为了提高效率，我避免了递归。而且，它没有使用任何特定的c++的东西-它在C上也能很好地工作。
我们尝试创建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++)
    ...

我将用一个数组替换i，j，...i [0]、i [1]、...、i [n-1]。
我的解决方案是：

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!
  }
}

好了，就这样。希望注解能清楚地说明它要做什么。我认为它应该非常高效--几乎和真正的嵌套for循环一样。大部分开销在开始时是一次性的，所以这应该比使用递归函数等更高效

具有多个循环的递归算法的基本结构如下：

void recursiveLoops(vector<int>& indexes, const vector<int>& endPerIndex, int currentIndex) {
    if (currentIndex == indexes.size()) {
        // This is where the real logic goes.
        // indexes[i] contain the value of the i-th index.
    } else {
        for (indexes[pos] = 0 ; indexes[pos] != endPerIndex[pos] ; indexes[pos]++) {
            // Recurse for the next level
            recursiveLoops(indexes, endPerIndex, pos+1);
        }
    }
}

从顶层调用recursiveLoops的设置需要两个向量-一个用于索引，另一个用于每层的迭代次数。下面的示例设置了三个嵌套循环，每层迭代5、6和9次：

vector<int> indexes(3, 0);
vector<int> endPerIndex;
endPerIndex.push_back(5);
endPerIndex.push_back(6);
endPerIndex.push_back(9);
recursiveLoops(indexes, endPerIndex, 0);

这是一个用普通的 C++ 编写的例子，首先我为每个维度创建一个向量maxes，如果所有索引之和为2，那么我print做了一些事情，在这个例子中，我循环z从0到1，y从0到2，x从0到3
你肯定可以把这个弄得更整洁。
这是：

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

int f(){ 
    return 2 ;
}

void inner(int depth,vector<int> & numbers,vector<int> & maxes){
  if (depth>0){
     for(int i=0;i<maxes[depth-1];i++){
        numbers[depth-1]=i;
        inner(depth-1, numbers,maxes) ;
     }
  }else{
     // calculate sum of x,y,z:
     cout << "values are ";
     for(int i=0;i<numbers.size();i++){
        cout <<numbers[i]<<" ";
     }
     int thesum(0);
     for(int i=0;i<numbers.size();i++){
        thesum+=numbers[i];
     }
     if (thesum==f()){
        cout << "did something! ";
     }
     cout<<endl;
   }
}

void donest(){
   vector<int>  numbers;
   numbers.resize(3);
   vector<int>  maxes;
   maxes.push_back(4);
   maxes.push_back(3);
   maxes.push_back(2);
   inner(numbers.size(),numbers,maxes);
}

int main(){
   donest();
}

结果：

values are 0 0 0 
values are 1 0 0 
values are 2 0 0  did something! 
values are 3 0 0 
values are 0 1 0 
values are 1 1 0  did something! 
values are 2 1 0 
values are 3 1 0 
values are 0 2 0  did something! 
values are 1 2 0 
values are 2 2 0 
values are 3 2 0 
values are 0 0 1 
values are 1 0 1  did something! 
values are 2 0 1 
values are 3 0 1 
values are 0 1 1  did something! 
values are 1 1 1 
values are 2 1 1 
values are 3 1 1 
values are 0 2 1 
values are 1 2 1 
values are 2 2 1 
values are 3 2 1

只需要计算每个递归函数的深度，并计数到f。

void myRecursiveFunc(int depth){
   if(depth == f)
      //do something
      return;
   else{
      myRecursiveFunc(depth + 1);
   }
}

如果你真的需要，你可以用三个不同的函数来表示x，y和z。

你对为什么要这样做非常含糊。对于初学者来说，一个可能的解决方案是用递归函数替换每个for循环。

void recursiveX(int zVal, int yVal, int xVal)
{
    if(zVal+yVal+xVal == f)...
    if(xVal != 0)
        recursiveX(zVal, yVal, xVal -1);
}

void recursiveY(int zVal, int yVal)
{
    recursiveX(zVal, yVal, 6);
    if(yVal != 0)
        recursiveY(zVal, yVal-1);
}

void recursiveZ(int val)
{
    recursiveY(val, 6);
    if(val != 0)
        recursiveZ(val-1);
}
...
recursiveZ(6);

最后你可以把所有这些合并成一个函数。然而，仅仅因为可能就使用递归从来都不是一个好主意。

你可以这样写，但是.... a.我不会。这是令人困惑的代码，不会给你带来任何好处。如果你想要这样做是因为你真正的用例有大量的嵌套循环，那么考虑一下不要这样做;这是一个严重设计气味。

void nested_loop(const int levels, const int comparator, const int level = 0, const int accumulator = 0)
{
   if (level < levels) {
      for (int i = 0; i < 6; i++) {
         nested_loop(levels, comparator, level + 1, accumulator + i);
      }
   }
   else {
      if (accumulator == comparator) {   // your if (z+y+x==f)
         //do something
      }
   }
}

int main() {
   const int levels = 3;
   const int f = 42;

   nested_loop(levels, f);
}

Live demo。

在“C”中使用while循环的变量循环。

概念

1.创建一个二维数组（arr[level][2]），其中第一个元素是start，第二个元素是end。
x[3][2] = {{0, 10}, {5, 20}, {2, 60}};
1.创建另一个包含起始元素的数组。
y[3] = {0, 5, 2};
1.我们创建了第二个数组，因为在循环过程中我们将更改“x”数组的第一个元素。
编号

#include <stdio.h>
int main(){
// bruteforce
int level = 10;
int start[10] = {0, 0, 0, 0};
int x[10][2] = {{0, 5}, {0, 5}, {0, 5}, {0, 5}};

for (int i = 1;i < level; ++i){
    x[i][1] = x[i][1] + 1;
}
while(3>2){
    // Your code here


   // 

    printf("%d %d %d %d\n", x[0][0], x[1][0], x[2][0], x[3][0]);

    // variable loop code
    // ==== Not To Modify ====
    int a = 0;
    int b = 0;
    for(int i = 0;i < level; ++i){
        if (x[i][0] >= x[i][1])
        {
                if(i != level-1){
                    x[i][0] = start[i];
                    x[i+1][0] = x[i+1][0] + 1;
                }else{
                    a = 1;
                }
                b = 1;
            
        }else{
            if(b == 0){
                x[0][0] = x[0][0] + 1;
                b = 1;
            }
        }
    }
    if(a == 1){
        break;
    }
}
return 0;
}

这是一个迟来的答案，但也许它会帮助一些人。
下面是我的解决方案在c++没有递归函数.：

int n_loops{3}; //number of nested for loops

int loops_idx[n_loops]; //like i,j,k but in an array
for (int i = 0; i < n_loops; i++)
    loops_idx[i]=0;

int max_idx[n_loops]{3,2,4}; // like in for(; i < counter ;), but the counters in an array

bool is_finished = false;
int debug_n_of_execution{0};

while (!is_finished)
{
    for (; loops_idx[0]<max_idx[0]; loops_idx[0]++)
    {
        /*
        some code with loops_idx array as i,j,k...
        */
        ++debug_n_of_execution;
        for (int i = 0; i < n_loops; i++)
            std::cout<<loops_idx[i]<<" ";
        std::cout << "\n";
    }

    --loops_idx[0]; //to cancel last increment
    //Here it will increment the last loop_idx which isn't equal to max_idx[i]-1
    //eg. after first above for loop loops_idx will be (max-1, 0, 0)
    //So it will be after this loop (0, 1, 0) and start from the beginning...
    for (int i = 0; i < n_loops+1; i++) //+1 to know if all loops are finished
    {
        if (i == n_loops)
        {is_finished= true; break;}

        if(loops_idx[i]==max_idx[i]-1)
            continue;
        
        ++loops_idx[i];
        for (int j = 0; j < i; j++) //make any previous loop = 0  
            loops_idx[j]=0;
        
        break;
    }
}

//just to check
int debug_perfect_n_of_execution{max_idx[0]};
for (int i = 1; i < n_loops; i++)
    debug_perfect_n_of_execution*= max_idx[i];
std::cout<<"Number of execution: "<<debug_n_of_execution<<" = "<<debug_perfect_n_of_execution;
assert(debug_n_of_execution==debug_perfect_n_of_execution);
std::cout << "\nTests Finished";

结果如下：

0 0 0 
1 0 0
2 0 0
0 1 0
1 1 0
2 1 0
0 0 1
1 0 1
2 0 1
0 1 1
1 1 1
2 1 1
0 0 2
1 0 2
2 0 2
0 1 2
1 1 2
2 1 2
0 0 3
1 0 3
2 0 3
0 1 3
1 1 3
2 1 3
Number of execution: 24 = 24
Tests Finished