1. Secondary pointer:
Two dimensional array Array of arrays
Secondary pointer? --- " Pointer to pointer
Concept: the secondary pointer stores the address of the primary pointer
(definition means space)
If there is space, there will be an address)
Format:
Define the format of the first level pointer:
Storage type * data type * pointer variable name;
Define the format of the secondary pointer:
Storage type * * data type * * pointer variable name;
Analysis:
Storage type: the storage type of the secondary pointer itself
Data type * *: the data type of the secondary pointer
Data type *: the type pointed to by the secondary pointer
Pointer variable name: see the meaning of name
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Summary:
(1) When is it necessary to define a secondary pointer When the value of this first level pointer is required
(2) How to determine the type of pointer?
-----Remove [variable name], and the rest is the type of the pointer itself
eg:
int *p; int *
int **pp; int **
(3) How to determine the type pointed to by the pointer?
----"[* pointer variable name] is removed, and the rest is the type pointed to by the pointer
(4) How to analyze the size of a pointer's one-time access space?
----"> depends on the type pointed to
char **pp; pp the size of one-time access space is 4
char *p; p one time access space is 1
int ****pppp;pppp can access 4 spaces at one time
#include <stdio.h>
int main(int argc, const char *argv[])
{
int a = 90;
int b = 78;
int *p = &a;
printf("*p = %d\n",*p);
//*p = 56;
//p = &b;
int **pp = &p;
*pp = &b;
printf("*p = %d\n",*p);
return 0;
}
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Combination of pointer and array:
Relationship between pointer and one-dimensional array:
Arithmetic operation of pointer: (+ - * /% + + -)
Let's take p and q as examples (Note: p and q are pointer variables of the same type)
p + N: represents the size of N data types moved by p in the direction of address increase (p + sizeof (data type) * n)
p - N: represents the size of N data types moved by p in the direction of address reduction (p - sizeof (data type) * n)
p + +: represents the size of one data type moved by p in the direction of address increase (p + sizeof (data type) * 1)
p --: represents the size of p moving 1 data type in the direction of address reduction (p - sizeof (data type) * 1)
p-q: represents the number of elements separated between two pointers (P - Q / sizeof (data type))
#include <stdio.h>
int main(int argc, const char *argv[])
{
//Define three pointer variables
int *p;
short *q;
char *s;
int *r = p+4;
printf("r-p = %d\n",r-p);
/*
p++;
printf("p = %p\n",p);
printf("p = %p\t p + 1 = %p\n",p,p+3);
printf("q = %p\t s + 1 = %p\n",q,q+1);
printf("s = %p\t s + 1 = %p\n",s,s+1);
*/
return 0;
}
The difference between p + + and p+1:
p++<===> p = p+1; Therefore, P + + will cause the pointer to change
p+1 does not cause the pointer to change
Array name:
(1) As an array name, it can represent the whole array
(2) It can also represent the first address of the array
How to print out array elements:
arr[i] <===> *(arr+i) <===> *(p+i) <===> *(p++) <===> p[i] <===> i[arr] <===> i[p]
#include <stdio.h>
int main(int argc, const char *argv[])
{
int arr[5] = {1,2,3,4,5};
printf("sizeof(arr) = %d\n",sizeof(arr));
//Define a pointer to the first element of the one-dimensional array
int *p = arr;
//printf("arr = %p\n",arr);
//printf("&arr[0] = %p\n",&arr[0]);
int i;
for(i=0;i<5;i++)
{
//printf("%d ",arr[i]);
//printf("%d ",*(arr+i));
//printf("%d ",*(p+i));
//printf("%d ",*(p++));
//printf("%d ",p[i]);
//printf("%d ",i[arr]);
printf("%d ",i[p]);
}
printf("\n");
return 0;
}
Analysis: the reason why we can use p[i] to traverse the array is that the compiler will automatically compile p[i] into * (p+i)
Summary:
For array names: arr[i]
For pointers: * (p+i) / *(p + +)
Exercise:
(1) Get a string from the keyboard and use the pointer to find the number of spaces in the string
#include <stdio.h>
#define N 20
int main(int argc, const char *argv[])
{
//Define a character array
char str[N] = {'\0'};
//Defines a character pointer to the first element of the array
char *p = str;
printf("Please enter a string:\n");
gets(p);
int count = 0;
//Traverse the string
while(*p != '\0')
{
if(' ' == *p)
{
count++;
}
p++;
}
printf("count = %d\n",count);
return 0;
}
(2) Use pointer to realize bubbling (change pointer pointing / do not change pointer pointing)
#include <stdio.h>
#define N 5
int main(int argc, const char *argv[])
{
//Sorting using pointers
int arr[N] = {0};
//Defines an integer pointer variable that points to the first element of the array
int *p = arr;
printf("Please enter:\n");
int i,j;
for(i=0;i<N;i++)
{
//scanf("%d",&arr[i]);
scanf("%d",p+i);
}
printf("Sort before: \n");
for(i=0;i<N;i++)
{
//printf("% ",arr[i]);
printf("%d ",*(p+i));
}
printf("\n");
for(i=0;i<N-1;i++)
{
p = arr;//Let p point back to the first element
for(j=0;j<N-1-i;j++)
{
if(*p > *(p+1))
{
int temp;
temp = *p;
*p = *(p+1);
*(p+1) = temp;
}
p++;
}
}
printf("After sorting is: \n");
p = arr;
for(i=0;i<N;i++)
{
//printf("% ",arr[i]);
printf("%d ",*(p+i));
}
printf("\n");
return 0;
}
Bubble sorting: compare from left to right
Note: find the location of bubbles according to the data
Select sort:
Suppose there are N numbers:
First trip: find the maximum number and exchange it with the number with position 0
The second trip: subtract 1 from the total number, and then find the largest clock to exchange with the number with position 1..
. . . . . . . . . . . .
Note: select to find data by location
#include <stdio.h>
#define M 5
int main(int argc, const char *argv[])
{
int arr[M] = {0};
int i;
for(i=0;i<M;i++)
{
scanf("%d",&arr[i]);
}
int j;
//int index;
//Select sort
for(i=0;i<M-1;i++)
{
//index = i;
for(j=i+1;j<M;j++)
{
/*if(arr[index] > arr[j])
{
index = j;
}
*/
if(arr[i] > arr[j])
{
int temp;
temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}
/*
//exchange
int temp;
temp = arr[i];
arr[i] = arr[index];
arr[index] = temp;
*/
}
printf("After sorting is:\n");
for(i=0;i<M;i++)
{
printf("%d ",arr[i]);
}
putchar('\n');
return 0;
}
Job:
1. Pointer implementation, define a string, and realize the inversion of string "break" -- > "kaerb"
#include<stdio.h>
#include<string.h>
#define M 5
int main(int argc, const char *argv[])
{
//Pointer implementation, string inversion
char arr[M] = {'\0'};
printf("Please enter a string:\n");
//Define pointer
char *p = arr; // p = &arr[0]
gets(p);
//Use strlen to measure the effective bytes of the arr array
int l;
l = strlen(arr);
//P (first address) + l -1 is the last address
p = p + l - 1;
int i;
for(i = 0;i < M;i++)
{
printf("%c",*(p-i));
}
putchar('\n');
return 0;
}
2. Use pointer to realize strcat function
#include<stdio.h>
#include<string.h>
#define M 15
#define N 20
char *mystrcat(char *p,char*q,int n)
{
if(NULL == p || NULL == q)
{
printf("NULL_ERROR!\n");
return NULL;
}
while(*p)
{
p++;
}
//N stands for n bits connecting arr2
while(n--)
{
*p++ = *q++;
}
*p = '\0';
return q;
}
int main(int argc, const char *argv[])
{
//Use pointer to realize the first n bits of strcat
char arr1[M] = {'\0'};
char arr2[N] = {'\0'};
printf("Please enter two strings:\n");
//gets input string
gets(arr1);
gets(arr2);
/*Defines a pointer to the first address of the array
char *p= &arr1[d];
char *q= &arr2[0];*/
//Defines the valid character length of arrays arr1 and arr2
int l1,l2;
l1 = strlen(arr1);
l2 = strlen(arr2);
if( M < l1 + l2 + 1)
{
printf("error!!!\n");
return -1;
}
/*while(*p)
{
p++;
}*/
/*Traversal arr2
while(*q)
{
*p++ = *q++;
}*/
char *s;
s = mystrcat(arr1,arr2,2);
printf("After connection:%s\n",arr1);
return 0;
}