10. Numbers, Characters, and Strings

While functions, variables, macros, and 25 special operators provide the basic building blocks of the language itself, the building blocks of your programs will be the data structures you use. As Fred Brooks observed in The Mythical Man-Month, “Representation is the essence of programming.”1

Common Lisp provides built-in support for most of the data types typically found in modern languages: numbers (integer, floating point, and complex), characters, strings, arrays (including multidimensional arrays), lists, hash tables, input and output streams, and an abstraction for portably representing filenames. Functions are also a first-class data type in Lisp—they can be stored in variables, passed as arguments, returned as return values, and created at runtime.

And these built-in types are just the beginning. They’re defined in the language standard so programmers can count on them being available and because they tend to be easier to implement efficiently when tightly integrated with the rest of the implementation. But, as you’ll see in later chapters, Common Lisp also provides several ways for you to define new data types, define operations on them, and integrate them with the built-in data types.

For now, however, you can start with the built-in data types. Because Lisp is a high-level language, the details of exactly how different data types are implemented are largely hidden. From your point of view as a user of the language, the built-in data types are defined by the functions that operate on them. So to learn a data type, you just have to learn about the functions you can use with it. Additionally, most of the built-in data types have a special syntax that the Lisp reader understands and that the Lisp printer uses. That’s why, for instance, you can write strings as "foo"; numbers as 123, 1/23, and 1.23; and lists as (a b c). I’ll describe the syntax for different kinds of objects when I describe the functions for manipulating them.

In this chapter, I’ll cover the built-in “scalar” data types: numbers, characters, and strings. Technically, strings aren’t true scalars—a string is a sequence of characters, and you can access individual characters and manipulate strings with a function that operates on sequences. But I’ll discuss strings here because most of the string-specific functions manipulate them as single values and also because of the close relation between several of the string functions and their character counterparts.