Question

I want the user to enter a 4 digit number and the program must tell what that 4 digit number was i.e generate that 4 digit number by Brute force attack.But at the line mentioned below the compiler says invalid indirection.I would also like to have some comments about they way I am implementing it,is it a good practise?

#include<stdio.h>
void BruteForceAttack(int *arr);
int main()
{
 int *arr,i;
 printf("Enter 4 digits ,press enter after entring each digit:\n");
 for(i=0;i<4;i++)
 scanf("%d",arr+i);
 BruteForceAttack(arr);
 getchar();
 return 0;
}
void BruteForceAttack(int *arr)
{
 int i,j,k,l;
 for(i=0;;i++)
 {
  for(j=0;;j++)
  {
   for(k=0;;k++)
   {
    for(l=0;;l++)
    {
      if((*(arr+0)==i)&&(*(arr+1==j))&&(*(arr+2==k))&&(*(arr+3)==l))   /*Here the compiler says invalid indirection*/
     {
      printf("The number is %d%d%d%d",i,j,k,l);
      return;
     }
    }
   }
  }
 }
}

Answer 1

Total of 3 problems:

Problem 1:

Your arr is a dangling pointer and you are dereferencing it in scanf.

You need:

int arr[4]; 

in place of

int *arr;

Problem 2:

The comparison involving j and k is incorrectly paranthesized:

&&(*(arr+1==j))&&(*(arr+2==k))

should be

&&(*(arr+1)==j)&&(*(arr+2)==k)
          ^              ^

Problem 3:

Even with above 2 fixes your program will run into infinite loop, because your for loops have no terminating conditions.

Since you are asking user to enter 4 digits, all your loop should go from 0 till 9 as:

for(i=0;i<10;i++)
        ^^^^^

Add similar check for other 3 loops aswell.