# Count N-digit numbers having sum of digits equal to a Prime Number

• Last Updated : 28 Sep, 2021

Given a positive integer N, the task is to count the number of N-digit numbers whose sum of digits is a prime number.

Examples:

Input: N = 1
Output: 4
Explanation: [2, 3, 5, 7] are single digit numbers whose sum of digits is equal to a prime number.

Input : N = 2
Output : 33

Naive Approach: The simplest approach to solve the given problem is to generate all possible N-digit numbers and count those numbers whose sum of digits is a prime number. After checking for all the numbers, print the value of the count as the resultant total count of numbers.

Time Complexity: O(N *10N)

Efficient Approach: The above approach can also be optimized by using Recursive Dynamic Programming because the above problem has overlapping subproblems and an optimal substructure. Follow the steps below to solve the problem:

• Initialize a global 2D array dp[N+1][N*10] with all values as -1 that stores the result of each recursive call.
• Compute prime numbers upto (N * 10) numbers by using Sieve of Eratosthenes.
• Define a recursive function, say countOfNumbers(index, sum, N) by performing the following steps.
• If the value of the index is N+1,
• If the sum is a prime number, return 1 as a valid N-digit number has been formed.
• Else return 0.
• If the result of the state dp[index][sum] is already computed, return this value dp[index][sum].
• If the current index is 1, then any digit from [1- 9] can be placed, else, [0-9] can be placed.
• After making a valid placement of digits, recursively call the countOfNumbers function for the next index, and sum up all recursive pending results into variable val.
• Store the value of val into the current state of the dp[index][sum] table.
• Return this state’s result val to it’s parent recursive call.
• Print the value returned by the function countOfNumbers(1, 0, N) as the result.

Below is the implementation of the above approach:

## C++

 `#include ``using` `namespace` `std;` `// Stores the dp states``int` `dp;` `// Check if a number is``// a prime or not``bool` `prime;` `// Function to generate all prime numbers``// that are less than or equal to n``void` `SieveOfEratosthenes(``int` `n)``{``    ``// Base cases.``    ``prime = prime = ``false``;``    ``for` `(``int` `p = 2; p * p <= n; p++) {` `        ``// If prime[p] is not changed,``        ``// then it is a prime``        ``if` `(prime[p] == ``true``) {` `            ``// Update all multiples``            ``// of as non-prime``            ``for` `(``int` `i = p * p; i <= n; i += p)``                ``prime[i] = ``false``;``        ``}``    ``}``}` `// Function to find the count of N-digit numbers``// such that the sum of digits is a prime number``int` `countOfNumbers(``int` `index, ``int` `sum, ``int` `N)``{` `    ``// If end of array is reached``    ``if` `(index == N + 1) {` `        ``// If the sum is equal to a prime number``        ``if` `(prime[sum] == ``true``) {``            ``return` `1;``        ``}` `        ``// Otherwise``        ``return` `0;``    ``}` `    ``int``& val = dp[index][sum];` `    ``// If the dp-states are already computed``    ``if` `(val != -1) {` `        ``return` `val;``    ``}` `    ``val = 0;` `    ``// If index = 1, any digit from [1 - 9] can be placed.``    ``// If N = 1, 0 also can be placed.``    ``if` `(index == 1) {` `        ``for` `(``int` `digit = (N == 1 ? 0 : 1); digit <= 9;``             ``++digit) {``            ``val += countOfNumbers(index + 1, sum + digit,``                                  ``N);``        ``}``    ``}` `    ``// Otherwise, any digit from [0 - 9] can be placed.``    ``else` `{``        ``for` `(``int` `digit = 0; digit <= 9; ++digit) {``            ``val += countOfNumbers(index + 1, sum + digit,``                                  ``N);``        ``}``    ``}` `    ``// Return the answer.``    ``return` `val;``}` `// Driver Code``int` `main()``{``    ``// Initializing dp array with -1``    ``memset``(dp, -1, ``sizeof` `dp);` `    ``// Initializing prime array to true``    ``memset``(prime, ``true``, ``sizeof``(prime));` `    ``// Find all primes less than or equal to 1000,``    ``// which is sufficient for N upto 100``    ``SieveOfEratosthenes(1000);` `    ``// Given Input``    ``int` `N = 6;` `    ``// Function call``    ``cout << countOfNumbers(1, 0, N);` `    ``return` `0;``}`

## Java

 `import` `java.util.Arrays;` `class` `GFG{` `// Stores the dp states``public` `static` `int``[][] dp = ``new` `int``[``100``][``1000``];` `// Check if a number is``// a prime or not``public` `static` `boolean``[] prime = ``new` `boolean``[``1005``];` `// Function to generate all prime numbers``// that are less than or equal to n``public` `static` `void` `SieveOfEratosthenes(``int` `n)``{``    ` `    ``// Base cases.``    ``prime[``0``] = prime[``1``] = ``false``;``    ``for``(``int` `p = ``2``; p * p <= n; p++)``    ``{``        ` `        ``// If prime[p] is not changed,``        ``// then it is a prime``        ``if` `(prime[p] == ``true``)``        ``{``            ` `            ``// Update all multiples``            ``// of as non-prime``            ``for``(``int` `i = p * p; i <= n; i += p)``                ``prime[i] = ``false``;``        ``}``    ``}``}` `// Function to find the count of N-digit numbers``// such that the sum of digits is a prime number``public` `static` `int` `countOfNumbers(``int` `index, ``int` `sum,``                                 ``int` `N)``{``    ` `    ``// If end of array is reached``    ``if` `(index == N + ``1``)``    ``{``        ` `        ``// If the sum is equal to a prime number``        ``if` `(prime[sum] == ``true``)``        ``{``            ``return` `1``;``        ``}` `        ``// Otherwise``        ``return` `0``;``    ``}` `    ``int` `val = dp[index][sum];` `    ``// If the dp-states are already computed``    ``if` `(val != -``1``)``    ``{``        ``return` `val;``    ``}` `    ``val = ``0``;` `    ``// If index = 1, any digit from [1 - 9] can be placed.``    ``// If N = 1, 0 also can be placed.``    ``if` `(index == ``1``)``    ``{``        ``for``(``int` `digit = (N == ``1` `? ``0` `: ``1``);``                ``digit <= ``9``; ++digit)``        ``{``            ``val += countOfNumbers(index + ``1``,``                                    ``sum + digit, N);``        ``}``    ``}` `    ``// Otherwise, any digit from [0 - 9] can be placed.``    ``else``    ``{``        ``for``(``int` `digit = ``0``; digit <= ``9``; ++digit)``        ``{``            ``val += countOfNumbers(index + ``1``,``                                    ``sum + digit, N);``        ``}``    ``}` `    ``// Return the answer.``    ``return` `val;``}` `// Driver Code``public` `static` `void` `main(String args[])``{``    ` `    ``// Initializing dp array with -1``    ``for``(``int``[] row : dp)``        ``Arrays.fill(row, -``1``);` `    ``// Initializing prime array to true``    ``Arrays.fill(prime, ``true``);` `    ``// Find all primes less than or equal to 1000,``    ``// which is sufficient for N upto 100``    ``SieveOfEratosthenes(``1000``);` `    ``// Given Input``    ``int` `N = ``6``;` `    ``// Function call``    ``System.out.println(countOfNumbers(``1``, ``0``, N));``}``}` `// This code is contributed by gfgking`

## Python3

 `# Python program for the above approach` `# Stores the dp states``dp ``=` `[[``-``1``]``*``100``]``*``1000` `# Check if a number is``# a prime or not``prime ``=` `[``True``] ``*` `1005` `# Function to generate all prime numbers``# that are less than or equal to n``def` `SieveOfEratosthenes(n) :``    ` `    ``# Base cases.``    ``prime[``0``] ``=` `prime[``1``] ``=` `False``    ``p ``=` `2``    ``while``(p ``*` `p <``=` `n):` `        ``# If prime[p] is not changed,``        ``# then it is a prime``        ``if` `(prime[p] ``=``=` `True``) :` `            ``# Update all multiples``            ``# of as non-prime``            ``for` `i ``in` `range``(p ``*` `p, n``+``1``, p) :``                ``prime[i] ``=` `False``        ` `        ``p ``+``=` `1` `# Function to find the count of N-digit numbers``# such that the sum of digits is a prime number``def` `countOfNumbers(index, ``sum``, N) :` `    ``# If end of array is reached``    ``if` `(index ``=``=` `N ``+` `1``) :` `        ``# If the sum is equal to a prime number``        ``if` `(prime[``sum``] ``=``=` `True``) :``            ``return` `1``        `  `        ``# Otherwise``        ``return` `0``    `  `    ``val ``=` `dp[index][``sum``]` `    ``# If the dp-states are already computed``    ``if` `(val !``=` `-``1``) :` `        ``return` `val``    `  `    ``val ``=` `0` `    ``# If index = 1, any digit from [1 - 9] can be placed.``    ``# If N = 1, 0 also can be placed.``    ``if` `(index ``=``=` `1``) :` `        ``for` `digit ``in` `range``(((``0``, ``1``) [N ``=``=` `1``])``+``1``, ``10``, ``1``) :``            ``val ``+``=` `countOfNumbers(index ``+` `1``, ``sum` `+` `digit, N)``        ` `    ``# Otherwise, any digit from [0 - 9] can be placed.``    ``else` `:``        ``for` `digit ``in` `range``(``0``, ``10``, ``1``) :``            ``val ``+``=` `countOfNumbers(index ``+` `1``, ``sum` `+` `digit, N)``    ` `    ``# Return the answer.``    ``return` `val` `# Driver Code` `# Find all primes less than or equal to 1000,``# which is sufficient for N upto 100``SieveOfEratosthenes(``1000``)` `# Given Input``N ``=` `6` `# Function call``print``(countOfNumbers(``1``, ``0``, N))` `# This code is contributed by avijitmondal1998.`

## C#

 `using` `System;` `public` `class` `GFG{` `// Stores the dp states``public` `static` `int``[,] dp = ``new` `int``[100,1000];` `// Check if a number is``// a prime or not``public` `static` `bool``[] prime = ``new` `bool``;` `// Function to generate all prime numbers``// that are less than or equal to n``public` `static` `void` `SieveOfEratosthenes(``int` `n)``{``    ` `    ``// Base cases.``    ``prime = prime = ``false``;``    ``for``(``int` `p = 2; p * p <= n; p++)``    ``{``        ` `        ``// If prime[p] is not changed,``        ``// then it is a prime``        ``if` `(prime[p] == ``true``)``        ``{``            ` `            ``// Update all multiples``            ``// of as non-prime``            ``for``(``int` `i = p * p; i <= n; i += p)``                ``prime[i] = ``false``;``        ``}``    ``}``}` `// Function to find the count of N-digit numbers``// such that the sum of digits is a prime number``public` `static` `int` `countOfNumbers(``int` `index, ``int` `sum,``                                 ``int` `N)``{``    ` `    ``// If end of array is reached``    ``if` `(index == N + 1)``    ``{``        ` `        ``// If the sum is equal to a prime number``        ``if` `(prime[sum] == ``true``)``        ``{``            ``return` `1;``        ``}` `        ``// Otherwise``        ``return` `0;``    ``}` `    ``int` `val = dp[index,sum];` `    ``// If the dp-states are already computed``    ``if` `(val != -1)``    ``{``        ``return` `val;``    ``}` `    ``val = 0;` `    ``// If index = 1, any digit from [1 - 9] can be placed.``    ``// If N = 1, 0 also can be placed.``    ``if` `(index == 1)``    ``{``        ``for``(``int` `digit = (N == 1 ? 0 : 1);``                ``digit <= 9; ++digit)``        ``{``            ``val += countOfNumbers(index + 1,``                                    ``sum + digit, N);``        ``}``    ``}` `    ``// Otherwise, any digit from [0 - 9] can be placed.``    ``else``    ``{``        ``for``(``int` `digit = 0; digit <= 9; ++digit)``        ``{``            ``val += countOfNumbers(index + 1,``                                    ``sum + digit, N);``        ``}``    ``}` `    ``// Return the answer.``    ``return` `val;``}` `// Driver Code``public` `static` `void` `Main(String []args)``{``    ` `    ``// Initializing dp array with -1``    ``for``(``int` `i = 0; i < 100; i++)``    ``{``        ``for` `(``int` `j = 0; j < 1000; j++) {``            ``dp[i,j] = -1;``        ``}``    ``}` `    ``// Initializing prime array to true``    ``for` `(``int` `i = 0; i < prime.Length; i++)``        ``prime[i] = ``true``;` `    ``// Find all primes less than or equal to 1000,``    ``// which is sufficient for N upto 100``    ``SieveOfEratosthenes(1000);` `    ``// Given Input``    ``int` `N = 6;` `    ``// Function call``    ``Console.WriteLine(countOfNumbers(1, 0, N));``}``}` `// This code is contributed by Princi Singh`

## Javascript

 ``

Output:

`222638`

Time Complexity: O(N2 * 10)
Auxiliary Space: O(N2)

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