How to find the nth number? - c

Question

For an assignment, I have to write code which accepts as input an integer n and outputs the nth 'superunusual' number.
The first few su-numbers are: 22, 23, 26, 33, ... So when the input is 1, the output should be 22. 2 gives 23 and 3 gives 26.
I already have a code that checks if the input number is a su-number, but I can't find a way to calculate the nth number.
So when I now input 22, it says that 22 is a superunusual number.
The code:
/* calculates largest prime factor */
int lprime(int n) {
int max = -1;
while (n % 2 == 0) {
max = 2;
n /= 2;
}
for (int i = 3; i*i <= n; i += 2) {
while (n % i == 0) {
max = i;
n = n / i;
}
}
if (n > 2) {
max = n;
}
return max;
}
/* check unusual number */
int unus(int n) {
/* find largest prime of number */
int factor = lprime(n);
/* Check if largest prime > sqrt(n) */
if ((factor*factor) > n) {
return 1; /* true */
}
else {
return 0; /* false */
}
}
/* delete digit from number */
int del(int num, int n) {
int d = log10(num)+1; /* checks amount of digits */
int revnew = 0;
int new = 0;
for (int i = 0; num != 0; i++) {
int dig = num % 10;
num = num / 10;
if(i == (d - n)) {
continue;
} else {
revnew = (revnew * 10) + dig;
}
}
for (int i = 0; revnew != 0; i++) {
new = (new*10) + (revnew % 10);
revnew = revnew / 10;
}
return new;
}
/* driver code */
int main(int argc, char* v[]) {
int m=22, n;
int x = 0;
int i = 1;
int counter = 0;
scanf("%d", &n);
int d = log10(m)+1;
while (counter < n) {
if (unus(m++)) {
counter++;
}
}
for(unus(m); i < d; i++) {
int nmin = del(m, i);
if (unus(nmin)) {
continue;
} else {
printf("%d is not supurunusual\n", (m-1));
x++;
}
}
if(x==0) {
printf("%d is superunusual!\n", (m-1));
}
return 0;
}
I hope you can understand my code. Otherwise I will explain it better.
Also, I'm quite new to coding, so please don't be to harsh...

Answer 1

You have a function to determine whether a number is unusual, but you do the check whether a number is super-unusual in the body of the main routine. If you extract that code into a proper function:
int is_superunusual(int m)
{
int d = log10(m) + 1;
if (unus(m) == 0) return 0;
for(int i = 0; i < d; i++) { // see footnote
int nmin = del(m, i);
if (unus(nmin) == 0) return 0;
}
return 1;
}
then you can use Eugene's code:
while (counter < n) {
if (is_superunusual(m++)) {
counter++;
}
}
printf("The su number #%d is %d\n", n, m - 1);
Your code tested for unusual numbers, not super-unusual numbers.
Footnote: If you take del(num, n) to mean "remove the nth digit from the end", you can do away with the log10 call in del. You must check all deletions anyway, so the order doesn't really matter here.