Create a recursive for statement in c++ - c++

Question

I'd like to know if it is actually possible to create / define an homemade for statement in C++. Something similar already have been asked here:
"How to create a for loop like command in C++? #user9282's answer"
What I ask for is if we can make a for that performs as much for as we want (n times).
As example, here is a basic for-loop statement :
for (int i = 0; i < 10; i++) { ... }
I'm wondering if the new for-loop could result more like this :
int x = 20; // individual loops
int y = 3; // = amount of for-loops performed (in this case, 3)
// maybe some code generating for-loops here or something...
// result:
for (int i = 0; i < x; i++)
{
for (int j = 0; j < x; j++)
{
for (int k = 0; k < x; k++)
{
// if "y" was equal to 5, there would be 2 more for-loops here
instructions; // we can use "i", "j", "k", ... here
}
}
}
Do you think this could be possible in c++ ?
[EDIT: made clearer the code above]
In one sentence: I want to create a statement (e.g. if, while, for, switch) that puts for-loops into for-loops (just like the code above has for-loops into for-loops) so we can access multiple increments (i, j, k, ...) in the same scope.

Answer 1

You can easily do that with recursive functions that contains a for loop. I would do it like this:
void foo() {
for (...) {
foo();
}
}
That way, you can do as many nested for loops as you want.
However, if you want define a recursive nested for loops in your code without defining an external function, you could use lambdas:
auto recursive = [](auto func) {
// This is needed to make a recursive lambda
return [=](auto... args){
func(func, args...);
};
};
auto nestedForLoop = recursive([](auto self){
// Here's your recursive loop!
for (...) {
self();
}
});
// You can simply call `nestedForLoop` to execute loop
nestedForLoop();

Answer 2

If you have n nested for loops with the same bound x, you are performing xn iterations. In that case, you can just use a single index and convert it into multiple indices for convenience. For n = 2, for example:
for (int z = 0; z < x * x; ++z) {
const int i = z / x;
const int j = z % x;
// ...
}
For n = 3:
for (int z = 0; z < x * x * x; ++z) {
// The last "% x" in this line is for illustration only.
const int i = (z / x / x) % x;
const int j = (z / x) % x;
const int k = z % x;
// ...
}
i, j, k, etc. are the digits of the current iteration number z converted to base x. You could generalise this into a recursive function, or one that unpacks the digits into a vector.

Answer 3

Quick answer:
#include <iostream>
using namespace std;
void recursive(int x, int y, int tempY) // 2d recursive for loop
{
if (x > 0)
{
if (y > 0)
{
cout << x << " " << y << " \n ";
recursive(x, y - 1, tempY);
}
else
{
y = tempY;
recursive(x - 1, y, tempY);
}
}
}
void recursive(int x, int y, int z, int tempY, int tempZ) // 3d recursive for loop
{
if (x > 0)
{
if (y > 0)
{
if (z > 0)
{
cout << x << " " << y << " " << z << " \n ";
recursive(x, y, z - 1, tempY, tempZ);
}
else
{
z = tempZ;
recursive(x, y - 1, z, tempY, tempZ);
}
}
else
{
y = tempY;
recursive(x - 1, y, z, tempY, tempZ);
}
}
}
int main()
{
recursive(1, 2, 2); // x = 1, y = 2
recursive(1, 2, 3, 2, 3); // x = 1, y = 2, z = 3
}

Answer 4

Quick answer:
void n_for_loop(int x, int size, int arr[])
{
static int* arrTemp(new int[size]);
if (x >= size)
{
for (int i = 0; i < size; i++)
{
cout << arrTemp[i] << " ";
}
cout << endl;
return;
}
for (int i = 0; i < arr[x]; i++)
{
arrTemp[x] = i;
n_for_loop(x + 1, size, arr);
}
}