Question

我编写了从classLength元素中打印length长度组合的代码。它工作正常，但我不明白为什么。

我的问题：

当
```
while(index[0] != length - classLength)
```
循环结束程序不打印最后一个组合，所以我必须在while循环后打印它。为什么？
我尝试在上面提到的while循环中更改条件。

而是length-classLength我写了lenght-classLength+1

运行程序时会导致无限循环。为什么会这样？
请注意
的注释部分
```
while(index[position] == maxIndex /*&& position >= 0*/)
```
while循环条件和评论if语句
```
/*if(position>=0)*/ index[position]++;
```
当代码的这些部分被取消注释并被注释时，程序将正常运行。我最初使用这些条件编写代码，因此程序无法访问具有不正确索引的数组。但是如果没有这些安全检查，这样做会很好。 为什么指数总是在范围内并且不会发生错误？

C ++代码

#include <iostream>
#include <fstream>
using namespace std;

    /**Print all combinations of k-th class of n elements**/

int main() {
    const int maxN = 100;
    int length = 0, classLength = 0;
    cout << "Input number of elements and class number: " << endl;
    cin >> length >> classLength;

    int array[maxN];
    cout << "Input elements: " << endl;
    for(int i = 0; i < length; i++)
        cin >> array[i];

    //first combination has indices 0, 1, ... , k-1
    int index[maxN];
    for(int i = 0; i < classLength; i++)
        index[i] = i;

    int ordNum = 1;
    ofstream out("output.txt");
    //length of class is classLength, last combination's last element's index is n-1
    //last combination's last element's index is n-1-k
    while(index[0] != length - classLength) {
        //print current combination
        out << ordNum++ << ")\t";
        for(int i = 0; i < classLength; i++)
            out << array[index[i]] << " ";
        out << endl;

        int position = classLength-1, maxIndex = length-1;
        //if index at current position is its max
        // value then we have to carry digit
        while(index[position] == maxIndex /*&& position >= 0*/) {
            index[position] = -1;
            position--, maxIndex--;
        }

        //when loop stops position is at index that
        //should be added the carried 1
        /*if(position>=0)*/
        index[position]++;

        //if a carry accured indices must be arranged so that
        //array index[] is non-decreasing
        for(int i = 1; i < classLength; i++)
            if(index[i] <= index[i-1])
                index[i] = index[i-1]+1;
    }

    out << ordNum++ << ")\t";
    for(int i = 0; i < classLength; i++)
        out << array[index[i]] << " ";
    out << endl;

    out.close();
    return 0;
}

代码不打印n个元素的最后k长度组合？

0 个答案: