Chapter 9

> 来源: Think Python 2e (Allen B. Downey)

> 原页: https://greenteapress.com/thinkpython/html/thinkpython009.html

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[ [](thinkpython010.html)

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Chapter 8 Strings

8.1 A string is a sequence

A string is a sequence of characters. You can access the characters one at a time with the bracket operator:

>>> fruit = 'banana'
>>> letter = fruit[1]

The second statement selects character number 1 from fruit and assigns it to fruit0.

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:

fruit0008

For most people, the first letter of fruit000 is b, not b. But for computer scientists, the index is an offset from the beginning of the string, and the offset of the first letter is zero.

fruit0012

So b is the 0th letter (“zero-eth”) of fruit001, b is the 1th letter (“one-eth”), and b is the 2th (“two-eth”) letter.

You can use any expression, including variables and operators, as an index, but the value of the index has to be an integer. Otherwise you get:

fruit0017

8.2 `b`00

b00 is a built-in function that returns the number of characters in a string:

fruit0020

To get the last letter of a string, you might be tempted to try something like this:

fruit0021

The reason for the fruit0022 is that there is no letter in fruit002 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 fruit0:

fruit0025

Alternatively, you can use negative indices, which count backward from the end of the string. The expression fruit0026 yields the last letter, fruit0027 yields the second to last, and so on.

8.3 Traversal with a `b`00 loop

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 fruit loop:

fruit0030

This loop traverses the string and displays each letter on a line by itself. The loop condition is fruit0031, so when fruit is equal to the length of the string, the condition is false, and the body of the loop is not executed. The last character accessed is the one with the index fruit0033, which is the last character in the string.

Exercise 1

Write a function that takes a string as an argument and displays the letters backward, one per line.

Another way to write a traversal is with a b00 loop:

fruit0035

Each time through the loop, the next character in the string is assigned to the variable b000. The loop continues until no characters are left.

The following example shows how to use concatenation (string addition) and a b00 loop to generate an abecedarian series (that is, in alphabetical order). 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:

fruit0038

The output is:

fruit0039

Of course, that’s not quite right because “Ouack” and “Quack” are misspelled.

Exercise 2

Modify the program to fix this error.

8.4 String slices

A segment of a string is called a slice. Selecting a slice is similar to selecting a character:

fruit0040

The operator fruit 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 counterintuitive, but it might help to imagine the indices pointing between the characters, as in Figure 8.1.

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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:

fruit0042

If the first index is greater than or equal to the second the result is an empty string, represented by two quotation marks:

fruit0043

An empty string contains no characters and has length 0, but other than that, it is the same as any other string.

Exercise 3

Given that fruit is a string, what does fruit004 mean?

8.5 Strings are immutable

It is tempting to use the b0 operator on the left side of an assignment, with the intention of changing a character in a string. For example:

fruit0047

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. An item is one of the values in a sequence.

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:

fruit0048

This example concatenates a new first letter onto a slice of fruit004. It has no effect on the original string.

8.6 Searching

What does the following function do?

fruit0050

In a sense, b000 is the opposite of the b0 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 b0.

This is the first example we have seen of a fruit0 statement inside a loop. If fruit0055, 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 b0.

This pattern of computation—traversing a sequence and returning when we find what we are looking for—is called a search.

Exercise 4

Modify b000 so that it has a third parameter, the index in b000 where it should start looking.

8.7 Looping and counting

The following program counts the number of times the letter b appears in a string:

fruit0060

This program demonstrates another pattern of computation called a counter. The variable fruit is initialized to 0 and then incremented each time an b is found. When the loop exits, fruit contains the result—the total number of b’s.

Exercise 5

Encapsulate this code in a function named fruit, and generalize it so that it accepts the string and the letter as arguments.

Exercise 6

Rewrite this function so that instead of traversing the string, it uses the three-parameter version of b000 from the previous section.

8.8 String methods

A method is similar to a function—it takes arguments and returns a value—but the syntax is different. For example, the method fruit takes a string and returns a new string with all uppercase letters:

Instead of the function syntax fruit0068, it uses the method syntax fruit0069.

fruit0070

This form of dot notation specifies the name of the method, fruit, and the name of the string to apply the method to, b000. The empty parentheses indicate that this method takes no argument.

A method call is called an invocation; in this case, we would say that we are invoking fruit on the b000.

As it turns out, there is a string method named b000 that is remarkably similar to the function we wrote:

fruit0076

In this example, we invoke b000 on b000 and pass the letter we are looking for as a parameter.

Actually, the b000 method is more general than our function; it can find substrings, not just characters:

fruit0080

It can take as a second argument the index where it should start:

fruit0081

And as a third argument the index where it should stop:

fruit0082

This search fails because b does not appear in the index range from b to b (not including b).

Exercise 7

There is a string method called fruit that is similar to the function in the previous exercise. Read the documentation of this method and write an invocation that counts the number of bs in fruit008.

Exercise 8

Read the documentation of the string methods at fruit0090. You might want to experiment with some of them to make sure you understand how they work. fruit and fruit00 are particularly useful.

The documentation uses a syntax that might be confusing. For example, in fruit0093, the brackets indicate optional arguments. So b00 is required, but fruit is optional, and if you include fruit, then b00 is optional.

8.9 The `b`0 operator

The word b0 is a boolean operator that takes two strings and returns b000 if the first appears as a substring in the second:

fruit0101

For example, the following function prints all the letters from fruit that also appear in fruit:

fruit0104

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:

fruit0105

8.10 String comparison

The relational operators work on strings. To see if two strings are equal:

fruit0106

Other relational operations are useful for putting words in alphabetical order:

fruit0107

Python does not handle uppercase and lowercase letters the same way that people do. All the uppercase letters come before all the lowercase letters, so:

fruit0108

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.

8.11 Debugging

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 b000 if one of the words is the reverse of the other, but it contains two errors:

fruit0110

The first b0 statement checks whether the words are the same length. If not, we can return fruit immediately and then, 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.

b and b are indices: b traverses fruit forward while b traverses fruit backward. If we find two letters that don’t match, we can return fruit immediately. If we get through the whole loop and all the letters match, we return b000.

If we test this function with the words “pots” and “stop”, we expect the return value b000, but we get an IndexError:

fruit0122

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.

fruit0123

Now when I run the program again, I get more information:

fruit0124

The first time through the loop, the value of b is 4, which is out of range for the string fruit0. The index of the last character is 3, so the initial value for b should be fruit0128.

If I fix that error and run the program again, I get:

fruit0129

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 fruit0130 is shows in Figure 8.2.

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Figure 8.2: State diagram.

I took a little license by arranging the variables in the frame and adding dotted lines to show that the values of b and b indicate characters in fruit and fruit.

Exercise 9

Starting with this diagram, execute the program on paper, changing the values of b and b during each iteration. Find and fix the second error in this function.

8.12 Glossary

object:

Something a variable can refer to. For now, you can use “object” and “value” interchangeably.

sequence:

An ordered set; that is, a set of values where each value is identified by an integer index.

item:

One of the values in a sequence.

index:

An integer value used to select an item in a sequence, such as a character in a string.

slice:

A part of a string specified by a range of indices.

empty string:

A string with no characters and length 0, represented by two quotation marks.

immutable:

The property of a sequence whose items cannot be assigned.

traverse:

To iterate through the items in a sequence, performing a similar operation on each.

search:

A pattern of traversal that stops when it finds what it is looking for.

counter:

A variable used to count something, usually initialized to zero and then incremented.

method:

A function that is associated with an object and called using dot notation.

invocation:

A statement that calls a method.

8.13 Exercises

Exercise 10

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.

fruit0137

A step size of -1 goes through the word backwards, so the slice fruit0 generates a reversed string.

Use this idiom to write a one-line version of fruit0139 from Exercise 6.

Exercise 11

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).

fruit0140

Exercise 12

ROT13 is a weak form of encryption that involves “rotating” each letter in a word by 13 places. To rotate a letter means to shift it through the alphabet, wrapping around to the beginning if necessary, so ’A’ shifted by 3 is ’D’ and ’Z’ shifted by 1 is ’A’.

Write a function called fruit0141 that takes a string and an integer as parameters, and that returns a new string that contains the letters from the original string “rotated” by the given amount.

For example, “cheer” rotated by 7 is “jolly” and “melon” rotated by -10 is “cubed”.

You might want to use the built-in functions b00, which converts a character to a numeric code, and b00, which converts numeric codes to characters.

Potentially offensive jokes on the Internet are sometimes encoded in ROT13. If you are not easily offended, find and decode some of them. Solution: fruit0144.

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