Algorithm: Detailded CyclicShift Solution One of Programming Pearls (2nd)

This artical will show you the detailed implementation of Cyclic Shift which is mentioned in Programming Pearls (2nd). In that book on Page 11, Column2.1, Question B,

Rotate a one-dimensional vector of n elements left by i positions. For instance, with n=8 and i=3, the vector abcdefgh is rotated to defghabc. Simple code uses an n-element intermediate vector to do the job in n steps. Can you rotate the vector in time proportional to n using only a few dozen bytes of storage?

author introduced one solution likes delicate juggling act. But I found this way is a little hard to understand. So based on standard answer behind book, I create a diagram and a implementation codes to clear this alforithm.

Diagram:





C++ Codes:

// CyclicShift1.cpp : Defines the entry point for the console application.
//

#include <iostream.h>
#include <string.h>

int gcd(int a,int b)
{
	if(a<b)
	{
		int temp = a ; 
		a = b ; 
		b = temp ;
	}

	while(b!=0)
	{
		int temp = a%b ;
		a = b ;
		b = temp ;
	}

	return a ;
}

void CyclicShift(char str[], int rotdist, int len)
{
	for(int i=0 ; i<gcd(rotdist,len); i++)
	{
		int t=str[i] ;
		int j=i ;
		int k=0 ;
		while(1)
		{
			k = k+rotdist ;
			if(k>=len)
				k = k-len ;
			if(k==i)
				break;
			str[j] = str[k] ;
			j = k ;
		}

		str[j] = t ;
	
	}

	

}

int main(int argc, char* argv[])
{
	char str[]={'a','b','c','d','e','f','g','h','/0'} ;
	int len = sizeof(str)/sizeof(char)-1 ;
	
	CyclicShift(str,3,len) ;

	cout<<str<<endl ;

	return 0;
}

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