# Think Python/Strings


## 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:

``` python
>>> 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:

``` python
>>> print 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.

``` python
>>> letter = fruit[0]
>>> print letter
b
```

So `b` is the 0th letter ("zero-eth") of `'banana'`, `a` is the 1th
letter ("one-eth"), and `n` 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:

``` python
>>> 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:

``` python
>>> 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`:

``` python
>>> last = fruit[length-1]
>>> print last
a
```

Alternatively, 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:

``` python
index = 0
while index &lt; 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 is not executed. The last character accessed is the one with
the index `len(fruit)-1`, which is the last character in the string.

```{=html}
<DIV CLASS="theorem">
```
**Exercise 1**  *Write a function that takes a string as an argument and
displays the letters backward, one per line.*

```{=html}
</DIV>
```
Another way to write a traversal is with a `for` loop:

``` python
for char in fruit:
    print char
```

Each time through the loop, the next character in the string is assigned
to the variable `char`. 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:

``` python
prefixes = 'JKLMNOPQ'
suffix = 'ack'

for letter in prefixes:
    print letter + suffix
```

The output is:

``` python
Jack
Kack
Lack
Mack
Nack
Oack
Pack
Qack
```

Of course, that's not quite right because "Ouack" and "Quack" are
misspelled.

### Exercise 2

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:

``` python
>>> s = 'Monty Python'
>>> print s[0:5]
Monty
>>> print s[6:13]
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 counterintuitive, but it might help to
imagine the indices pointing *between* the characters, as in the
following diagram:

```{=html}
<DIV CLASS="center">
```
`<IMG SRC="book011.png">`{=html}

```{=html}
</DIV>
```
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:

``` python
>>> 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:

``` python
>>> 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.

### Exercise 3

Given that **\'`fruit`**\' is a string, what does **\'`fruit[:]`**\'
mean?\'\'

## 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:

``` python
>>> greeting = 'Hello, world!'
>>> greeting[0] = 'J'
TypeError: 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. 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:

``` python
>>> greeting = 'Hello, world!'
>>> new_greeting = 'J' + greeting[1:]
>>> print 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?

``` python
def find(word, letter):
    index = 0
    while index &lt; len(word):
        if word[index] == letter:
            return index
        index = index + 1
    return -1
```

In a sense, `find` is the opposite 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 a called a **search**.

### Exercise 4

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:

``` 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.

```{=html}
<DIV CLASS="theorem">
```
**Exercise 5**   *Encapsulate this code in a function named
**\'`count`**\', and generalize it so that it accepts the string and the
letter as arguments.*

```{=html}
</DIV>
```
```{=html}
<DIV CLASS="theorem">
```
**Exercise 6**  *Rewrite this function so that instead of traversing the
string, it uses the three-parameter version of **\'`find`**\' from the
previous section.*

```{=html}
</DIV>
```
## `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 `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()`.

``` python
>>> word = 'banana'
>>> new_word = word.upper()
>>> print 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 argument.

A method call is called an **invocation**; in this case, we would say
that we are invoking `upper` on the `word`.

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

``` python
>>> word = 'banana'
>>> index = word.find('a')
>>> print 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:

``` python
>>> word.find('na')
2
```

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

``` python
>>> word.find('na', 3)
4
```

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

``` python
>>> 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`).

```{=html}
<DIV CLASS="theorem">
```
**Exercise 7**  ***\' There is a string method called**\'`count`**\'
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**\'`a`s in***`banana`***.*

```{=html}
</DIV>
```
## 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:

``` python
>>> 'a' in 'banana'
True
>>> 'seed' in 'banana'
False
```

For example, the following function prints all the letters from `word1`
that also appear in `word2`:

``` 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:

``` python
>>> in_both('apples', 'oranges')
a
e
s
```

## String comparison

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

``` python
if word == 'banana':
    print  'All right, bananas.'
```

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

``` 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 that
people do. All the uppercase letters come before all the lowercase
letters, so:

``` python
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:

``` 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 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.

`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:

``` python
>>> 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.

``` 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:

``` python
>>> 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:

``` python
>>> 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` looks like this:

```{=html}
<DIV CLASS="center">
```
`<IMG SRC="book012.png">`{=html}

```{=html}
</DIV>
```
I took a little 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`.

```{=html}
<DIV CLASS="theorem">
```
**Exercise 8**  ***\' Starting with this diagram, execute the program on
paper, changing the values of**\'`i`**\' and**\'`j`during each
iteration. Find and fix the second error in this function.*

```{=html}
</DIV>
```
## Glossary

```{=html}
<DL CLASS="description">
```
```{=html}
<DT CLASS="dt-description">
```
**object:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
Something a variable can refer to. For now, you can use "object" and
"value" interchangeably.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**sequence:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
An ordered set; that is, a set of values where each value is identified
by an integer index.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**item:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
One of the values in a sequence.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**index:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
An integer value used to select an item in a sequence, such as a
character in a string.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**slice:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
A part of a string specified by a range of indices.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**empty string:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
A string with no characters and length 0, represented by two quotation
marks.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**immutable:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
The property of a sequence whose items cannot be assigned.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**traverse:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
To iterate through the items in a sequence, performing a similar
operation on each.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**search:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
A pattern of traversal that stops when it finds what it is looking for.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**counter:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
A variable used to count something, usually initialized to zero and then
incremented.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**method:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
A function that is associated with an object and called using dot
notation.

```{=html}
</DD>
```
```{=html}
<DT CLASS="dt-description">
```
**invocation:**

```{=html}
</DT>
```
```{=html}
<DD CLASS="dd-description">
```
A statement that calls a method.

```{=html}
</DD>
```
```{=html}
</DL>
```
## Exercises

### Exercise 9

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.

``` python
''>>> 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 **\'6.6**\'.

### Exercise 10

Read the documentation of the string methods at
**\'`docs.python.org/lib/string-methods.html`**\'. 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.

### 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.\'\'

``` 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 12

ROT13 is a weak form of encryption that involves "rotating" each letter
in a word by 13 places[^1]. 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`rotate_word`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 **\'`ord`**\', which
converts a character to a numeric code, and **\'`chr`**\', 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.

## Notes


[^1]: See `<TT>`{=html}wikipedia.org/wiki/ROT13
