Black Horse Programmer---C base 9 "string Input Output" "String correlation Function" "pointer" "Pointer variable initial" "two level pointer"

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"Input and output of strings"

1. String output:

%s-------output characters from the given address until it encounters a stop;

printf ("%s\n", &a[3]);

printf ("%s\n", a);

Char a[]= "hellowo\0rd!";

2. String input:

Receives a string from the keyboard, stored in an array of characters with the first address of a

scanf ("%s", a);

printf ("%s\n", a);

If you use%s to enter attention to whitespace issues

If the input string has a space, then the character after the space cannot be received, the input encounters a space to end;

The length of the input string is less than the length of the array and will be out of bounds;

3. Use of the string terminator:

Terminator;

The Terminator can be used to determine the length of the character;

No terminator%s could not be stopped;

　　　　

Improper use of the, will only print part;

4, the string length calculation method :

1) int n=sizeof (a), not affected by the effect of the;

2) The string function calculates the length strlen (str1);

3) as the condition of the "," to judge, encountered in the end of the ";" is the length of the string;

5. The difference between a character array and an ordinary array:

There is no essential difference:

Char[],int[]

The array name is a pointer address, a constant, and the element has a type;

"String correlation function"

1. String function:

#include <stdio.h> standard output input function header file

1) puts () outputs a string

Char a[]= "Hello, world!";

Puts (a);

Line wrap, () inside is a character array address;

2) gets () Enter a string: gets (b); inside is the array name address;

Warning:this program uses gets (), which is unsafe.

There is an unsafe warning, there is a cross-border problem, you can receive the space, try not to cross the border;

#include <string.h> string function header file

3) strcat () connect a string

Char a[100]= "Hello", b[]= ", world!";

Strcat (A, b);

Puts (a);

Connect B to the back of A; B to cover the last character of a.

You have to add a header file, #include <string.h>

The lengths of A and B to become new a,a should be large enough to keep the a+b at least.

4) strcpy; string copy function;

strcpy (A, b); Copy the string of B into a string, where a is large enough and may be out of bounds;

5) stecmp; string comparison function;

strcmp (A, b); Compares the size of two strings;

Rules: Compare their ASCII values and compare the order in which they appear in the dictionary

The process of comparison: one by one;

Return value: Greater than 0--a>b, less than 0---a<b, equal to 0---a=b; the value is the number of two string elements of the difference;

6) The length calculation function of the strlen string;

int I=strlen (a); does not contain \ n, array-length-independent, and character-element-related

Pointer

1. Pointer concept:

The memory unit number is the address, according to the address can find the contents, all call this address pointer; Pointer is a memory unit number address;

A memory cell address is a pointer, where the data is the contents of the unit;

2, the advantages of pointers:

1) provide a flexible means of modifying variables for functions;

2) Let the function have multiple return values;

3) can improve the efficiency of sub-procedures

4) Providing support for dynamic data structures

3. How to access variables

1) Direct access gets the variable value by the variable name;

2) Indirect access gets the value of the variable by the pointer (address);

4. Pointer variable:

　　　　The variable that holds the pointer, the variable that holds the address, and a variable that holds the pointer

The pointer is an address and is a constant;

A pointer variable is one that holds an address and is a variable;

　　　　

　　

5. Define pointer variables:

Three parts:

1) pointer type description, which defines the variable as a pointer variable

2) pointer variable name

3) Variable value (pointer)

General form:

Type declarator * variable name; Type declaration is the type of storage that represents the pointer to a variable

Int *a;

6, the definition of matters needing attention:

1) "*" must have, if not the equivalent of defining a common variable;

2) Pointer variables can not be used to hold values and characters, only address;

3) Store the address of the storage data type to be consistent;

4) The same as the variable also has a global and local points;

"Pointer variable initial"

1. Initialize:

1) Define the initialization at the same time:

Full initialization:

int i=0;

int *a1=&i,*a2=&i; The A1A2 all pointed to I;

Partial initialization

int *a1=&i,*a2;

2) First define after initialization:

int i=0;

int *a;

a=&i;

3) If a pointer is defined, but you do not know who to point the pointer to

int *a1=null;null is empty 0;

int *a1=0; nothing points;

Common errors:

int *a1=1000; The address should be stored;

The direction of the pointer can be changed;

int *a1=*a2;

If no initialization is a wild pointer;

2. Use * To obtain the contents of the corresponding storage area of the pointer:

&: take address symbol;

*: pointer operation symbol;

int i = 10;

int *a;

A = &i;

int b = *a;

* pointer variable, function: Gets the memory that the pointer variable points to the stored content

　　　　　　

Two Uses of "*":

1) used to define a pointer variable: int *a1=&i

2) Get the value int b = *a;

3) Set the pointer variable to the contents of the storage space

int i=10;

int *a=&i;

*a=100;

If a pointer variable is not initialized, the pointer variable holds a garbage number, which is called the wild Pointer, and the pointer variable

Two number exchange:

1#include <stdio.h>2 3 voidChangeint*P1,int*p2);4 5 voidChangeint*P1,int*p2) {6 7*P1 = *p1^*P2;8 9*P2 = *p1^*P2;Ten  One*P1 = *p1^*P2; A  - } -  the intMainintargcConst Char*argv[]) { -  -     intA =Ten, B = -; -  +     int*P1 = &a, *P2 = &b; -  + Change (P1, p2); A  atprintf"a=%d\nb=%d\n", *p1,*p2); -  -     return 0; -  -}

3, the common use of pointers to the scene:

1) indirectly access the variables in the caller in the function:

2) allow the function to have multiple return values;

"Level Two pointer"

1, Level Two pointers:

If a pointer variable holds the address of another pointer variable, the pointer variable is a pointer variable that points to the pointer. Also for level two pointers;

int*p=&a;

int**p1=&p;

　　　　

　　

* First-level pointer,* * level two pointer,* * * three pointer;

2. The pointer distinguishes the type;

Under the same compiler, the space occupied by a pointer is determined and occupies 8 bytes in the system.

Defines what type of pointer is pointing to what type of variable;

Black Horse Programmer---C base 9 "string Input Output" "String correlation Function" "pointer" "Pointer variable initial" "two level pointer"