Moving elements in an array

[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;
     }
}

[Jordan]

Without doing the code for you, I suggest using a debugger and stepping through the function. You will then be able to see where the object out of bounds is coming from.

[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]

Originally Posted by WingedPanther

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?