Summary of the third class

The operator

Monocular operators

There are many monocular operators , Here are just a few explanations

1. sizeof
   It can calculate types and variables , Such as printf("%d\n",sizeof(int));printf("%d\n",sizeof(a));
   This can be calculated , The result of running is 4（ Define an integer variable a）.
2. ++a,a++
   Novices of this monocular operator often get confused , Or you don't understand the principle thoroughly , But with the accumulation of time, this kind of thing goes directly to the back , I don't think we need to go too deep , All in all , To sum up, the following two sentences .

In front of ++, First add , After use
After ++, First use , Add after

3. Cast
   
   
   Think of this situation , The system will have an alarm , Because it was originally a decimal , And you have to assign it to the variable of shaping , So report a mistake , Then how to solve ？
   At this time, the cast type appears
   This "（int）“ It means to put 3.252 Cast to Int type .
   Only one more ”（int）", So the warning is gone , Does it work ？

Relational operator

What we should pay attention to here is "=“ and ”= =“ The difference between , Novices often make mistakes , I sometimes neglect it carelessly , If it is ignored, the result will be different from that expected .

The result we want to run is "eiei”, But the result is "heihei", At first glance, there seems to be no mistake , And the mistake is "=“ and ”= =“ The difference between , One is assignment , One is equal . Think about the expected results , Just think of the following .
therefore , Make a distinction between ”=“ and ”= =" The difference between .

Logical operators

There is no further study here “ Short circuit phenomenon ”（ That's what our school teacher said in class ）, Simply put
“&&” ： If "&&“ The previous expression is false , that ”&&“ The following expression is not executed , If the following expression has assignment or other operations on variables , Then you won't be able to calculate , It will deviate from the expected results .
“||” : If ”||“ The previous expression is true , So the same ”||" The following expression will not operate , If the following expression also has the operation on variables , Then I can't calculate , There will be deviations .
This is something that some people who are good at learning programs sometimes don't notice , This leads to program errors .

Conditional operators

The conditional operator is also called the trinocular operator , yes C The only ternary operator in language , The usage is also very simple .
（exp It means expression ）exp1 Set up ？ Set up and Export exp2, Otherwise output exp3.

Comma expression

The comma expression is （exp1,exp2,exp3,…） Its principle is to calculate from left to right , But the result of the whole comma expression is the result of the last expression . Such as ：

The result of the operation is 4
First calculate from left to right ,（c=3,b=13,d=4,4） This is how the program works , Take a look .
as for c Why did it start with 3, And then 4, This is what I said at the beginning of my article

First use , Add after

Common keywords

There are many common keywords , I won't list them one by one here , Quarks can be found on a search , Only a few keywords are described here .

1. typedef
   This is actually type define（ The type definition ）, Simply put, it is to redefine a type , Its purpose is to simplify the name of complex types , There are other people who do not adapt to this type of name can replace the name . For example, one who doesn't learn C Language people , In his language, plastic surgery is not called int, Then he can redefine .

like this , Is to redefine .
Here is the general "unsigned int" Defined as "uint", Is it a lot simpler ？
typedef+ Space + Defined type + The name of the definition

2. static
   
   As shown in the figure ,static There are three usages that are not explained in depth here , Explain it in a special article .

3. extern
   Declare external symbols （ External symbols are in other source files ）
   Again and without further explanation , and static Put them together in an article to explain .