnce ( of the University of Michign School of Informtion nd open.umich.edu nd mde vilble under Cretive Commons Attribution 4.0 License. Plese mintin this lst slide in ll copies of the document to comply with the ttribution requirements of the license. If you mke chnge, feel free to dd your nme nd orgniztion to the list of contributors on this pge s you republish the mterils.... Initil Development: Chrles Severnce, University of Michign School of Informtion Insert new Contributors nd Trnsltors here