[some special grammars about C]

About C Some special grammars of

Sentence block

• All statements written in a brace , It is called a statement block . Usually used in if、 loop After the statement , When there is only one statement after , It saves .
• special , It can be written almost anywhere , But the scope of action is limited , It only takes effect in the statement block ：

    int a = 1;
    {
    
        int a = 2;
        printf("%d\n", a);  
    }
    printf("%d\n", a);

Such as removing braces , Will compile errors .

    int a = 1;
    int a = 2;
    printf("%d\n", a);
    printf("%d\n", a);

printf And Ternary expression In combination with

For some usual needs , Want to judge according to different results , Output different data , So you may have the following code ：

if(i>0)
    printf("%d Greater than 0",i);
else
    printf("%d Less than or equal to 0",i);

For just one function , It's too cumbersome , actual printf The following syntax is supported , Using ternary expressions , The code can be much simpler .

    printf("%d", 1 > 0 ? 1 : 0);
    printf(1 < 0 ? "%d" : "%c", 65);

printf Execution order of

int a = 1;    
printf("%d,%d,%d\n", a, a + 1, a++);

• If you press from left to right , The output should be : 1,2,1
• But the actual output : 2,3,1

    int a = 1;
    printf("%d,%d,%d\n", a = a + 1, a + 1, a);

• It comes from right to left output , Guess that the output is ：2,2,1
• But the reality is ：2,2,2
• reason : For variables a , It should be after all expressions are executed , Print together come out . Instead of printing one by one .
• So for a=a+1、a A similar expression , Will be printed with the same value , Its value is after executing all expressions from right to left , Variable a Value .
• And for something like a+1 The expression of , Variables are not printed a Value , It is not assigned to a Variable , It can only be printed as a constant .

printf The return value of

Take a chestnut :

printf("%d-%d-%d-%d-%d", printf("1"),printf("22"),printf("333"),printf("4444"),printf("55555"));

• Output results : 5555544443332211-2-3-4-5
• According to the output , Also proved that printf The function is executed from right to left , First output 555554444333221, Output again printf The return value of , Namely 1-2-3-4-5, according to printf Parameters in function , It's easy to find rules and find printf The return value of ： The number of printed characters .

if Condition judgment in

• about if Conditional expressions in statements , There are many ways to express , There are different ways , But use the same . example ：i!=0 It can be directly written as a i, It can also have the same effect , And more concise. . because C Specified in the language , In numerical terms 0 Express " false ", The rest are expressed as “ really ”. But notice 0 Different representations of , character '\0'、NULL All represent values 0, But the character zero '0' , It does not mean numerical value 0, stay ASCII In code , Characters zero with numeric value 48 Express .
• So the following expressions are not executed

if(0)
    printf("0");
if('\0')
    printf("\'\\0\'");
if(NULL)
    printf("NULL");

if And else The problem of pairing

• else Only with the front lately Unmatched Of if pairing , one-on-one . Pay attention to three points ：1、 stay else Before ;2、 leave else lately ;3、 Unpaired if.
• It has nothing to do with indentation ！！！ This is not Python.

switch Medium break

• Basic grammar : General form

switch(express) {
    
    case 1:
		printf("1");
		break;
	case 2:
		printf("2");
		break;
	case 3:
		printf("3");
		break;
	default
        printf("default");
}

• express It should be an integer expression ,case Selected value , Can only be constant , Be careful ： Characters are also plastic , And case The values of are mutually exclusive .
• When the first one case After the value of successfully matches the expression , To perform its case The following sentence , Again by break Jump out of , Do not execute the following statement .

switch (express){
    
    case 1:
    case 2:
        printf("2");
    default:
        printf("default");
    case 3:
        printf("3");
        break;
    }

• More special ,break It saves ,case There can be no statement after , Sentences can be saved ,default The position of the statement is arbitrary . however , If default Prior to case The value of matches , Will continue to execute backwards default What's in the sentence ,default The statement of no longer has no value matching , Just executed .
• For all case The statement in , There are two situations that will be implemented ：1、case The value of matches the expression ;2、 At present case Previous case The value of the statement matches the expression , In the statement before execution None break Jump out of switch;
• about default There are also two situations in which statements will be executed ：1、 be-all case Do not match the value of the expression ;2、 Previous case The value of the statement matches the expression , In the statement before execution None break Jump out of switch;
• Such as switch stay Loop statement in , stay switch Medium break Only right switch take effect , Can't jump out of the loop .

for Cycle default

• C In language , Altogether 3 A loop statement , among while And do……while Must contain an expression , and for Empty can be saved in the loop . however for Semicolons in a loop cannot be omitted ！
• Infinite loop writing :

for(;;)
    printf("for If the expression in is not written, it defaults to ' really '");

• In almost all cases , The three loops can be used interchangeably . According to the actual needs, generally choose a more concise one .

define Macro definition

• define Just a simple character replacement , Pay special attention to the difference between brackets .
• Can be nested , With parameters , It can be a statement .