原位循环C ++实践 [英] In Place rotation C++ Practice

问题描述

我有一个工作旋转函数为我的itemsint数组。下面的代码完成，除了im传输值不必要。我试图实现inplace旋转。我的意思是，ptrs将递增或递减，而不是抓取值从数组。我需要提高效率水平这种方法对于这种方法。任何建议？

I have a working rotating function going for my "items" int array. The code below gets it done, except that im transferring values out unnecessarily. Im trying to acheive the "inplace" rotation. What I mean by that is where the ptrs would increment or decrement instead of grabbing values out of the array..By which I need to "up" the efficiency level in that way for this method..Any Suggestions?

void quack::rotate(int nRotations)
{
 if ( count <= 1 ) return;
 else  // make sure our ptrs are where we want them.
 {
  intFrontPtr = &items[0].myInt;
  intBackPtr  = &items[count-1].myInt;
 }
 for (int temp = 0; nRotations != 0;)
 {
  if ( nRotations > 0 )
  {
     temp = *intFrontPtr;
    *intFrontPtr = *intBackPtr;
    *intBackPtr  = temp; // Connect temps for the rotation
   --intBackPtr; // Move left [...<-] into the array
  }
  else if ( nRotations < 0 ) 
  {
   temp = *intBackPtr;
   *intBackPtr  = *intFrontPtr;
   *intFrontPtr = temp; // Connect temps for the rotation
   ++intFrontPtr; // Move right [->...] into the array
  }
  if ( intBackPtr  == &items[0].myInt  || 
    intFrontPtr == &items[count-1].myInt ) 
  {
   intFrontPtr = &items[0].myInt; 
   intBackPtr  = &items[count-1].myInt; // need to re-set
   if ( nRotations > 0 ) nRotations--;  // Which ways did we rotate?
   else nRotations++;
  }
 }
 }

实践c + +和知道他们是许多功能浮动周围，已编程来做这已经...我想建立自己的。我认为我已经把它在语法上，但效率总是在我奋斗的地方。

Oh yes, Im trying to practice c++ and know their are many functions floating around that are programmed to do this already...Im trying to "build my own". I think i've got it down syntactically, but the efficiency is always where i struggle. As, a novice, I would greatly appreciate critisim towards this aspect..

推荐答案

这是一个新的技巧， array（我第一次看到它在编程珍珠）

There is an old trick for rotating elements in an array (I first saw it in Programming Pearls)

假设你想向左旋转一个数组三个元素。

Say you want to rotate an array to the left by three elements.

首先逆转前三个元素，然后反转其余元素，然后反转整个数组。

First reverse the first three elements, next reverse the remaining elements, and then reverse the entire array.

Starting Array:
1 2 3 4 5 6 7

After reversing the first three elements
3 2 1 4 5 6 7

After reversing the remaining elements
3 2 1 7 6 5 4

Finally reverse the entire array to get the final rotated array
4 5 6 7 1 2 3

可以在阵列中反转数组，这样就不需要任何额外的内存。

Reversing portions of the array can be done in place so you don't need any extra memory.

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