Task: Humble numbers
C: #include <limits.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

bool isHumble(int i) {
    if (i <= 1) return true;
    if (i % 2 == 0) return isHumble(i / 2);
    if (i % 3 == 0) return isHumble(i / 3);
    if (i % 5 == 0) return isHumble(i / 5);
    if (i % 7 == 0) return isHumble(i / 7);
    return false;
}

int main() {
    int limit = SHRT_MAX;
    int humble[16];
    int count = 0;
    int num = 1;
    char buffer[16];

    memset(humble, 0, sizeof(humble));

    for (; count < limit; num++) {
        if (isHumble(num)) {
            size_t len;
            sprintf_s(buffer, sizeof(buffer), "%d", num);
            len = strlen(buffer);
            if (len >= 16) {
                break;
            }
            humble[len]++;

            if (count < 50) {
                printf("%d ", num);
            }
            count++;
        }
    }
    printf("\n\n");

    printf("Of the first %d humble numbers:\n", count);
    for (num = 1; num < 10; num++) {
        printf("%5d have %d digits\n", humble[num], num);
    }

    return 0;
}

Java: import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class HumbleNumbers {

    public static void main(String[] args) {
        System.out.println("First 50 humble numbers:");
        System.out.println(Arrays.toString(humble(50)));
        Map<Integer,Integer> lengthCountMap = new HashMap<>();
        BigInteger[] seq = humble(1_000_000);
        for ( int i = 0 ; i < seq.length ; i++ ) {
            BigInteger humbleNumber = seq[i];
            int len = humbleNumber.toString().length();
            lengthCountMap.merge(len, 1, (v1, v2) -> v1 + v2);
        }
        List<Integer> sorted = new ArrayList<>(lengthCountMap.keySet());
        Collections.sort(sorted);
        System.out.printf("Length  Count%n");
        for ( Integer len : sorted ) {
            System.out.printf("    %2s  %5s%n", len, lengthCountMap.get(len));
        }
    }
    
    private static BigInteger[] humble(int n) {
        BigInteger two = BigInteger.valueOf(2);
        BigInteger twoTest = two;
        BigInteger three = BigInteger.valueOf(3);
        BigInteger threeTest = three;
        BigInteger five = BigInteger.valueOf(5);
        BigInteger fiveTest = five;
        BigInteger seven = BigInteger.valueOf(7);
        BigInteger sevenTest = seven;
        BigInteger[] results = new BigInteger[n];
        results[0] = BigInteger.ONE;
        int twoIndex = 0, threeIndex = 0, fiveIndex = 0, sevenIndex = 0;
        for ( int index = 1 ; index < n ; index++ ) {
            results[index] = twoTest.min(threeTest).min(fiveTest).min(sevenTest);
            if ( results[index].compareTo(twoTest) == 0 ) {
                twoIndex++;
                twoTest = two.multiply(results[twoIndex]);
            }
            if (results[index].compareTo(threeTest) == 0 ) {
                threeIndex++;
                threeTest = three.multiply(results[threeIndex]);
            }
            if (results[index].compareTo(fiveTest) == 0 ) {
                fiveIndex++;
                fiveTest = five.multiply(results[fiveIndex]);
            }
            if (results[index].compareTo(sevenTest) == 0 ) {
                sevenIndex++;
                sevenTest = seven.multiply(results[sevenIndex]);
            }
        }
        return results;
    }

}