# 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.

Live Demo

#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.