Unique Combinations of Digits in C

Question

I need to design an algorithm in C to calculate unique combinations of digits for 0 to 1,000,000. For example, when 13 appears, 31 would not be included in this sequence. Can anyone help me find an algorithm to describe this? The first few numbers in the series would be:

 1,2,3,4,5,6,7,8,9,11,12,13,14,15,16,17,18,19,22,23,24,25,26,27,28,29,33,etc

Thanks!

edit - Sorry, forgot to mention that zero isn't included

Answer 1

#include <stdio.h>
int main(void) {
    int i, n;
    for (n = 1; n < 1000000; n++) {
        for (i = n;;) {
            if (i / 10 % 10 > i % 10) break;
            if ((i /= 10) == 0) { printf("%d\n", n); break; }
        }
    }
}

5004 numbers in the series from 0 to 1000000

A much quicker version:

#include <stdio.h>
#include <stdlib.h>

static long long enumerate(char *p, int i, int n, int digit, int silent) {
    long long count = 0;
    if (i >= n) {
        if (!silent) printf("%s\n", p);
        return 1;
    }
    for (p[i] = digit; p[i] <= '9'; p[i]++)
        count += enumerate(p, i + 1, n, p[i], silent);
    return count;
}

int main(int argc, char **argv) {
    char array[256];
    int i, n;
    int max = (argc > 1) ? strtol(argv[1], NULL, 0) : 6;
    int silent = 0;
    long long count = 0;
    if (max < 0) {
        max = -max;
        silent = 1;
    }
    array[sizeof(array)-1] = '\0';
    for (n = 1; n <= max; n++) {
        count += enumerate(array + sizeof(array) - 1 - n, 0, n, '1', silent);
        if (silent)
            printf("%lld combinations between 0 and 1E%d\n", count, n);
    }
}

invoke with a positive number to enumerate combinations and a negative number to just count them.