9 replies to this topic

[Fischerspooner]

Hi,

I'm currently studying Java on my own and I wanted to try to "rotate" elements in an array.
So the start would be:
{5,3,8,7,7,2,0,6,1,4}
But if I enter "rotate 1" it should give me
{4,5,3,8,7,7,2,0,6,1}

But I keep running into ArrayIndexOutOfBounds errors if I check my pseudocode in Java.

I don't want some code, just a hint or two would be very much appreciated:)

This is what I have already:

for(t=DIM-1 ; t > rot ; t--)
{
     if(t < DIM)
     {
        k = table[t];
        table[t-1] = table[t];
        table[t+1]= k;
     }
     else
     {
        k = table[t];
        table[t-1] = table[t-DIM];
        table[t+1] = k;
     }
}

[Fischerspooner]

I ran it through the debugger but I still cant find my mistake. I'm just overlooking something.

Some pseudocode:

temp = table[i+rot]; // remember what was on i+rot
table[i] = table[i+rot] // change the place of element i to i+rot
if (i+rot < table.length)
start from 0

My head is all messed up :/

[WingedPanther]

At a guess, you are trying to push beyond the beginning/end of your array. One note: t < DIM is always true.

[Fischerspooner]

I used a second array to rotate and it works. Is it even possible with one array?

for(t=0 ; t < DIM ; t++)
{
   if(t+rot < DIM)
  {
       table2[t+rot] = table[t];
  }
  else
  {
       table2[t+rot-DIM] = table[t];
   }
}

[WingedPanther]

It's possible, but you have to be careful around the beginning/end of the array.

[Fischerspooner]

WingedPanther said:

It's possible, but you have to be careful around the beginning/end of the array.

Could you post some code? I know I didn't ask for code but I'm eager to learn...:)

[WingedPanther]

#include <iostream>


int GCF(int a,int b)

{

 int quot, mod;

 do

 {

  quot = a/b;

  mod = a%b;

  a = b;

  b = quot;

 } while (mod != 0);

 return a;

}

    

int main()

{

 const int DIM=9;

 int table[DIM];

 int rot=3;

 int loops = GCF(DIM,rot);

 for (int i=0;i<DIM;i++) table[i] = i+1;

 for (int i=0;i<DIM;i++) std::cout << table[i] << " ";

 std::cout << "\n";

 for (int i=0;i<loops;i++)

 {

  int temp=table[i];

  for (int j=(i+rot)%DIM;j!=i;j=(j+rot)%DIM) 

  {

   table[(j-rot)%DIM]=table[j];

  }

  table[DIM+(i-rot)%DIM]=temp;

 }

 for (int i=0;i<DIM;i++) std::cout << table[i] << " ";

 std::cout << "\n";

 return 0;

}

[Fischerspooner]

Thanks, I'll study it:)

[WingedPanther]

Here's an (possibly) interesting thing to think about: What are the time/space advantages/costs of each method of rotating an array?