c++ - 变量嵌套 for 循环

Question

我正在尝试弄清楚如何使用递归来执行 n 级嵌套 for 循环。 例如，如果 n=3，就会有 3 个“级别”

for(z=0;z<6;z++){
   for(y=0;y<6;y++){
      for(x=0;x<6;x++){
         if (z+y+x==f){
            //do something
         } 
      }
   }
}

等等。

我似乎无法弄清楚如何将 if 循环放在最后一个 for 循环中以及如何从 if 语句访问前一个 for 循环的变量。我知道变量嵌套循环的问题已经被问过很多次了，我已经浏览了所有这些问题。但似乎没有人帮助我。

请记住，我仍然是 c++ 的初学者，有人可以提供一种使用递归来实现此目的的简单方法，为我指明正确的方向吗？

用例如下:

Write a program to input the number of dice m. The program will output the total number of possible cases, the number of possible cases for each possible n and the n with the highest probability. Note: only one input m is read in. n is computed by the program

Example if user enters m=2 then program should output

The total number of possible cases is 36.
The possibilities are
2 1
3 2
4 3
.
.
.
12 1

Answer 1

为了提高效率，我避免了递归。此外，它不使用任何特定的 C++ 东西 - 它也可以在 C 上正常工作。

我们正在尝试创建 N 个嵌套的“for”循环。 而不是使用

for(int i = 0; i<max; i++)
  for (int j = 0; j<max; j++)
    ...

我将用数组替换 i、j、...:i[0]、i[1]、...、i[n-1]。

这是我的解决方案:

const int n = /*Insert N here: how many loops do you need?*/;
int i[n+1]; // if "n" is not known before hand, then this array will need to be created dynamically.
//Note: there is an extra element at the end of the array, in order to keep track of whether to exit the array.

for (int a=0; a<n+1; a++) {
  i[a]=0;
}

int MAX = 79; //That's just an example, if all of the loops are identical: e.g. "for(int i=0; i<79; i++)". If the value of MAX changes for each loop, then make MAX an array instead: (new) int MAX [n]; MAX[0]=10; MAX[1]=20;...;MAX[n-1]=whatever.

int p = 0; //Used to increment all of the indicies correctly, at the end of each loop.
while (i[n]==0) {//Remember, you're only using indicies i[0], ..., i[n-1]. The (n+1)th index, i[n], is just to check whether to the nested loop stuff has finished.

  //DO STUFF HERE. Pretend you're inside your nested for loops. The more usual i,j,k,... have been replaced here with i[0], i[1], ..., i[n-1].


  //Now, after you've done your stuff, we need to increment all of the indicies correctly.
  i[0]++;
  // p = 0;//Commented out, because it's replaced by a more efficient alternative below.
  while(i[p]==MAX) {//(or "MAX[p]" if each "for" loop is different. Note that from an English point of view, this is more like "if(i[p]==MAX". (Initially i[0]) If this is true, then i[p] is reset to 0, and i[p+1] is incremented.
    i[p]=0;
    i[++p]++; //increase p by 1, and increase the next (p+1)th index
    if(i[p]!=MAX)
      p=0;//Alternatively, "p=0" can be inserted above (currently commented-out). This one's more efficient though, since it only resets p when it actually needs to be reset!
  }
}

到此为止。希望评论能清楚地说明它的意思。我认为它应该非常有效 - 几乎与真正的嵌套 for 循环一样多。大部分开销在一开始都是一次性的，所以这应该比使用递归函数等更有效