# Chapter 8 Strings 字符串 Strings are not like integers, floats, and booleans. A string is a sequence, which means it is an ordered collection of other values. In this chapter you’ll see how to access the characters that make up a string, and you’ll learn about some of the methods strings provide. > 字符串和整形、浮点数以及布尔值很不一样。一个字符串是一个序列，意味着是对其他值的有序排列。在本章你将学到如何读取字符串中的字符，你还会学到一些字符串相关的方法。 ## 8.1 A string is a sequence 字符串是序列 A string is a sequence of characters. You can access the characters one at a time with the bracket operator: > 字符串就是一串有序的字符。你可以通过方括号操作符，每次去访问字符串中的一个字符： ```Python >>> fruit = 'banana' >>> fruit = 'banana' >>> letter = fruit[1] >>> letter = fruit[1] ``` The second statement selects character number 1 from fruit and assigns it to letter. > 第二个语句选择了 fruit 这个字符串的序号为1的字符，并把这个字符赋值给了 letter 这个变量。 （译者注：思考一下这里的 letter 是一个什么类型的变量。） The expression in brackets is called an index. The index indicates which character in the sequence you want (hence the name). But you might not get what you expect: > 方括号内的内容叫做索引。索引指示了你所指定的字符串中字符的位置（就跟名字差不多）。 > 但你可能发现得到的结果和你预期的有点不一样： ```Python >>> letter >>> letter 'a' ``` For most people, the first letter of 'banana' is b, not a. But for computer scientists, the index is an offset from the beginning of the string, and the offset of the first letter is zero. > 大多数人都认为banana 的第『1』个字符应该是 b，而不是 a。但对于计算机科学家来说，索引是字符串从头的偏移量，所以真正的首字母偏移量应该是0. ```Python >>> letter = fruit[0] >>> letter = fruit[0] >>> letter >>> letter 'b' ``` So b is the 0th letter (“zero-eth”) of 'banana', a is the 1th letter (“one-eth”), and n is the 2th letter (“two-eth”). > 所以 b 就是字符串 banana 的第『0』个字符，而 a 是第『1』个，n 就是第『2』个了。 As an index you can use an expression that contains variables and operators: > 你可以在方括号内的索引中使用变量和表达式： ```Python >>> i = 1 >>> i = 1 >>> fruit[i] >>> fruit[i] 'a' >>> fruit[i+1] >>> fruit[i+1] 'n' ``` But the value of the index has to be an integer. Otherwise you get: > 但要注意的事，索引的值必须是整形的。否则你就会遇到类型错误了： ```Python >>> letter = fruit[1.5] >>> letter = fruit[1.5] TypeError: string indices must be integers ``` ## 8.2 len 长度 len is a built-in function that returns the number of characters in a string: > len 是一个内置函数，会返回一个字符串中字符的长度： ```Python >>> fruit = 'banana' >>> fruit = 'banana' >>> len(fruit) 6 >>> len(fruit) 6 ``` To get the last letter of a string, you might be tempted to try something like this: > 要得到一个字符串的最后一个字符，你可能会想到去利用 len 函数： ```Python >>> length = len(fruit) >>> length = len(fruit) >>> last = fruit[length] >>> last = fruit[length] IndexError: string index out of range ``` The reason for the IndexError is that there is no letter in ’banana’ with the index 6. Since we started counting at zero, the six letters are numbered 0 to 5. To get the last character, you have to subtract 1 from length: > 出现索引错误的原因就是banana 这个字符串在第『6』个位置是没有字母的。因为我们从0开始数，所以这一共6个字母的顺序是0到5号。因此要得到最后一次字符，你需要在字符串长度的基础上减去1才行： ```Python >>> last = fruit[length-1] >>> last = fruit[length-1] >>> last >>> last 'a' ``` Or you can use negative indices, which count backward from the end of the string. The expression fruit[-1] yields the last letter, fruit[-2] yields the second to last, and so on. > 或者你也可以用负数索引，意思就是从字符串的末尾向前数几位。fruit[-1]这个表达式给你最后一个字符，fruit[-2]给出倒数第二个，依此类推。 ## 8.3 Traversal with a for loop 用 for 循环进行遍历 A lot of computations involve processing a string one character at a time. Often they start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal. One way to write a traversal is with a while loop: > 很多计算过程都需要每次从一个字符串中拿一个字符。一般都是从头开始，依次得到每个字符，然后做点处理，然后一直到末尾。这种处理模式叫遍历。写一个遍历可以使用 while 循环： ```Python index = 0 while index < len(fruit): letter = fruit[index] print(letter) index = index + 1 ``` This loop traverses the string and displays each letter on a line by itself. The loop condition is index < len(fruit), so when index is equal to the length of the string, the condition is false, and the body of the loop doesn’t run. The last character accessed is the one with the index len(fruit)-1, which is the last character in the string. > 这个循环遍历了整个字符串，然后它再把买一个字符显示在一行上面。循环条件是 index 这个变量小于字符串 fruit 的长度，所以当 index 与字符串长度相等的时候，条件就不成立了，循环体就不运行了。最后一个字符被获取的时候，index 正好是len(fruit)-1，这就已经是该字符串的最后一个字符了。 As an exercise, write a function that takes a string as an argument and displays the letters backward, one per line. > 下面就练习一下了，写一个函数，接收一个字符串做参数，然后倒序显示每一个字符，每行显示一个。 Another way to write a traversal is with a for loop: > 另外一种遍历的方法就是 for 循环了： ```Python for letter in fruit: print(letter) ``` Each time through the loop, the next character in the string is assigned to the variable letter. The loop continues until no characters are left. > 每次循环之后，字符串中的下一个字符都会赋值给变量 letter。循环在进行到没有字符剩余的时候就停止了。 The following example shows how to use concatenation (string addition) and a for loop to generate an abecedarian series (that is, in alphabetical order). > 下面的例子展示了如何使用级联（字符串加法）以及一个 for 循环来生成一个简单的序列（用字母表顺序）。 In Robert McCloskey’s book Make Way for Ducklings, the names of the ducklings are Jack, Kack, Lack, Mack, Nack, Ouack, Pack, and Quack. This loop outputs these names in order: > 在 Robert McCloskey 的一本名叫《Make Way for Ducklings》的书中，小鸭子的名字依次为：Jack, Kack, Lack, Mack, Nack, Ouack, Pack, 和Quack。下面这个循环会依次输出他们的名字： ```Python prefixes = 'JKLMNOPQ' suffix = 'ack' for letter in prefixes: print(letter + suffix) ``` The output is: > 输出结果如下： ```Python Jack Kack Lack Mack Nack Oack Pack Qack ``` Of course, that’s not quite right because “Ouack” and “Quack” are misspelled. As an exercise, modify the program to fix this error. > 当然了，有点不准确的地方，因为有“Ouack”和 “Quack”两处拼写错了。做个练习，修改一下程序，改正这个错误。 ## 8.4 String slices 字符串切片 A segment of a string is called a slice. Selecting a slice is similar to selecting a character: > 字符串的一段叫做切片。从字符串中选择一部分做切片，与选择一个字符有些相似： ```Python >>> s = 'Monty Python' >>> s = 'Monty Python' >>> s[0:5] >>> s[0:5] 'Monty' >>> s[6:12] >>> s[6:12] 'Python' ``` The operator [n:m] returns the part of the string from the “n-eth” character to the “m-eth” character, including the first but excluding the last. This behavior is counter intuitive, but it might help to imagine the indices pointing between the characters, as in Figure 8.1. > [n:m]这种操作符，会返回字符串中从第『n』个到第『m』个的字符，包含开头的第『n』个，但不包含末尾的第『m』个。这个设计可能有点违背直觉，但可能有助于想象这个切片在字符串中的方向，如图8.1。 * * *  Figure 8.1: Slice indices. * * * If you omit the first index (before the colon), the slice starts at the beginning of the string. If you omit the second index, the slice goes to the end of the string: > 如果你忽略了第一个索引（就是冒号前面的那个），切片会默认从字符串头部开始。如果你忽略了第二个索引，切片会一直包含到最后一位： ```Python >>> fruit = 'banana' >>> fruit = 'banana' >>> fruit[:3] >>> fruit[:3] 'ban' >>> fruit[3:] >>> fruit[3:] 'ana' ``` If the first index is greater than or equal to the second the result is an empty string, represented by two quotation marks: > 如果两个索引相等，得到的就是空字符串了，用两个单引号来表示： ```Python >>> fruit = 'banana' >>> fruit = 'banana' >>> fruit[3:3] >>> fruit[3:3] '' ``` An empty string contains no characters and has length 0, but other than that, it is the same as any other string. Continuing this example, what do you think fruit[:] means? Try it and see. > 空字符串不包含字符，长度为0，除此之外，都与其他字符串是一样的。 > 那么来练习一下，你觉得 fruit[:]这个是什么意思？在程序中试试吧。 ## 8.5 Strings are immutable 字符串不可修改 It is tempting to use the [] operator on the left side of an assignment, with the intention of changing a character in a string. For example: > 大家总是有可能想试试把方括号在赋值表达式的等号左侧，试图去更改字符串中的某一个字符。比如： ```Python >>> greeting = 'Hello, world!' >>> greeting = 'Hello, world!' >>> greeting[0] = 'J' >>> greeting[0] = 'J' TypeError: 'str' object does not support item assignment ``` The “object” in this case is the string and the “item” is the character you tried to assign. For now, an object is the same thing as a value, but we will refine that definition later (Section 10.10). > 『object』是对象的意思，这里指的是字符串类 string，然后『item』是指你试图赋值的字符串中的字符。目前来说，一个对象就跟一个值差不多，但后续在第十章第十节我们再对这个定义进行详细讨论。 The reason for the error is that strings are immutable, which means you can’t change an existing string. The best you can do is create a new string that is a variation on the original: > 产生上述错误的原因是字符串是不能被修改的，这意味着你不能对一个已经存在的字符串进行任何改动。你顶多也就能建立一个新字符串，新字符串可以基于旧字符串进行一些改动。 ```Python >>> greeting = 'Hello, world!' >>> greeting = 'Hello, world!' >>> new_greeting = 'J' + greeting[1:] >>> new_greeting = 'J' + greeting[1:] >>> new_greeting >>> new_greeting 'Jello, world!' ``` This example concatenates a new first letter onto a slice of greeting. It has no effect on the original string. > 上面的例子中，对 greeting 这个字符串进行了切片，然后添加了一个新的首字母过去。这并不会对原始字符串有任何影响。（译者注：也就是 greeting 这个字符串的值依然是原来的值，是不可改变的。） ## 8.6 Searching 搜索 What does the following function do? > 下面这个函数是干啥的？ ```Python def find(word, letter): index = 0 while index < len(word): if word[index] == letter: return index index = index + 1 return -1 ``` In a sense, find is the inverse of the [] operator. Instead of taking an index and extracting the corresponding character, it takes a character and finds the index where that character appears. If the character is not found, the function returns -1. > 简单来说，find 函数，也就是查找，是方括号操作符[]的逆运算。方括号是之道索引然后提取对应的字符，而查找函数是选定一个字符去查找这个字符出现的索引位置。如果字符没有被报道，函数就返回-1。 This is the first example we have seen of a return statement inside a loop. If word[index] == letter, the function breaks out of the loop and returns immediately. If the character doesn’t appear in the string, the program exits the loop normally and returns -1. > 这是我们见过的第一个返回语句位于循环体内的例子。如果word[index]等于letter，函数就跳出循环立刻返回。如果字符在字符串里面没出现，程序正常退出循环并且返回-1。 This pattern of computation—traversing a sequence and returning when we find what we are looking for—is called a search. As an exercise, modify find so that it has a third parameter, the index in word where it should start looking. > 这种计算-遍历一个序列然后返回我们要找的东西的模式就叫做搜索了。 > 做个练习，修改一下 find 函数，加入第三个参数，这个参数为查找开始的字符串位置。 ## 8.7 Looping and counting 循环和计数 The following program counts the number of times the letter a appears in a string: > 下面这个程序计算了字母 a 在一个字符串中出现的次数： ```Python word = 'banana' count = 0 for letter in word: if letter == 'a': count = count + 1 print(count) ``` This program demonstrates another pattern of computation called a counter. The variable count is initialized to 0 and then incremented each time an a is found. When the loop exits, count contains the result—the total number of a’s. > 这一程序展示了另外一种计算模式，叫做计数。变量 count 被初始化为0，然后每次在字符串中找到一个 a，就让 count 加1.当循环退出的时候，count 就包含了 a 出现的总次数。 As an exercise, encapsulate this code in a function named count, and generalize it so that it accepts the string and the letter as arguments. Then rewrite the function so that instead of traversing the string, it uses the three-parameter version of find from the previous section. > 做个练习，把上面的代码封装进一个名叫 count 的函数中，泛化一下，一遍让他接收任何字符串和字幕作为参数。 > 然后再重写一下这个函数，这次不再让它遍历整个字符串，而使用上一节中练习的三参数版本的 find 函数。 ## 8.8 String methods 字符串方法 Strings provide methods that perform a variety of useful operations. A method is similar to a function—it takes arguments and returns a value—but the syntax is different. For example, the method upper takes a string and returns a new string with all uppercase letters. Instead of the function syntax upper(word), it uses the method syntax word.upper(). > 字符串提供了一些方法，这些方法能够进行很多有用的操作。方法和函数有些类似，也接收参数然后返回一个值，但语法稍微不同。比如，upper 这个方法就读取一个字符串，返回一个全部为大写字母的新字符串。 > 与函数的 upper(word)语法不同，方法的语法是 word.upper()。 ```Python >>> word = 'banana' >>> word = 'banana' >>> new_word = word.upper() >>> new_word = word.upper() >>> new_word 'BANANA' >>> new_word 'BANANA' ``` This form of dot notation specifies the name of the method, upper, and the name of the string to apply the method to, word. The empty parentheses indicate that this method takes no arguments. > 这种用点号分隔的方法表明了使用的方法名字为 upper，使用这个方法的字符串的名字为 word。后面括号里面是空白的，表示这个方法不接收参数。 A method call is called an invocation; in this case, we would say that we are invoking upper on word. > 方法的调用被叫做——调用（译者注：这真是扯淡，中文都混淆成调用，英文里面 invocation 和 invoke 都有祈祷的意思，和 call 有显著的意义差别，但中文都混淆成调用，这种例子不胜枚举，所以大家尽量多读原版作品。）；在这里，我们就说调用了 word 的 upper 方法。 As it turns out, there is a string method named find that is remarkably similar to the function we wrote: > 结果我们发现string 有一个方法叫做 find，跟我们写过的函数 find 有惊人的相似： ```Python >>> word = 'banana' >>> word = 'banana' >>> index = word.find('a') >>> index = word.find('a') >>> index >>> index 1 ``` In this example, we invoke find on word and pass the letter we are looking for as a parameter. > 在这里我们调用了 word 的 find 方法，然后给定了我们要找的字母 a 作为一个参数。 Actually, the find method is more general than our function; it can find substrings, not just characters: > 实际上，这个 find 方法比我们的 find 函数功能更通用；它不仅能查找字符，还能查找字符串： ```Python >>> word.find('na') >>> word.find('na') 2 ``` By default, find starts at the beginning of the string, but it can take a second argument, the index where it should start: > 默认情况下 find 方法从字符串的开头来查找，不过可以给它一个第二个参数，让它从指定位置查找： ```Python >>> word.find('na', 3) >>> word.find('na', 3) 4 ``` This is an example of an optional argument; find can also take a third argument, the index where it should stop: > 这是一个可选参数的例子；find 方法还能接收第三个参数，可以指定查找终止的位置： ```Python >>> name = 'bob' >>> name = 'bob' >>> name.find('b', 1, 2) >>> name.find('b', 1, 2) -1 ``` This search fails because b does not appear in the index range from 1 to 2, not including 2. Searching up to, but not including, the second index makes find consistent with the slice operator. > 这个搜索失败了，因为 b 并没有在索引1到2且不包括2的字符中间出现。搜索到指定的第三个变量作为索引的位置，但不包括该位置，这就让 find 方法与切片操作符相一致。 ## 8.9 The in operator 运算符 in The word in is a boolean operator that takes two strings and returns True if the first appears as a substring in the second: > in 这个词在字符串操作中是一个布尔操作符，它读取两个字符串，如果前者的字符串为后者所包含，就返回真，否则为假： ```Python >>> 'a' in 'banana' >>> 'a' in 'banana' True >>> 'seed' in 'banana' >>> 'seed' in 'banana' False ``` For example, the following function prints all the letters from word1 that also appear in word2: > 举个例子，下面的函数显示所有同时在 word1和 word2当中出现的字母： ```Python def in_both(word1, word2): for letter in word1: if letter in word2: print(letter) ``` With well-chosen variable names, Python sometimes reads like English. You could read this loop, “for (each) letter in (the first) word, if (the) letter (appears) in (the second) word, print (the) letter.” Here’s what you get if you compare apples and oranges: > 选好变量名的话，Python 有时候读起来就跟英语差不多。你读一下这个循环，就能发现，『对第一个 word 当中的每一个字母letter，如果这个字母也在第二个 word 当中出现，就输出这个字母 letter。』 ```Python >>> in_both('apples', 'oranges') >>> in_both('apples', 'oranges') a e s ``` ## 8.10 String comparison 字符串对比 The relational operators work on strings. To see if two strings are equal: > 关系运算符对于字符串来说也可用。比如可以看看两个字符串是不是相等： ```Python if word == 'banana': print('All right, bananas.') ``` Other relational operations are useful for putting words in alphabetical order: > 其他的关系运算符可以来把字符串按照字母表顺序排列： ```Python 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.') ``` Python does not handle uppercase and lowercase letters the same way people do. All the uppercase letters come before all the lowercase letters, so: Your word, Pineapple, comes before banana. > Python 对大小写字母的处理与人类常规思路不同。所有大写字母都在小写字母之前，所以顺序上应该是： > Your word，然后是 Pineapple，然后才是 banana。 A common way to address this problem is to convert strings to a standard format, such as all lowercase, before performing the comparison. Keep that in mind in case you have to defend yourself against a man armed with a Pineapple. > 一个解决这个问题的普遍方法是把字符串转换为标准格式，比如都转成小写的，然后再进行比较。一定要记得哈，以免你遇到一个用 Pineapple 武装着自己的家伙的时候手足无措。 ## 8.11 Debugging 调试 When you use indices to traverse the values in a sequence, it is tricky to get the beginning and end of the traversal right. Here is a function that is supposed to compare two words and return True if one of the words is the reverse of the other, but it contains two errors: > 使用索引来遍历一个序列中的值的时候，弄清楚遍历的开头和结尾很不容易。下面这个函数用来对比两个单词，如果一个是另一个的倒序就返回真，但这个函数代码中有两处错误： ```Python def is_reverse(word1, word2): if len(word1) != len(word2): return False i = 0 j = len(word2) while j > 0: if word1[i] != word2[j]: return False i = i+1 j = j-1 return True ``` The first if statement checks whether the words are the same length. If not, we can return False immediately. Otherwise, for the rest of the function, we can assume that the words are the same length. This is an example of the guardian pattern in Section 6.8. > 第一个 if 语句是检查两个词的长度是否一样。如果不一样长，当场就返回假。对函数其余部分，我们假设两个单词一样长。这里用到了守卫模式，在第6章第8节我们提到过。 i and j are indices: i traverses word1 forward while j traverses word2 backward. If we find two letters that don’t match, we can return False immediately. If we get through the whole loop and all the letters match, we return True. If we test this function with the words “pots” and “stop”, we expect the return value True, but we get an IndexError: > i 和 j 都是索引：i 从头到尾遍历单词 word1，而 j 逆向遍历单词word2.如果我们发现两个字母不匹配，就可以立即返回假。如果经过整个循环，所有字母都匹配，就返回真。 > 如果我们用这个函数来处理单词『pots』和『stop』，我们希望函数返回真，但得到的却是索引错误： ```Python >>> is_reverse('pots', 'stop') >>> is_reverse('pots', 'stop') ... File "reverse.py", line 15, in is_reverse if word1[i] != word2[j]: IndexError: string index out of range ``` For debugging this kind of error, my first move is to print the values of the indices immediately before the line where the error appears. > 为了改正这个错误，第一步就是在出错的那行之前先输出索引的值。 ```Python while j > 0: print(i, j) # print here if word1[i] != word2[j]: return False i = i+1 j = j-1 ``` Now when I run the program again, I get more information: > 然后我再次运行函数，得到更多信息了： ```Python >>> is_reverse('pots', 'stop') >>> is_reverse('pots', 'stop') 0 4 ... IndexError: string index out of range ``` The first time through the loop, the value of j is 4, which is out of range for the string 'pots'. The index of the last character is 3, so the initial value for j should be len(word2)-1. If I fix that error and run the program again, I get: > 第一次循环完毕的时候，j 的值是4，这超出了『pots』这个字符串的范围了（译者注：应该是0-3）。最后一个索引应该是3，所以 j 的初始值应该是 len(word2)-1。 ```Python >>> is_reverse('pots', 'stop') >>> is_reverse('pots', 'stop') 0 3 1 2 2 1 True ``` This time we get the right answer, but it looks like the loop only ran three times, which is suspicious. To get a better idea of what is happening, it is useful to draw a state diagram. During the first iteration, the frame for is_reverse is shown in Figure 8.2. > 这次我们得到了正确的结果，但似乎循环只走了三次，这有点奇怪。为了弄明白带到怎么回事，我们可以画一个状态图。在第一次迭代的过程中，is_reverse 的框架如图8.2所示。 * * *  Figure 8.2: State diagram. * * * I took some license by arranging the variables in the frame and adding dotted lines to show that the values of i and j indicate characters in word1and word2. Starting with this diagram, run the program on paper, changing the values ofi and j during each iteration. Find and fix the second error in this function. > 我通过设置变量框架中添加虚线表明，i和j的值显示在人物word1and word2拿许可证。 从这个图上运行的程序，文件，更改这些值I和J在每一次迭代过程。发现并解决此函数中的二次错误。 ## 8.12 Glossary 术语列表 object: Something a variable can refer to. For now, you can use “object” and “value” interchangeably. > 对象：一个值能够指代的东西。目前为止，你可以把对象和值暂且作为一码事来理解。 sequence: An ordered collection of values where each value is identified by an integer index. > 序列：一系列值的有序排列，每一个值都有一个唯一的整数序号。 item: One of the values in a sequence. > 元素：一列数值序列当中的一个值。 index: An integer value used to select an item in a sequence, such as a character in a string. In Python indices start from 0. > 索引：一个整数值，用来指代一个序列中的特定一个元素，比如在字符串里面就指代一个字符。在 Python 里面索引从0开始计数。 slice: A part of a string specified by a range of indices. > 切片：字符串的一部分，通过一个索引区间来取得。 empty string: A string with no characters and length 0, represented by two quotation marks. > 空字符串：没有字符的字符串，长度为0，用两个单引号表示。 immutable: The property of a sequence whose items cannot be changed. > 不可更改：一个序列中所有元素不能被改变的性质。 traverse: To iterate through the items in a sequence, performing a similar operation on each. > 遍历：在一个序列中依次对每一个元素进行某种相似运算的过程。 search: A pattern of traversal that stops when it finds what it is looking for. > 搜索：一种遍历的模式，找到要找的内容的时候就停止。 counter: A variable used to count something, usually initialized to zero and then incremented. > 计数：一种用来统计某种东西数量的变量，一般初始化为0，然后逐次递增。 invocation: A statement that calls a method. > 方法调用：调用方法的语句。 optional argument: A function or method argument that is not required. > 可选参数：一个函数或者方法中有一些参数是可选的，非必需的。 ## 8.13 Exercises 练习 ### Exercise 1 练习1 Read the documentation of the string methods at [here](http://docs.python.org/2/library/stdtypes.html#string-methods). You might want to experiment with some of them to make sure you understand how they work. strip and replace are particularly useful. > 阅读 [这里](http://docs.python.org/2/library/stdtypes.html#string-methods)关于字符串的文档。你也许会想要试试其中一些方法，来确保你理解它们的意义。比如 strip 和 replace 都特别有用。 The documentation uses a syntax that might be confusing. For example, in find(sub[, start[, end]]), the brackets indicate optional arguments. So sub is required, but start is optional, and if you include start, then end is optional. > 文档的语法有可能不太好理解。比如在find 这个方法中，方括号表示了可选参数。所以 sub 是必须的参数，但 start 是可选的，如果你包含了 start，end 就是可选的了。 ### Exercise 2 练习2 There is a string method called count that is similar to the function in Section 8.7. Read the documentation of this method and write an invocation that counts the number of 'a's in 'banana'. > 字符串有个方法叫 count，与咱们在8.7中写的 count 函数很相似。 阅读一下这个方法的文档，然后写一个调用这个方法的代码，统计一下 banana 这个单词中 a 出现的次数 。 ### Exercise 3 练习3 A string slice can take a third index that specifies the “step size;” that is, the number of spaces between successive characters. A step size of 2 means every other character; 3 means every third, etc. > 字符串切片可以使用第三个索引，作为步长来使用；步长的意思就是取字符的间距。一个步长为2的意思就是每隔一个取一个字符；3的意思就是每次取第三个，以此类推。 ```Python >>> fruit = 'banana' >>> fruit = 'banana' >>> fruit[0:5:2] >>> fruit[0:5:2] 'bnn' ``` A step size of -1 goes through the word backwards, so the slice [::-1]generates a reversed string. Use this idiom to write a one-line version of is_palindrome from Exercise 3. > 步长如果为-1，意思就是倒序读取字符串，所以[::-1]这个切片就会生成一个逆序的字符串了。 > 使用这个方法把练习三当中的is_palindrome写成一个一行代码的版本。 ### Exercise 4 练习4 The following functions are all intended to check whether a string contains any lowercase letters, but at least some of them are wrong. For each function, describe what the function actually does (assuming that the parameter is a string). > 下面这些函数都试图检查一个字符串是不是包含小写字母，但他们当中肯定有些是错的。描述一下每个函数真正的行为（假设参数是一个字符串）。 ```Python def any_lowercase1(s): for c in s: if c.islower(): return True else: return False def any_lowercase2(s): for c in s: if 'c'.islower(): return 'True' else: return 'False' def any_lowercase3(s): for c in s: flag = c.islower() return flag def any_lowercase4(s): flag = False for c in s: flag = flag or c.islower() return flag def any_lowercase5(s): for c in s: if not c.islower(): return False return True ``` ### Exercise 5 练习5 A Caesar cypher is a weak form of encryption that involves “rotating” each letter by a fixed number of places. To rotate a letter means to shift it through the alphabet, wrapping around to the beginning if necessary, so ’A’ rotated by 3 is ’D’ and ’Z’ rotated by 1 is ’A’. > 凯撒密码是一种简单的加密方法，用的方法是把每个字母进行特定数量的移位。对一个字母移位就是把它根据字母表的顺序来增减对应，如果到末尾位数不够就从开头算剩余的位数，『A』移位3就是『D』，而『Z』移位1就是『A』了。 To rotate a word, rotate each letter by the same amount. For example, “cheer” rotated by 7 is “jolly” and “melon” rotated by -10 is “cubed”. In the movie 2001: A Space Odyssey, the ship computer is called HAL, which is IBM rotated by -1. > 要对一个词进行移位，要把每个字母都移动同样的数量。比如『cheer』这个单词移位7就是『jolly』，而『melon』移位-10就是『cubed』。在电影《2001 太空漫游》中，飞船的电脑叫 HAL，就是 IBM 移位-1。 Write a function called rotate_word that takes a string and an integer as parameters, and returns a new string that contains the letters from the original string rotated by the given amount. > 写一个名叫 rotate_word 的函数，接收一个字符串和一个整形为参数，返回将源字符串移位该整数位得到的新字符串。 You might want to use the built-in function ord, which converts a character to a numeric code, and chr, which converts numeric codes to characters. Letters of the alphabet are encoded in alphabetical order, so for example: > 你也许会用得上内置函数 ord，它把字符转换成数值代码，然后还有个 chr 是用来把数值代码转换成字符。字母表中的字母都被编译成跟字母表中同样的顺序了，所以如下所示： ```Python >>> ord('c') - ord('a') >>> ord('c') - ord('a') 2 ``` Because 'c' is the two-eth letter of the alphabet. But beware: the numeric codes for upper case letters are different. > c 是字母表中的第『2』个（译者注：从0开始数哈）的位置，所以上述结果是2。但注意：大写字母的数值代码是和小写的不一样的。 Potentially offensive jokes on the Internet are sometimes encoded in ROT13, which is a Caesar cypher with rotation 13. If you are not easily offended, find and decode some of them. [Solution](http://thinkpython2.com/code/rotate.py). > 网上很多有冒犯意义的玩笑都是用 ROT13加密的，也就是移位13的凯撒密码。如果你不太介意，找一下这些密码解密一下吧。[样例代码](http://thinkpython2.com/code/rotate.py).