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Count the numbers < N which have equal number of divisors as K
• Last Updated : 07 May, 2021

Given two integers N and K, the task is to count all the numbers < N which have equal number of positive divisors as K.
Examples:

Input: n = 10, k = 5
Output:
2, 3 and 7 are the only numbers < 10 which have 2 divisors (equal to the number of divisors of 5)
Input: n = 500, k = 6
Output: 148

Approach:

• Compute the number of divisor of each number < N and store the result in an array where arr[i] contains the number of divisors of i.
• Traverse arr[], if arr[i] = arr[K] then update count = count + 1.
• Print the value of count in the end.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `// Function to return the count of the``// divisors of a number``int` `countDivisors(``int` `n)``{``    ``// Count the number of``    ``// 2s that divide n``    ``int` `x = 0, ans = 1;``    ``while` `(n % 2 == 0) {``        ``x++;``        ``n = n / 2;``    ``}``    ``ans = ans * (x + 1);` `    ``// n must be odd at this point.``    ``// So we can skip one element``    ``for` `(``int` `i = 3; i <= ``sqrt``(n); i = i + 2) {` `        ``// While i divides n``        ``x = 0;``        ``while` `(n % i == 0) {``            ``x++;``            ``n = n / i;``        ``}``        ``ans = ans * (x + 1);``    ``}` `    ``// This condition is to``    ``// handle the case when``    ``// n is a prime number > 2``    ``if` `(n > 2)``        ``ans = ans * 2;` `    ``return` `ans;``}` `int` `getTotalCount(``int` `n, ``int` `k)``{``    ``int` `k_count = countDivisors(k);` `    ``// Count the total elements``    ``// that have divisors exactly equal``    ``// to as that of k's``    ``int` `count = 0;``    ``for` `(``int` `i = 1; i < n; i++)``        ``if` `(k_count == countDivisors(i))``            ``count++;` `    ``// Exclude k from the result if it``    ``// is smaller than n.``    ``if` `(k < n)``       ``count = count - 1;` `    ``return` `count;``}` `// Driver code``int` `main()``{``    ``int` `n = 500, k = 6;``    ``cout << getTotalCount(n, k);``    ``return` `0;``}`

## Java

 `// Java implementation of the approach` `public` `class` `GFG{` `    ``// Function to return the count of the``    ``// divisors of a number``    ``static` `int` `countDivisors(``int` `n)``    ``{``        ``// Count the number of``        ``// 2s that divide n``        ``int` `x = ``0``, ans = ``1``;``        ``while` `(n % ``2` `== ``0``) {``            ``x++;``            ``n = n / ``2``;``        ``}``        ``ans = ans * (x + ``1``);``    ` `        ``// n must be odd at this point.``        ``// So we can skip one element``        ``for` `(``int` `i = ``3``; i <= Math.sqrt(n); i = i + ``2``) {``    ` `            ``// While i divides n``            ``x = ``0``;``            ``while` `(n % i == ``0``) {``                ``x++;``                ``n = n / i;``            ``}``            ``ans = ans * (x + ``1``);``        ``}``    ` `        ``// This condition is to``        ``// handle the case when``        ``// n is a prime number > 2``        ``if` `(n > ``2``)``            ``ans = ans * ``2``;``    ` `        ``return` `ans;``    ``}``    ` `    ``static` `int` `getTotalCount(``int` `n, ``int` `k)``    ``{``        ``int` `k_count = countDivisors(k);``    ` `        ``// Count the total elements``        ``// that have divisors exactly equal``        ``// to as that of k's``        ``int` `count = ``0``;``        ``for` `(``int` `i = ``1``; i < n; i++)``            ``if` `(k_count == countDivisors(i))``                ``count++;``    ` `        ``// Exclude k from the result if it``        ``// is smaller than n.``        ``if` `(k < n)``        ``count = count - ``1``;``    ` `        ``return` `count;``    ``}``    ` `    ``// Driver code``     ``public` `static` `void` `main(String []args)``    ``{``        ``int` `n = ``500``, k = ``6``;``        ``System.out.println(getTotalCount(n, k));``    ``}``    ``// This code is contributed by Ryuga``}`

## Python3

 `# Python3 implementation of the approach` `# Function to return the count of``# the divisors of a number``def` `countDivisors(n):``    ` `    ``# Count the number of 2s that divide n``    ``x, ans ``=` `0``, ``1``    ``while` `(n ``%` `2` `=``=` `0``):``        ``x ``+``=` `1``        ``n ``=` `n ``/` `2``    ``ans ``=` `ans ``*` `(x ``+` `1``)` `    ``# n must be odd at this point.``    ``# So we can skip one element``    ``for` `i ``in` `range``(``3``, ``int``(n ``*``*` `1` `/` `2``) ``+` `1``, ``2``):``        ` `        ``# While i divides n``        ``x ``=` `0``        ``while` `(n ``%` `i ``=``=` `0``):``            ``x ``+``=` `1``            ``n ``=` `n ``/` `i``        ``ans ``=` `ans ``*` `(x ``+` `1``)` `    ``# This condition is to handle the``    ``# case when n is a prime number > 2``    ``if` `(n > ``2``):``        ``ans ``=` `ans ``*` `2` `    ``return` `ans` `def` `getTotalCount(n, k):``    ``k_count ``=` `countDivisors(k)` `    ``# Count the total elements that``    ``# have divisors exactly equal``    ``# to as that of k's``    ``count ``=` `0``    ``for` `i ``in` `range``(``1``, n):``        ``if` `(k_count ``=``=` `countDivisors(i)):``            ``count ``+``=` `1` `    ``# Exclude k from the result if it``    ``# is smaller than n.``    ``if` `(k < n):``        ``count ``=` `count ``-` `1` `    ``return` `count` `# Driver code``if` `__name__ ``=``=` `'__main__'``:``    ``n, k ``=` `500``, ``6``    ``print``(getTotalCount(n, k))` `# This code is contributed``# by 29AjayKumar`

## C#

 `// C# implementation of the approach``using` `System;``                    ` `public` `class` `GFG{`` ` `    ``// Function to return the count of the``    ``// divisors of a number``    ``static` `int` `countDivisors(``int` `n)``    ``{``        ``// Count the number of``        ``// 2s that divide n``        ``int` `x = 0, ans = 1;``        ``while` `(n % 2 == 0) {``            ``x++;``            ``n = n / 2;``        ``}``        ``ans = ans * (x + 1);``     ` `        ``// n must be odd at this point.``        ``// So we can skip one element``        ``for` `(``int` `i = 3; i <= Math.Sqrt(n); i = i + 2) {``     ` `            ``// While i divides n``            ``x = 0;``            ``while` `(n % i == 0) {``                ``x++;``                ``n = n / i;``            ``}``            ``ans = ans * (x + 1);``        ``}``     ` `        ``// This condition is to``        ``// handle the case when``        ``// n is a prime number > 2``        ``if` `(n > 2)``            ``ans = ans * 2;``     ` `        ``return` `ans;``    ``}``     ` `    ``static` `int` `getTotalCount(``int` `n, ``int` `k)``    ``{``        ``int` `k_count = countDivisors(k);``     ` `        ``// Count the total elements``        ``// that have divisors exactly equal``        ``// to as that of k's``        ``int` `count = 0;``        ``for` `(``int` `i = 1; i < n; i++)``            ``if` `(k_count == countDivisors(i))``                ``count++;``     ` `        ``// Exclude k from the result if it``        ``// is smaller than n.``        ``if` `(k < n)``        ``count = count - 1;``     ` `        ``return` `count;``    ``}``     ` `    ``// Driver code``     ``public` `static` `void` `Main()``    ``{``        ``int` `n = 500, k = 6;``        ``Console.WriteLine(getTotalCount(n, k));``    ``}   ``}` `// This code is contributed by 29AjayKumar`

## PHP

 ` 2``    ``if` `(``\$n` `> 2)``        ``\$ans` `= ``\$ans` `* 2;` `    ``return` `\$ans``;``}` `function`  `getTotalCount(``\$n``, ``\$k``)``{``    ``\$k_count` `= countDivisors(``\$k``);` `    ``// Count the total elements``    ``// that have divisors exactly equal``    ``// to as that of k's``    ``\$count` `= 0;``    ``for` `(``\$i` `= 1; ``\$i` `< ``\$n``; ``\$i``++)``        ``if` `(``\$k_count` `== countDivisors(``\$i``))``            ``\$count``++;` `    ``// Exclude k from the result if it``    ``// is smaller than n.``    ``if` `(``\$k` `< ``\$n``)``    ``\$count` `= ``\$count` `- 1;` `    ``return` `\$count``;``}` `// Driver code` `    ``\$n` `= 500;``    ``\$k` `= 6;``    ``echo`  `getTotalCount(``\$n``, ``\$k``);` `#This code is contributed by Sachin..``?>`

## Javascript

 ``
Output:
`148`

Optimization:
The above solution can be optimized using Sieve technique. Please refer Count number of integers less than or equal to N which has exactly 9 divisors for details.

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