What are the predefined identifiers in C?

C language predefined identifiers are compiler-defined special identifiers for specific purposes. Common predefined identifiers include data type specifiers (such as int, float), control statements (such as if, else), and functions (such as printf, scanf). Predefined identifiers cannot be redefined or renamed, and intentionally using them as variable names will cause an error. Misuse of keywords (such as fore, inti) is a common mistake, and you should develop good programming habits to avoid mistakes. Preprocessing instructions (such as #define, #include) are also predefined identifiers and are used to control compilation. Understanding predefined identifiers helps to write concise and elegant C code.

C language predefined identifier? This question is awesome! It looks simple on the surface, but in fact there are many ways to do it. Do you think it's just a few simple keywords? wrong! This is the cornerstone of C language. Only by understanding them can you truly master the essence of this language.

Let's first talk about what is a predefined identifier. Simply put, it is an identifier with a specific meaning that the C compiler has defined in advance. You can't use them to name variables, functions, or anything else unless you really want to cause trouble for yourself. It's like Lego bricks, and predefined identifiers are those special blocks of building blocks that have fixed shapes and functions that you can't change at will.

Common predefined identifiers, such as int , float , char , void type specifiers, you must have seen them, and they define the data types of variables. There are also control statements such if , else , for , and while , which determine the execution process of the program. For example, return is responsible for the return value of the function; sizeof is to obtain the number of bytes of the data type; struct is to define the structure; union is to define the union... These are all predefined by the compiler, just use it directly.

But this is just the tip of the iceberg. Some header files also define a large number of predefined identifiers, such as printf and scanf in <stdio.h></stdio.h> , malloc , free in <stdlib.h></stdlib.h> , etc. These functions are library functions, providing many commonly used functions, saving us the trouble of writing our own code. However, you have to know that they are predefined and cannot be renamed or redefined at will.

Here are some tips to help you better understand these predefined identifiers. Try it, deliberately use int as the variable name to see how the compiler will report an error. You will find that the compiler will tell you int has been defined and cannot be defined repeatedly. This will allow you to more deeply understand the particularity of predefined identifiers.

When it comes to pitfalls, the most common thing is to misuse keywords. For example, if you may accidentally write for fore , or write int as inti , the compiler will report an error. This seems like a minor error, but it is very difficult to debug. Therefore, it is very important to develop good programming habits, carefully check the code, and avoid spelling errors.

To go deeper, some preprocessing directives, such as #define and #include , are not predefined identifiers in the strict sense, but they are also predefined instructions by the compiler and used to control the compilation process. Understanding these instructions is essential to writing efficient, maintainable code.

Finally, I would like to remind you that there are a lot of predefined identifiers in the C language standard library, and it is impossible to remember them all. But it is most important to know their existence and how to find them. Proficiency in using these predefined identifiers allows you to write more concise and elegant C code. Read more code and practice more, and you will slowly master these skills. Remember, programming is a job of real knowledge!

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