Smalltalk and Object Orientation: an Introduction - Free

x inspect.
This has some similarities to a hash table found in some other languages (e.g. Common LISP) or in
libraries available for other languages (e.g. C). The great advantage of Smalltalk is that everyone has
the same type of Dictionary. In Pascal o r C almost everyone would have to re -invent their own or
purchase a library to get the same functionality. This, of course, leads to problems of consistency
between implementations.
Another form of the Dictionary collection also exists. This form is termed the
IdentityDictionary. Like the IdentitySet it uses the == test rather than the = test. This
means that for an object to be returned, the object used as the key must be the same object as was used
to create the key and not just an equivalent object. Like the IdentitySet it is more efficient that the
standard Dictionary class and may be used in certain circumstances.
11.8 Strings
Strings are represented in Smalltalk as collections of characters. Unlike in languages such as C,
there is no need to provide special string processing functions such as strcpy(). Instead, we can treat
strings as collections (and iterate over their contents) or as “strings” in their own right. This is because
we are inheriting the collection class (and the su bclasses between the collection class and the class
String) methods and are using the methods defined in the class String.
Useful String operations include the concatenation of strings, searching for substrings and pattern
matching between s trings. For example, sameAs: aString answers whether the receiver and
aString match precisely (ignoring case differences).
To concatenate two strings together use a comma. For example:
myName := 'John' , ' E ' , 'Hunt'.
Notice that you do not have to specify the length of the string. Remember they are just objects and a
variable can hold any object.
Substring operations are essential in any language and Smalltalk provides a variety of features for
searching for substrings and pat tern matching between strings. For example, the findString:
startingAt: message. This message searches the receiving string for the string passed as a
parameter to the method. For example:
'John Hunt was here' findString: 'Hunt' startingAt: 1.
There are two pattern matching methods defined for strings, sameCharacters: and match:. The
first method counts the number of beginning characters which are the same. The second method is more
powerful and can include two types of wild card ( # which can represent any single character and *
which can represent zero or more characters). For example:
‘Dr. * Hunt’ match: ‘Dr. John Hunt’.
This expression evaluates to true. There is a variant of the match: method which can either ignore or
consider case; match:ignoreCase: (which takes either true or false as the second argument).
Instances of class String also respond to boolean operators such as ‘=’. Strings can also be converted into lower or upper
case using asLowerCase and asUpperCase. Finally, as st rings are types of collection they can
respond to the iteration message described above. These can be used for performing some operation on
each character in a string in turn.
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