Computing with Strings. Learning Outcomes. Python s String Type 9/23/2012

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1 Computing with Strings CMSC 201 Fall 2012 Instructor: John Park Lecture Section 01 Discussion Sections 02-08, 16, 17 1 Learning Outcomes To understand the string data type and how strings are represented in memory To become familiar with the way strings can be manipulated, by using built-in functions and the string library To understand how indexing works on Python lists and strings To understand how characters are stored To be aware of the chart and how to get the ordinal of a character and vice versa. To be aware that formatted output is possible to produce nice-looking reports and tables To be able to write programs that process textual information 2 Python s String Type As you know, we call a sequence of printable characters a string Unlike many other languages, string literals in Python can be denoted using either single or double quotes >>> mystring1 = 'Hello' >>> mystring = "Goodbye" >>> mystring 'Goodbye' This allows us to easily embed quoted text: >>> fact = '"The Cat In the Hat" was written by Dr. Suess.' >>> print fact "The Cat In the Hat" was written by Dr. Suess. 3 1

2 String Representation Indexing component characters of a string in Python is similar to indexing elements in a list. Here's how we can picture a list of length 5, showing indices of 0 through 4. 0: 1: 2: 3: 4: Note that a string is not the same as a list of characters but they are both sequences 4 String Representation Many of the operations you applied to lists actually apply to sequences in general, and therefore, to strings: Indexing: 0 thru n-1 Negative indices: string[-1] is last character Slicing: you can create a substring from a string by using an index range, much like extracting sublists from lists 5 Slicing Strings You can specify a slice, or subsequence, of a string in the same way as a list, by giving two numbers in the index (say m & n); the subsequence will include the characters from position m up through position n-1 E.g.: Hello [1:4] 0: H 1: e 2: l 3: l 4: o 6 2

3 String Operators Operation Python Operator Concatenation + Repetition * Indexing [ ] Slicing [ : ] 7 String Operators Examples using these operators: >>> 'snow' + 'ball' 'snowball' >>> 'hello' * 3 'hellohellohello' Oops. Let's see if we can do better... >>> 'hello ' * 3 + '!' 'hello hello hello!' 8 String Operators >>> len('sample') 6 >>> 'sample'[6] Traceback (most recent call last): File "", line 1, in? IndexError: string index out of range >>> 'sample'[5] 'e' >>> 'sample'[-3] 'p' 9 3

4 Iterating Over Strings Like lists, strings are sequences (of characters), so we can use a for-loop to iterate over its elements: >>> for ch in 'hello':... print ch... h e l l o >>> 10 String Library Python provides a collection of preimplemented functions (a library) for working on strings In order to use the string library, you have to import it : import string 11 String Library Function string.capitalize(s) string.capwords(s) string.count(s, sub) string.find(s, sub) string.join(list) string.upper(s) string.lower(s) string.split(s, c) string.strip(s) Purpose Returns s with only the first letter capitalized Returns s with the first letter of each word capitalized Returns the number of times sub occurs in s Finds the first occurrence of sub in s and returns its position or -1 if it is not found Concatenates a list of strings together to make one string Returns a copy of s in all capital letters Returns a copy of s in all lower case letters Returns a list of strings made by splitting s on each occurence of c Removes all whitespace from the beginning and end of s 12 4

5 String Library Examples lower() makes the string all lower case upper() makes the string all upper case >>> import string >>> string.lower("hello World") 'hello world >>> string.upper("hello World") 'HELLO WORLD' count() counts the number of times sub occurs in s >>> string.count("isn't this it", "is") 2 13 String Library Examples strip() removes all whitespace from the beginning and end of a string. >>> string.strip(" Hello World \n") 'Hello World >>> string.strip(" \n \n \n \t \n ") '' split() makes a string into a list of strings. Each element was separated by whitespace in the original. For example: >>> string.split("hello World from U M B C") ['Hello', 'World', 'from', 'U', 'M', 'B', 'C'] 14 String Library Examples Strings are objects in Python, so the following syntax also works : >>> str = "Hello World from U M B C >>> str.split() ['Hello', 'World', 'from', 'U', 'M', 'B', 'C'] or even: >>> "Hello World from U M B C".split() ['Hello', 'World', 'from', 'U', 'M', 'B', 'C'] 15 5

6 How to Handle String Input Recall: input() will pre-process user s input, interpreting it as a number, or variable name, or expression, or raw_input() will not pre-process the input, always returning it as a string How do you then post-process the string into another type? 16 How Are Characters Stored? As binary, of course. Everything is stored in binary! The technique used for characters is that each character is assigned a number (an integer), and that number is stored in binary. Recall that an integer is 4 bytes big, with 8 bits/byte, that means an integer needs 32 bits of memory. That allows us to count up to But do we really need all that space to represent a character? 18 Characters If you think about the English language and everything that's necessary to communicate, there are very few individual characters : Type of Character Upper-case characters 26 Lower-case characters 26 Digits 10 Arithmetic Operators 9 Punctuation Marks 24 Total 95 Number 19 6

7 Characters This means that in order to assign a number to each character, we only need to count to 95. How many bits does that take? Bits Values Bits Values So by storing characters in just one byte (8 bits), the amount of memory needed for each character is only 1/4 of the amount of space needed by an int. This is a tremendous amount of saved space for storing text. So all characters are only one byte big. 20 Fragments Trivia Question: What is a half-byte (4bits) called? A nibble What about 2 bits? A crumb! 21 == American Standard Code for Information Interchange Derived from telegraphic codes. First version published in 1963 defines 128 characters Codes define common printable characters: Includes lower-case letters (codes ), upper-case letters (codes 65-90), digits (codes 48-57), punctuation, symbols, whitespace 33 nonprinting control characters: Includes codes in range 0-31 (control codes for communication devices, printers, etc., e.g. line feed, form feed) and 127 (DELETE) 22 7

8 Just as integers can be cast into floats, characters can be cast into their values values can be cast back into their characters >>> chr(104) 'h' >>> ord('h') Here is a full table. Let's print out our own chart of just the printable characters how would we do that? >>> for num in range(32, 127):... print num, '=', chr(num) = 33 =! 34 = 35 = # 36 = $ 37 = % 38 = & 39 = 24 As you've seen, if we print just one character per line, our chart is extremely long and narrow, so let's print 6 characters across instead. Since we haven't covered if yet, we can't use it in our solution. So how can we do it? >>> for num in range(32, 127, 6):... print num, '=', chr(num),... print num + 1, '=', chr(num + 1),... print num + 2, '=', chr(num + 2),... print num + 3, '=', chr(num + 3),... print num + 4, '=', chr(num + 4),... print num + 5, '=', chr(num + 5)

9 which produces the output: 32 = 33 =! 34 = " 35 = # 36 = $ 37 = % 38 = & 39 = ' 40 = ( 41 = ) 42 = * 43 = + 44 =, 45 = - 46 =. 47 = / 48 = 0 49 = 1 50 = 2 51 = 3 52 = 4 53 = 5 54 = 6 55 = 7 56 = 8 57 = 9 58 = : 59 = ; 60 = < 61 = = 62 = > 63 =? = A 66 = B 67 = C 68 = D 69 = E 70 = F 71 = G 72 = H 73 = I 74 = J 75 = K 76 = L 77 = M 78 = N 79 = O 80 = P 81 = Q 82 = R 83 = S 84 = T 85 = U 86 = V 87 = W 88 = X 89 = Y 90 = Z 91 = [ 92 = \ 93 = ] 94 = ^ 95 = _ 96 = ` 97 = a 98 = b 99 = c 100 = d 101 = e 102 = f 103 = g 104 = h 105 = i 106 = j 107 = k 108 = l 109 = m 110 = n 111 = o 112 = p 113 = q 114 = r 115 = s 116 = t 117 = u 118 = v 119 = w 120 = x 121 = y 122 = z 123 = { 124 = 125 = } 126 = ~ 127 = 26 Almost there But can we make the columns line up better? Use formatted output! >>> for num in range(32, 127, 6):... print %3d = %c % (num, chr(num)),... print %3d = %c % (num + 1, chr(num + 1)),... print %3d = %c % (num + 2, chr(num + 2)),... print %3d = %c % (num + 3, chr(num + 3)),... print %3d = %c % (num + 4, chr(num + 4)),... print %3d = %c % (num + 5, chr(num + 5)) which produces the output: 32 = 33 =! 34 = " 35 = # 36 = $ 37 = % 38 = & 39 = ' 40 = ( 41 = ) 42 = * 43 = + 44 =, 45 = - 46 =. 47 = / 48 = 0 49 = 1 50 = 2 51 = 3 52 = 4 53 = 5 54 = 6 55 = 7 56 = 8 57 = 9 58 = : 59 = ; 60 = < 61 = = 62 = > 63 =? = A 66 = B 67 = C 68 = D 69 = E 70 = F 71 = G 72 = H 73 = I 74 = J 75 = K 76 = L 77 = M 78 = N 79 = O 80 = P 81 = Q 82 = R 83 = S 84 = T 85 = U 86 = V 87 = W 88 = X 89 = Y 90 = Z 91 = [ 92 = \ 93 = ] 94 = ^ 95 = _ 96 = ` 97 = a 98 = b 99 = c 100 = d 101 = e 102 = f 103 = g 104 = h 105 = i 106 = j 107 = k 108 = l 109 = m 110 = n 111 = o 112 = p 113 = q 114 = r 115 = s 116 = t 117 = u 118 = v 119 = w 120 = x 121 = y 122 = z 123 = { 124 = 125 = } 126 = ~ 127 = 28 9

10 Exercise Write a program that prints the value of each letter in a string all on the same line? For this exercise and all future exercises, you must have a file-header comment Your code should be in main() Ask the user to enter a string of less than 10 characters Get a string from the user using raw_input You may assume the string is of less than 10 characters