Largest number with prime digits in C++

In this tutorial, we are going to write a program that finds the largest number with prime digits that is less than n.

Let's see the steps to solve the problem.

• Write a loop that iterates from 0 to n.
  • If the current digit is not prime.
    • While the digit is less 2, decrement the i value. If the i value is negative, then make it 0.
    • Update the current index value with the next smallest prime digit.
    • From the next index, make every digit to 7.
• Return n.

Example

Let's see the code.

#include <bits/stdc++.h>
using namespace std;
bool isPrime(char c) {
   return c == '2' || c == '3' || c == '5' || c == '7';
}
void decrease(string& n, int i) {
   if (n[i] <= '2') {
      n.erase(i, 1);
      n[i] = '7';
   }else if (n[i] == '3') {
      n[i] = '2';
   }else if (n[i] <= '5') {
      n[i] = '3';
   }else if (n[i] <= '7') {
      n[i] = '5';
   }else {
      n[i] = '7';
   }
   return;
}
string getPrimeDigitsNumber(string n) {
   for (int i = 0; i < n.length(); i++) {
      if (!isPrime(n[i])) {
         while (n[i] <= '2' && i >= 0) {
            i--;
         }
         if (i < 0) {
            i = 0;
         }
         decrease(n, i);
         for (int j = i + 1; j < n.length(); j++) {
            n[j] = '7';
         }
         break;
      }
   }
   return n;
}
int main() {
   string n = "7464";
   cout << getPrimeDigitsNumber(n) << endl;
   return 0;
}

Output

If you run the above code, then you will get the following result.

7377

Conclusion

If you have any queries in the tutorial, mention them in the comment section.