Accessing a one-dimensional array with a pointer

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In order to find

• Pointer is required ： In the set of integers r Medium order search and given value key Equal elements .
• Use a one-dimensional integer array a[n] Store integer set , The sequential search starts with the first element of the array , Compare each element in turn , Until you find key until , If all the elements of the array are compared , Description search failed

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Array implementation

#include <stdio.h>
#define N 6
int main()
{
    
	int a[N] = {
     2,4,8,6,5,3 }, i, key, index = 0;
	key = 8;
	for(i=0;i<N;i++)
		if (a[i] == key)
		{
    
			index = i;
			break;
		}
	if (i == N)
		index = -1;	// Did not find 
	if (index == -1)
		printf(" To find the failure ！\n");
	else
		printf(" Find success , Elements %d The sequence number in the set is %d\n", key, index);
	return 0;
}

Pointer implementation

#include <stdio.h>
#define N 6
int main()
{
    
	int a[N] = {
     2,4,8,6,5,3 }, i, key, index = 0;
	int* p = a;
	key = 8;
	for (; p < a + N; p++)
		if (*p == key) {
    
			index = p - a;
			break;
		}
	if (p == a + N)
		index = -1;
	if (index == -1)
		printf(" To find the failure ！\n");
	else
		printf(" Find success , Elements %d The sequence number in the set is %d\n", key, index);
	return 0;
}

There are three ways to access a one bit array element

• 1、 Access array elements by subscript ：

int a[10];
for (int i = 0;i < 10;i++)
    a[i] = i;

• 2、 Access array elements by address

int a[10];
for (int i = 0;i < 10; i++)
    *(a+i) = i;				//a+i It's No i Elements 

• 3、 Access array elements through pointers ：

int a[10];
int *p=a;
for (int i = 0;i < 10; i++)
    *(p+i)=i;		//*(p+i) It can be used p[i] Instead of 

(a+i) and (p+i) It's different .a Can't move ,p Mobile p++ a Is the first address of the saved array , Constant pointer

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p++ Self increasing operation is better than p+i Arithmetic is much faster , Therefore, its execution efficiency is higher than that of address mode

int a[10];
int *p = a;
for (int i = 0; p < (a+10); i++,p++)			// expression 3 It's a comma expression 
    *p=i;

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Because the array occupies a continuous memory space , therefore , The arithmetic operation of pointers is usually used for arrays .

int num,a[5]={
    1,2,3,4,5};
int *p=a,*q;				// The pointer p Point to elements a[0]
q=p+4;						// amount to q = p + 4 × sizeof(int),  take q Point to a[4]
num=q-p;					// The pointer q and p The number of elements between , The value is 4
num=(int)q-(int)p;			// The pointer q and p The number of bytes between , The value is 16（ This is rarely used ）

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Relational operation of pointer variables

Compare pointers of the same type .

Between different types of pointers 、 Pointer and non 0 The relation operation between integers has no practical significance .

hypothesis p and q Are two pointer variables of the same type , be

• p == q： Judgment pointer p and q Whether to point to the same storage unit
• p > q ： The pointer p The storage unit referred to is in the pointer q Behind the storage unit
• p != NULL ： Judgment pointer p Null pointer or not

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Cited example

Input 10 It's an integer , Exchange the smallest number with the first number , Exchange the largest number with the last number .

• Example of program running results （ Blue font indicates input ）：
• Please enter 10 It's an integer ：2 3 4 23 5 78 0 1 6 12
• The result of the exchange is ：0 3 4 23 5 12 2 1 5 78
• Please enter 10 It's an integer ：1 25 7 40 5 6 12 8 30 19
• The result of the exchange is ：1 25 7 19 5 6 12 8 30 40

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The above program is realized by array, that is, array parameter transfer .

Write 3 A function ：(1) Input 10 Number ;(2) To deal with ;（3） Output 10 Number .

This program uses array and pointer to realize .

Array implementation

// In general, we seldom pass an array parameter directly , It is often necessary to pass two parameters , It's an array , How long is the second transmission group 
void process(int arr[],int num)
{
    
    int temp;
    int maxNum = arr[0];
    int minNum = arr[0];
    int maxPos = 0;
    int minPos = 0;
    for (int i=1; i<num; i++)
    {
    
        if (arr[i] < minNum)
        {
    
            minNum = arr[i];
            minPos = i;
        }
        if (arr[i] > maxNum)
        {
    
            maxNum = arr[i];
            maxPos = i;
        }
    }
    // If the minimum number is not the first number , Exchange the minimum number with the first number 
    if (minPos != 0)
        swap(&arr[0],&arr[minPos]);
    // If the maximum number is not the last number , Exchange the maximum number and the last number 
    if (maxPos != num-1)
    	swap(&arr[num-1],&arr[maxPos]);
}

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The main function

int main()
{
    
    int a[10];
    inputArray(a,10);
    process(a,10);
    outputArray(a,10);
    return 0;
}

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Pointer implementation

void process(int *arr,int num)
{
    
    int temp;
    int *p;					//p The pointer moves backward to scan the array 
    int *min,*max;			//min and max The pointer records the minimum and maximum addresses 
    p=max=min=arr;			//p,max,min Both point to the first number of the array 
   	for (; p<arr+num ; p++)
    {
    
        if (*p<*min)
            min = p;
        if (*p>*max)
            max = p;
    }
    if (min != arr)
    	swap(arr,min);
    if (max != arr+num-1)
        swap((arr+num-1),max);
}

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The main function

// Call statement 
process(a,10);

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One dimensional array as a function parameter

The characteristics of seat parameters of one-dimensional array are ： Parameter array and argument array are the same array .

in other words , The main program and subroutine have different names , Shared the same space . So in the subroutine , adopt arr【i】 Any changes to the array , Will correspondingly affect the corresponding arguments a【i】

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The practical application

describe

There are multiple consecutive spaces in a sentence , Filter extra spaces , Just leave a space .

Input

a line , A string （ Length not exceeding 200）, There are no spaces at the beginning and the end of a sentence

Output

Filtered sentences

The sample input ：

Hello world.This is c language.

Sample output

Hello world.This is c language.

str1:	T	h	i	s			i	s				c		l	a	n	g	u	a	g	e	\0

str2:	T	h	i	s		i	s		c		l	a	n	g	u	a	g	e	\0

void operate(char *p1,char *p2)
{
    
    while (*p1)
    {
    
        if (*p1!=' ')
        {
    
            *p2++=*p1++;		//* Priority of 
        }
        else
        {
    
            *p2++ = ' ';
            while(*p1 == ' ')
                p1++;
            /*  It can also be written as  while (*p1++ == ' ') ; */
        }
    }
    *p2 = '\0';		// hold str2 Initialize to '\0' There is no need to write this , Considering that other situations will be used , May not initialize , Plus this will be very safe 
}
int mian()
{
    
    char str1[201]={
    0};
    char str2[201]={
    0};
    gets(str1);
    operate(str1,str2);
    printf("%s",str2);
}