Setting Up the Structure Declaration

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A structure declaration is the master plan that describes how a structure is put together. The declaration looks like this:

 Begin:struct book {     char title[MAXTITL];     char author[MAXAUTL];     float value; };

This declaration describes a structure made up of two character arrays and one float variable. It does not create an actual data object, but it describes what constitutes such an object. (Occasionally, we'll refer to a structure declaration as a template because it outlines how data will be stored. If you've heard of templates in C++, that's a different use of the word.) Let's look at the details. First comes the keyword struct . It identifies what comes next as a structure. Next comes an optional tag ”the word book ”which is a shorthand label you can use to refer to this structure. Therefore, later on we have this declaration:

 struct book libry;

It declares libry to be a structure variable using the book structure design. Next in the structure declaration, the list of structure members are enclosed in a pair of braces. Each member is described by its own declaration, complete with a terminating semicolon. For instance, the title portion is a char array with MAXTITL elements. A member can be any C data type ”and that includes other structures!

A semicolon after the closing brace ends the definition of the structure design. You can place this declaration outside any function (externally), as we have done, or inside a function definition. If the declaration is placed inside a function, then its tag can be used only inside that function. If the declaration is external, it is available to all the functions following the declaration in the file. For example, in a second function, you could define

 struct book dickens;

and that function would have a variable dickens that followed the form of the book design.

The tag name is optional, but you must use one when you set up structures as we did, with the structure design defined one place and the actual variables defined elsewhere. We will return to this point soon, after we look at defining structure variables .

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