Print 1 to 100 in C++, without loop and recursion

3.5

Following is a C++ program that prints 1 to 100 without loop and without recursion.

#include <iostream>
using namespace std;

template<int N>
class PrintOneToN
{
public:
    static void print()
    {
        PrintOneToN<N-1>::print();  // Note that this is not recursion
        cout << N << endl;
    }
};

template<>
class PrintOneToN<1>
{
public:
    static void print()
    {
        cout << 1 << endl;
    }
};
int main()
{
    const int N = 100;
    PrintOneToN<N>::print();
    return 0;
}

Output:

1
2
3
..
..
98
99
100

The program prints all numbers from 1 to n without using a loop and recursion. The concept used in this program is Template Metaprogramming.
Let us see how this works. Templates in C++ allow non-datatypes also as parameter. Non-datatype means a value, not a datatype. For example, in the above program, N is passed as a value which is not a datatype. A new instance of a generic class is created for every parameter and these classes are created at compile time. In the above program, when compiler sees the statement “PrintOneToN<>::print()” with N = 100, it creates an instance PrintOneToN<100>. In function PrintOneToN<100>::print(), another function PrintOneToN<99>::print() is called, therefore an instance PrintOneToN<99> is created. Similarly, all instances from PrintOneToN<100> to PrintOneToN<2> are created. PrintOneToN<1>::print() is already there and prints 1. The function PrintOneToN<2> prints 2 and so on. Therefore we get all numbers from 1 to N printed on the screen.



Following is another approach to print 1 to 100 without loop and recursion.

#include<iostream>
using namespace std;

class A
{
public:
    static int a;
    A()
    {  cout<<a++<<endl;  }
};

int A::a = 1;

int main()
{
    int N = 100;
    A obj[N];
    return 0;
}

The output of this program is same as above program. In the above program, class A has a static variable ‘a’, which is incremented with every instance of A. The default constructor of class A prints the value of ‘a’. When we create an array of objects of type A, the default constructor is called for all objects one by one. Value of ‘a’ is printed and incremented with every call. Therefore, we get all values from 1 to 100 printed on the scree.

Thanks to Lakshmanan for suggesting this approach.



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