将指针作为函数的参数传递

Question

I need help with understanding how pointers are working in this code below as i am really confused.The code is about finding maximum and the minimum value from a given array.I know that the problem can also be approached by using a structure but i want to understand use of pointers

#include<stdio.h>
int max2,min2,max,min,maximum,minimum;
int Maxmin(int a[],int *max,int *min,int i,int j)
{
    if(i==j)
    {
        return *max=*min=a[i];
    }
    else if(i==j-1)
    {
        if(a[i]<a[j])
        {
            *max=a[j];
            *min=a[i];
        }
        else
        {
            *max=a[i];
            *min=a[j];
        }
    }
    else
    {
        int mid=(i+j)/2;
        Maxmin(a,max,min,i,mid);
        Maxmin(a,&max2,&min2,mid+1,j);

    }
    if(*max<max2)
    {
        *max=max2;
    }

    if(*min>min2)
    {
        *min=min2;
    }

}

int main()
{
    int a[]={4,30,2,100,230};
    Maxmin(a,&max,&min,0,4);
    printf("%d %d",max,min);
}

Answer 1

From the caller side:

max and min are int variables. &max and &min are the memory addresses for these int variables.

From the callee side:

int *max and int *min are declared as pointers to int in Maxmin . Let's see it as if pointers contain a memory address. Pointers to int contain the memory address of an int variable.

*max and *min is the way to access the memory address content (dereferencing the pointer). Since you have a pointer to an int, you'll read an int out of that operation.

The key to this code is to know that, if you modify *max or *min from within Maxmin , you will be changing the value of max and min outside the function, because you're working directly with the memory contents of those global variables.

Answer 2

First of all you need to differentiate between * , & symbols used for pointers and their usage.

* operator Can be used in two ways

1. * as can be used to define a pointer Eg. int * ptr This thing will create a pointer and can store the address of type int

2. * as Dereference Operator printf("%d",*ptr); prints the value who's address the pointer is storing.
   Eg. int a = 10; int *ptr=&a; printf("%d",*ptr) ; Dereference the value of pointer ans prints the value as 10

   & in Terms of pointers is used only as address of some variable.

Now , In your code when you use *max and *min , it refers to the value of pointers used in function max , min , which ultimately refers to global variables as you have passed global variable's address in main function And when & is used in function for &max2 and &min2 it means address of global variables.

The code might be giving a wrong Output because its comparing the value of all the four global variables that can have a junk value . So its better to initialize your global variables like this int max=-10000 ,min=10000 ,min2=10000 ,max2=-10000 . This will always give a correct output.