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Pike works with data items, also called values, such as the integers 17 and -4717, or the string of characters "Hello world!". Each data item has, or "is", a certain type: 17 and -4717 both have the type integer, and "Hello world!" has the type string. If we want to refer to the data type integer in a Pike program, we write int. For the type string, we write string.

Each variable, method, and method parameter also has a type. The types of these determines which data items you can put in the variable, return from the method, or use as arguments to the method. An integer variable, created with

int i;

can only contain integer values. If we try to put something else, such as a string, in this variable, Pike will try to discover the discrepancy and complain about it. This is called type checking.

string and int are built-in types in Pike. You can also create your own data types, or classes. We have already seen some examples of this, for example the class Query, which is part of the module Protocols.HTTP, and which is used to contain the data we get when we retrieve a page from the World Wide Web.

We will examine each individual data type in some detail in a later chapter, but this chapter will give an introduction to data types in general.

Different Kinds of Types

The data types in Pike can be divided into three categories: the basic types (int, float, and string), the container types (array, mapping, multiset), and then the three types program, object and function. We will start by looking at the basic types.