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Arrays and Bitwise

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Arrays and Bitwise

 
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  #1
Jul 3rd, 2008
So I got such exercise:
You have array of 1000 elements they all are random numbers from 1 to 1000 and there is only one duplicate number. You can access each array member only once, can you find duplicate value? And you can't create another array. You can create some variables to store temporary data.

and how divide number by 7 using bitwise, how to divide by 8 i know it only takes shift to left by 3 i.e. number << 3, but how divide by 7?
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Re: Arrays and Bitwise

 
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  #2
Jul 3rd, 2008
The first problem is easy
I'll give you the code for it
C++ Syntax (Toggle Plain Text) 
  1. #include <iostream>
  2. #include <cstdlib>
  3. int main()
  4. {
  5. int A[1000];
  6.  
  7. // for loop to fill the array with random elements between 1 to 1000
  8. for( int i = 0; i < 1000; i++)
  9. {
  10. A[i] = 1 + rand() %1000;
  11. }
  12.  
  13. // for loop to find the duplicate integer
  14. for( int j = 0; j < 1000; j++ )
  15. {
  16. if( A[j] == A[j])
  17. cout << "the duplicate integer found " << A[j];
  18. }
  19.  
  20. return 0;
  21.  
  22. } // end main
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Re: Arrays and Bitwise

 
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  #3
Jul 3rd, 2008
That doesn't make sense, because A[j] == A[j] you are comparing same values, so they always will be equal...

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Originally Posted by Q8iEnG View Post
The first problem is easy
I'll give you the code for it
C++ Syntax (Toggle Plain Text) 
  1. #include <iostream>
  2. #include <cstdlib>
  3. int main()
  4. {
  5. int A[1000];
  6.  
  7. // for loop to fill the array with random elements between 1 to 1000
  8. for( int i = 0; i < 1000; i++)
  9. {
  10. A[i] = 1 + rand() %1000;
  11. }
  12.  
  13. // for loop to find the duplicate integer
  14. for( int j = 0; j < 1000; j++ )
  15. {
  16. if( A[j] == A[j])
  17. cout << "the duplicate integer found " << A[j];
  18. }
  19.  
  20. return 0;
  21.  
  22. } // end main
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Re: Arrays and Bitwise

 
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  #4
Jul 3rd, 2008
can you use std::sort(A, A+1000)
and if A[j] == A[j+1] cout << A[j]
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Re: Arrays and Bitwise

 
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  #5
Jul 3rd, 2008
Well that might be solution, but i'm not sure. Maybe there is some way without sorting ?

And i made mistake, i was wondering how to multiply number by 7 not divide.

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Originally Posted by ivailosp View Post
can you use std::sort(A, A+1000)
and if A[j] == A[j+1] cout << A[j]
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Re: Arrays and Bitwise

 
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  #6
Jul 3rd, 2008
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And i made mistake, i was wondering how to multiply number by 7 not divide.
cpp Syntax (Toggle Plain Text) 
  1. 	int x = 21;
  2. 	x = (x << 2) + (x << 1) + x; //multiply number by 7
  3. 	cout << x;
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Re: Arrays and Bitwise

 
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  #7
Jul 3rd, 2008
I'm really distracted today, there was mentioned that i can't use addition or subtraction

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Originally Posted by ivailosp View Post
cpp Syntax (Toggle Plain Text) 
  1. 	int x = 21;
  2. 	x = (x << 2) + (x << 1) + x; //multiply number by 7
  3. 	cout << x;
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Re: Arrays and Bitwise

 
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  #8
Jul 3rd, 2008
First question is interesting one, I researched on it, but the below mentioned link has figured out some really good stuff regarding your first question.

http://aperiodic.net/phil/archives/G...-elements.html.

will try your 2nd question tomorrow .
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Re: Arrays and Bitwise

 
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  #9
Jul 3rd, 2008
how to sum numbers...
it was hard to write this :/
EDIT: small fix
cpp Syntax (Toggle Plain Text) 
  1. #include <iostream>
  2. using namespace std;
  3.  
  4. int sum_numbers(int x, int y) {
  5. 	while(y = (x&y) << ((x ^= y)?1:1));
  6. 	return x;
  7. }
  8.  
  9. int main() {
  10. 	int x = 24;
  11. 	int y = 3;
  12. 	for(;y < 100;++y)
  13. 	cout << sum_numbers(x, y) << '-';
  14. 	return 0;
  15. }
Last edited by ivailosp; Jul 3rd, 2008 at 11:46 am.
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Re: Arrays and Bitwise

 
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  #10
Jul 3rd, 2008
and here is the file code for multiply number by 7

cpp Syntax (Toggle Plain Text) 
  1. #include <iostream>
  2. using namespace std;
  3.  
  4. int sum_numbers(int x, int y) {
  5. 	while(y = (x&y) << ((x ^= y)?1:1));
  6. 	return x;
  7. }
  8.  
  9. int multiply_by_seven(int x){
  10. 	return sum_numbers(sum_numbers((x << 2), (x << 1)), x);
  11. }
  12.  
  13. int main() {
  14. 	for (int i = 0; i <= 100; ++i){
  15. 		cout << multiply_by_seven(i) << endl;
  16. 	}
  17. 	return 0;
  18. }
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