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 ： This issue focuses on ： some C Summary of mistakes prone points of language brush questions

   I hope everyone can study and work happily every day . 

One . About global variables

Two . About scanf

3、 ... and . Pointer practice

  Four . "A" and 'A' The difference between

5、 ... and . About data storage

6、 ... and . About two-dimensional arrays

7、 ... and . Operator priority

One . About global variables

Can't you use global variables before they are defined ？

extern int a;

int main()
{
	printf("a = %d\n", a);
	return 0;
}

int a = 10;

Let's see , First int a The definition of is in main After the function , Can be used by declaration .

The essence ： First, global variables have been allocated memory space when the program is running , So it can be used before definition .

Two . About scanf

Add the appropriate input statement at the horizontal line , Enter... From the keyboard while the program is running ：How are you?< enter >, The final output is ： How are you?How

int main()
{
	char *a = "How are you?", b[20];
    ——————————————————————————————
	printf("%s %s\n", a, b);
	return 0;
}

A：gets(b)       B:scanf("%s",b)

  gets The end of is line feed （ '\n' ）

 scanf The end is a space enter Tab key , That is, a continuous character .

So we should choose B:scanf("%s",b)

3、 ... and . Pointer practice

	int a[10] = { 1, 2, 3, 5, 5, 6, 7, 8, 9, 10 };
	int x = 0;
	int *pa = a;

With the above definition , Then in the following assignment , Which one doesn't subscript the array to 3 The element of is assigned to x Well ？

A：x = pa[3]                            B:x= *(a+3)

C:  x =a[3]                                D:x = *pa+3

Under the analysis of ：

A： correct ,pa Although it's a pointer , But it can be accessed in the form of an array .

B:  correct ,a It's the first element address , Move successively 3 individual int Size , Just access to the subscript 3 The elements of , Assignment in dereference

C: correct , In the form of an array , Mark the subscript as 3 The value of is assigned to x 了 .

D: error , The first is to directly dereference a Is the first element of the array , And then in +3, This is not true .

  Four . "A" and 'A' The difference between

First character 'A' Is a single character , and "A" It's two characters ,A and \0, They are different .

5、 ... and . About data storage

Look directly at the code ：

	unsigned short A = 10;
	printf("~A = %u\n", ~A);
	char c = 128;
	printf("c = %d\n", c);

Let's analyze it A：

Deposit in A In space is 00000000 00001010 , Then on A Carry out bit inversion to become 11111111 11110101

And then according to %u Print , Is to shape and print according to the unsigned, that is 11111111 11111111 11111111 11110101

The result of printing is 4294967285

Then analyze c ：

c yes 128, The storage space is 10000000, yes %d Print , So plastic surgery is promoted to

11111111 11111111 11111111 10000000, This is the complement ,

Convert to inverse code 10000000 000000000 00000000 01111111

The original code is 10000000 000000000 00000000 10000000,

The final print is -128

6、 ... and . About two-dimensional arrays

The following output results ：

	char arr[2][4];
	strcpy(arr[0], "you");
	strcpy(arr[1], "me");
	arr[0][3] = '&';
	printf("%s \n", arr);

Let's analyze first ：

The data put after the first copy is ,

  The data put after the second copy is ,

  At this time, if we directly take %s If you print it , Then it will print to '\0' end , But we did the assignment operation , hold arr[0][3] in '\0', Replace with ' $ ', So the result is " you&me ".

7、 ... and . Operator priority

*p++ Is a self increasing pointer p Or self increasing pointer p What it points to ？

Look at a code ：

	int a = 10;
	int *p = NULL;
	int **pp = NULL;
	p = &a;
	pp = &p;
	printf("%d\n", *pp);
	printf("%d\n", *p++);
	printf("%d\n", *pp);

When we run it directly, we can get , Is self increasing, is a pointer variable p.

 
There are two schools of explanation ：

1. stay C And pointer are interpreted as , Under the same priority , The combination is from right to left .

2. On the priority diagram , For post ++ Has priority over dereference ( * ).

Let's discuss this in the discussion area ！