How to Think Like a Computer Scientist: Strings

A compound data type

So far we have seen five types: int, float, bool, NoneType and str. Strings are qualitatively different from the other four because they are made up of smaller pieces---characters.

Types that comprise smaller pieces are called compound data types. Depending on what we are doing, we may want to treat a compound data type as a single thing, or we may want to access its parts. This ambiguity is useful.

The bracket operator selects a single character from a string.

>>> fruit = "banana"
>>> letter = fruit[1]
>>> print letter

The expression fruit[1] selects character number 1 from fruit. The variable letter refers to the result. When we display letter, we get a surprise:

The first letter of "banana" is not a. Unless you are a computer scientist. For perverse reasons, computer scientists always start counting from zero. The 0th letter (zero-eth) of "banana" is b. The 1th letter (one-eth) is a, and the 2th (two-eth) letter is n.

If you want the zero-eth letter of a string, you just put 0, or any expression with the value 0, in the brackets:

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

The expression in brackets is called an index. An index specifies a member of an ordered set, in this case the set of characters in the string. The index indicates which one you want, hence the name. It can be any integer expression.

Length

The len function 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]       # ERROR!

That won't work. It causes the runtime error IndexError: string index out of range. The reason is that there is no 6th letter in "banana". Since we started counting at zero, the six letters are numbered 0 to 5. To get the last character, we have to subtract 1 from length:

length = len(fruit)
last = fruit[length-1]

Alternatively, we 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 and the `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 encode a traversal is with a while statement:

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 is not executed. The last character accessed is the one with the index len(fruit)-1, which is the last character in the string.

Using an index to traverse a set of values is so common that Python provides an alternative, simpler syntax---the for loop:

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 and a for loop to generate an abecedarian series. Abecedarian refers to a series or list in which the elements appear in alphabetical order. For example, 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 of this program is:

Jack
Kack
Lack
Mack
Nack
Oack
Pack
Qack

Of course, that's not quite right because Ouack and Quack are misspelled. You'll fix this as an exercise below.

String slices

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

>>> s = "Peter, Paul, and Mary"
>>> print s[0:5]
Peter
>>> print s[7:11]
Paul
>>> print s[17:21]
Mary

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; it makes more sense if you imagine the indices pointing between the characters, as in the following diagram:

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. Thus:

>>> fruit = "banana"
>>> fruit[:3]
'ban'
>>> fruit[3:]
'ana'

What do you think s[:] means?

String comparison

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

if word == "banana":
    print  "Yes, we have no bananas!"

Other comparison 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 "Yes, we have no bananas!"

You should be aware, though, that Python does not handle upper- and lowercase letters the same way that people do. All the uppercase letters come before all the lowercase letters. As a result:

Your word, Zebra, 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. A more difficult problem is making the program realize that zebras are not fruit.

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'            # ERROR!
print greeting

Instead of producing the output Jello, world!, this code produces the runtime error TypeError: 'str' object doesn't support item assignment.

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!"
newGreeting = 'J' + greeting[1:]
print newGreeting

The solution here is to concatenate a new first letter onto a slice of greeting. This operation has no effect on the original string.

A `find` function

What does the following function do?

def find(strng, ch):
    index = 0
    while index < len(strng):
        if strng[index] == ch:
            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 strng[index] == ch, the function returns immediately, breaking out of the loop prematurely.

If the character doesn't appear in the string, then the program exits the loop normally and returns -1.

This pattern of computation is sometimes called a eureka traversal because as soon as we find what we are looking for, we can cry Eureka! and stop looking.

Looping and counting

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

fruit = "banana"
count = 0
for char in fruit:
    if char == '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. (To increment is to increase by one; it is the opposite of decrement, and unrelated to excrement, which is a noun.) When the loop exits, count contains the result---the total number of a's.

The `string` module

The string module contains useful functions that manipulate strings. As usual, we have to import the module before we can use it:

>>> import string

The string module includes a function named find that does the same thing as the function we wrote. To call it we have to specify the name of the module and the name of the function using dot notation.

>>> fruit = "banana"
>>> index = string.find(fruit, "a")
>>> print index
1

This example demonstrates one of the benefits of modules---they help avoid collisions between the names of built-in functions and user-defined functions. By using dot notation we can specify which version of find we want.

Actually, string.find is more general than our version. First, it can find substrings, not just characters:

>>> string.find("banana", "na")
2

Also, it takes an additional argument that specifies the index it should start at:

>>> string.find("banana", "na", 3)
4

Or it can take two additional arguments that specify a range of indices:

>>> string.find("bob", "b", 1, 2)
-1

In this example, the search fails because the letter b does not appear in the index range from 1 to 2 (not including 2).

Character classification

It is often helpful to examine a character and test whether it is upper- or lowercase, or whether it is a character or a digit. The string module provides several constants that are useful for these purposes.

The string string.lowercase contains all of the letters that the system considers to be lowercase. Similarly, string.uppercase contains all of the uppercase letters. Try the following and see what you get:

>>> print string.lowercase
>>> print string.uppercase
>>> print string.digits

We can use these constants and find to classify characters. For example, if find(lowercase, ch) returns a value other than -1, then ch must be lowercase:

def isLower(ch):
    return string.find(string.lowercase, ch) != -1

Alternatively, we can take advantage of the in operator, which determines whether a character appears in a string:

def isLower(ch):
    return ch in string.lowercase

As yet another alternative, we can use the comparison operator:

def isLower(ch):
    return 'a' <= ch <= 'z'

If ch is between a and z, it must be a lowercase letter.

Another constant defined in the string module may surprise you when you print it:

>>> print string.whitespace

Whitespace characters move the cursor without printing anything. They create the white space between visible characters (at least on white paper). The constant string.whitespace contains all the whitespace characters, including space, tab (\t), and newline (\n).

There are other useful functions in the string module, but this book isn't intended to be a reference manual. On the other hand, the Python Library Reference is. Along with a wealth of other documentation, it's available from the Python website, http://www.python.org.

Glossary

compound data type:: A data type in which the values are made up of components, or elements, that are themselves values.
traverse:: To iterate through the elements of a set, performing a similar operation on each.
index:: A variable or value used to select a member of an ordered set, such as a character from a string.
slice:: A part of a string specified by a range of indices.
mutable:: A compound data types whose elements can be assigned new values.
counter:: A variable used to count something, usually initialized to zero and then incremented.
increment:: To increase the value of a variable by one.
decrement:: To decrease the value of a variable by one.
whitespace:: Any of the characters that move the cursor without printing visible characters. The constant string.whitespace contains all the white-space characters.

Exercises

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

Modify:

prefixes = "JKLMNOPQ"
suffix = "ack"
   
for letter in prefixes:
    print letter + suffix

so that Ouack and Quack are spelled correctly.

Modify the find function so that it takes a third parameter, the index in the string where it should start looking.

Encapsulate

fruit = "banana"
count = 0
for char in fruit:
    if char == 'a':
    count = count + 1
print count

in a function named countLetters, and generalize it so that it accepts the string and the letter as arguments.

Now rewrite the countLetters function so that instead of traversing the string, it uses your three-parameter version of find.
Which version of isLower do you think will be fastest? Can you think of other reasons besides speed to prefer one version or the other?

Strings