Strings

=======

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.

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 letter.

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:

::

>>> 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.

::

>>> letter = fruit[0]

>>> 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”).

As an index you can use an expression that contains variables and

operators:

::

>>> i = 1

>>> fruit[i]

'a'

>>> fruit[i+1]

'n'

But the value of the index has to be an integer. Otherwise you get:

::

>>> letter = fruit[1.5]

TypeError: string indices must be integers

len

---

len is a built-in function that returns the number of characters in a

string:

::

>>> fruit = 'banana'

>>> len(fruit)

6

To get the last letter of a string, you might be tempted to try

something like this:

::

>>> length = len(fruit)

>>> 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:

::

>>> last = fruit[length-1]

>>> 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.

Traversal with a for 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 while loop:

::

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.

As an exercise, 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 for loop:

::

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.

The following example shows how to use concatenation (string addition)

and a for 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:

::

prefixes = 'JKLMNOPQ'

suffix = 'ack'

for letter in prefixes:

print(letter + suffix)

The output is:

::

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.

String slices

-------------

A segment of a string is called a **slice**. Selecting a slice is

similar to selecting a character:

::

>>> s = 'Monty Python'

>>> s[0:5]

'Monty'

>>> s[6:12]

'Python'

The operator 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 [fig.banana].

.. figure:: figs/banana.pdf

:alt: Slice indices.

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:

::

>>> fruit = 'banana'

>>> fruit[:3]

'ban'

>>> 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:

::

>>> fruit = 'banana'

>>> 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.

Strings are immutable

---------------------

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:

::

>>> greeting = 'Hello, world!'

>>> 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 [equivalence]).

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:

::

>>> greeting = 'Hello, world!'

>>> new_greeting = 'J' + greeting[1:]

>>> new_greeting

'Jello, world!'

This example concatenates a new first letter onto a slice of greeting.

It has no effect on the original string.

Searching

---------

What does the following function do?

::

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.

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.

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.

Looping and counting

--------------------

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

a string:

::

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.

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.

String methods

--------------

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().

::

>>> word = 'banana'

>>> new_word = word.upper()

>>> 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.

A method call is called an **invocation**; in this case, we would say

that we are invoking upper on word.

As it turns out, there is a string method named find that is remarkably

similar to the function we wrote:

::

>>> word = 'banana'

>>> index = word.find('a')

>>> index

1

In this example, we invoke find on word and pass the letter we are

looking for as a parameter.

Actually, the find method is more general than our function; it can find

substrings, not just characters:

::

>>> 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:

::

>>> 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:

::

>>> name = 'bob'

>>> 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.

The in operator

---------------

The word in is a boolean operator that takes two strings and returns

True if the first appears as a substring in the second:

::

>>> 'a' in 'banana'

True

>>> 'seed' in 'banana'

False

For example, the following function prints all the letters from word1

that also appear in word2:

::

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:

::

>>> in_both('apples', 'oranges')

a

e

s

String comparison

-----------------

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

equal:

::

if word == 'banana':

print('All right, bananas.')

Other relational operations are useful for putting words in alphabetical

order:

::

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.

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.

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 True if one of the

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

::

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 [guardian].

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:

::

>>> 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.

::

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:

::

>>> 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:

::

>>> 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 [fig.state4].

.. figure:: figs/state4.pdf

:alt: State diagram.

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

word1 and word2.

Starting with this diagram, run the program on paper, changing the

values of i and j during each iteration. Find and fix the second error

in this function. [isreverse]

.. _glossary08:

Glossary

--------

.. include:: glossary/08.txt

Exercises

---------

Read the documentation of the string methods at

http://docs.python.org/3/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.

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.

There is a string method called count that is similar to the function in

Section [counter]. Read the documentation of this method and write an

invocation that counts the number of a’s in ``'banana'``.

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.

::

>>> fruit = 'banana'

>>> 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 [palindrome].

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).

::

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

[exrotate] 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’.

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.

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.

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:

::

>>> 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.

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.