How to check whether a no is factorial or not? - c

Question

I have a problem, then given some input number n, we have to check whether the no is factorial of some other no or not.
INPUT 24, OUTPUT true
INPUT 25, OUTPUT false
I have written the following program for it:-
int factorial(int num1)
{
if(num1 > 1)
{
return num1* factorial(num1-1) ;
}
else
{
return 1 ;
}
}
int is_factorial(int num2)
{
int fact = 0 ;
int i = 0 ;
while(fact < num2)
{
fact = factorial(i) ;
i++ ;
}
if(fact == num2)
{
return 0 ;
}
else
{
return -1;
}
}
Both these functions, seem to work correctly.
When we supply them for large inputs repeatedly, then the is_factorial will be repeatedly calling factorial which will be really a waste of time.
I have also tried maintaining a table for factorials
So, my question, is there some more efficient way to check whether a number is factorial or not?

Answer 1

It is wasteful calculating factorials continuously like that since you're duplicating the work done in x! when you do (x+1)!, (x+2)! and so on.
One approach is to maintain a list of factorials within a given range (such as all 64-bit unsigned factorials) and just compare it with that. Given how fast factorials increase in value, that list won't be very big. In fact, here's a C meta-program that actually generates the function for you:
#include <stdio.h>
int main (void) {
unsigned long long last = 1ULL, current = 2ULL, mult = 2ULL;
size_t szOut;
puts ("int isFactorial (unsigned long long num) {");
puts (" static const unsigned long long arr[] = {");
szOut = printf (" %lluULL,", last);
while (current / mult == last) {
if (szOut > 50)
szOut = printf ("\n ") - 1;
szOut += printf (" %lluULL,", current);
last = current;
current *= ++mult;
}
puts ("\n };");
puts (" static const size_t len = sizeof (arr) / sizeof (*arr);");
puts (" for (size_t idx = 0; idx < len; idx++)");
puts (" if (arr[idx] == num)");
puts (" return 1;");
puts (" return 0;");
puts ("}");
return 0;
}
When you run that, you get the function:
int isFactorial (unsigned long long num) {
static const unsigned long long arr[] = {
1ULL, 2ULL, 6ULL, 24ULL, 120ULL, 720ULL, 5040ULL,
40320ULL, 362880ULL, 3628800ULL, 39916800ULL,
479001600ULL, 6227020800ULL, 87178291200ULL,
1307674368000ULL, 20922789888000ULL, 355687428096000ULL,
6402373705728000ULL, 121645100408832000ULL,
2432902008176640000ULL,
};
static const size_t len = sizeof (arr) / sizeof (*arr);
for (size_t idx = 0; idx < len; idx++)
if (arr[idx] == num)
return 1;
return 0;
}
which is quite short and efficient, even for the 64-bit factorials.
If you're after a purely programmatic method (with no lookup tables), you can use the property that a factorial number is:
1 x 2 x 3 x 4 x ... x (n-1) x n
for some value of n.
Hence you can simply start dividing your test number by 2, then 3 then 4 and so on. One of two things will happen.
First, you may get a non-integral result in which case it wasn't a factorial.
Second, you may end up with 1 from the division, in which case it was a factorial.
Assuming your divisions are integral, the following code would be a good starting point:
int isFactorial (unsigned long long num) {
unsigned long long currDiv = 2ULL;
while (num != 1ULL) {
if ((num % currDiv) != 0)
return 0;
num /= currDiv;
currDiv++;
}
return 1;
}
However, for efficiency, the best option is probably the first one. Move the cost of calculation to the build phase rather than at runtime. This is a standard trick in cases where the cost of calculation is significant compared to a table lookup.
You could even make it even mode efficient by using a binary search of the lookup table but that's possibly not necessary given there are only twenty elements in it.

Answer 2

If the number is a factorial, then its factors are 1..n for some n.
Assuming n is an integer variable, we can do the following :
int findFactNum(int test){
for(int i=1, int sum=1; sum <= test; i++){
sum *= i; //Increment factorial number
if(sum == test)
return i; //Factorial of i
}
return 0; // factorial not found
}
now pass the number 24 to this function block and it should work. This function returns the number whose factorial you just passed.

Answer 3

You can speed up at least half of the cases by making a simple check if the number is odd or even (use %2). No odd number (barring 1) can be the factorial of any other number

Answer 4

#include<stdio.h>
main()
{
float i,a;
scanf("%f",&a);
for(i=2;a>1;i++)
a/=i;
if(a==1)
printf("it is a factorial");
else
printf("not a factorial");
}

Answer 5

You can create an array which contains factorial list:
like in the code below I created an array containing factorials up to 20.
now you just have to input the number and check whether it is there in the array or not..
#include <stdio.h>
int main()
{
int b[19];
int i, j = 0;
int k, l;
/*writing factorials*/
for (i = 0; i <= 19; i++) {
k = i + 1;
b[i] = factorial(k);
}
printf("enter a number\n");
scanf("%d", &l);
for (j = 0; j <= 19; j++) {
if (l == b[j]) {
printf("given number is a factorial of %d\n", j + 1);
}
if (j == 19 && l != b[j]) {
printf("given number is not a factorial number\n");
}
}
}
int factorial(int a)
{
int i;
int facto = 1;
for (i = 1; i <= a; i++) {
facto = facto * i;
}
return facto;
}

Answer 6

public long generateFactorial(int num){
if(num==0 || num==1){
return 1;
} else{
return num*generateFactorial(num-1);
}
}
public int getOriginalNum(long num){
List<Integer> factors=new LinkedList<>(); //This is list of all factors of num
List<Integer> factors2=new LinkedList<>(); //List of all Factorial factors for eg: (1,2,3,4,5) for 120 (=5!)
int origin=1; //number representing the root of Factorial value ( for eg origin=5 if num=120)
for(int i=1;i<=num;i++){
if(num%i==0){
factors.add(i); //it will add all factors of num including 1 and num
}
}
/*
* amoong "factors" we need to find "Factorial factors for eg: (1,2,3,4,5) for 120"
* for that create new list factors2
* */
for (int i=1;i<factors.size();i++) {
if((factors.get(i))-(factors.get(i-1))==1){
/*
* 120 = 5! =5*4*3*2*1*1 (1!=1 and 0!=1 ..hence 2 times 1)
* 720 = 6! =6*5*4*3*2*1*1
* 5040 = 7! = 7*6*5*4*3*2*1*1
* 3628800 = 10! =10*9*8*7*6*5*4*3*2*1*1
* ... and so on
*
* in all cases any 2 succeding factors inf list having diff=1
* for eg: for 5 : (5-4=1)(4-3=1)(3-2=1)(2-1=1)(1-0=1) Hence difference=1 in each case
* */
factors2.add(i); //in such case add factors from 1st list " factors " to " factors2"
} else break;
//else if(this diff>1) it is not factorial number hence break
//Now last element in the list is largest num and ROOT of Factorial
}
for(Integer integer:factors2){
System.out.print(" "+integer);
}
System.out.println();
if(generateFactorial(factors2.get(factors2.size()-1))==num){ //last element is at "factors2.size()-1"
origin=factors2.get(factors2.size()-1);
}
return origin;
/*
* Above logic works only for 5! but not other numbers ??
* */
}