Why is my CheckFactorial script not working? [closed] - c

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#include <stdio.h>
int checkiffactorial();
int factorial(int n);
int number;
int main()
{
int answer, n, i;
printf("Enter a number: ");
scanf("%d", &number);
answer = checkiffactorial();
if (answer == 1)
{
printf("It's a factorial");
}
else
{
printf("It's not a factorial");
}
}
int checkiffactorial()
{
static int whichnumber = 1;
int currnumber;
if (whichnumber > number)
{
return 0;
}
if(whichnumber <= number)
{
currnumber = factorial(whichnumber);
if (currnumber == factorial(number))
{
return 1;
}
whichnumber++;
checkiffactorial();
}
}
int factorial(int n)
{
int i;
int fac;
for(i=1; i<=n; ++i)
{
fac = fac * i;
}
}
Why is this code not working?
My C code is meant for you to input a number and check if that number can be a factorial.
Like for example: if you enter 6 it should be Yes because 3! = 6 but if you enter 8 it would not work.
I d'ont think it's a duplicate because the method i did it was different.
Please note i'm not really good at C so any extra tips could be appreciated.

Answer 1

You need to correct 3 mistakes to make this program work.
You're comparing factorial of whichnumber with factorial of number that's wrong.
currnumber = factorial(whichnumber);
if (currnumber == factorial(number)) //<----never be true
{
return 1;
}
You should compare factorial of whichnumber with number
currnumber = factorial(whichnumber);
if (currnumber == number) //<----should check whether it's same with the number
{
return 1;
}
2 . You should initialize fac variable in factorial function otherwise it will take some garbage value.
int fac=1; //<-----initialize this variable
3. You should return the value of fact after calculating the factorial.
return fac; //<-----should return value
Here is the modified code:
#include <stdio.h>
int checkiffactorial();
int factorial(int n);
int number;
int main()
{
int answer, n, i;
printf("Enter a number: ");
scanf("%d", &number);
answer = checkiffactorial();
if (answer == 1)
{
printf("It's a factorial");
}
else
{
printf("It's not a factorial");
}
}
int checkiffactorial()
{
static int whichnumber = 1;
int currnumber;
if (whichnumber > number)
{
return 0;
}
if(whichnumber <= number)
{
currnumber = factorial(whichnumber);
if (currnumber == number) //<----should check whether it's same with the number
{
return 1;
}
whichnumber++;
checkiffactorial();
}
}
int factorial(int n)
{
int i;
int fac=1; //<-----initialize this variable
for(i=1; i<=n; ++i)
{
fac = fac * i;
}
return fac; //<-----should return value
}

Answer 2

It has been pointed out to you that you don't return value from your functions and use uninitialsed values. These errors are easy to make, but they are also easy to catch: Enable warnings for your compiler and they will tell you about these things.
Suvojit's answer tells you what is wrong with your factorial function. Unfortunately, more things are wrong with your factorial check:
You make the number you check a global variable. This should really be an argument to the function, so that you can call it like you should: is_factorial(n).
You make your counter a static variable. This is like a global variable, but with the restriction that it is only known in this function, which means that you cannot change it from outside. If your program want to check several numbers, the second call starts where you left off earlier, which leads to wrong results.
Of course that's what you want in your implementation, because you use a recursive algorithm. In this case, that's not a good choice; use a loop.
Your condition when to stop the iteration (or when to break the loop) checks the number against the number you took the factorial of. You should test this against the factorial itself.
Note that a typical int has 32 bits and can represent positive values up to 2³¹. The factorial 13! already exceeds this limit. So you must check your number against 12 values.
You don't need the factorial function for this, you can build these values as you go, because n! = (n − 1)! · n. (You can use the factorial function, but will do the same calculations over and over again, which is wasteful. It doesn't matter for this toy problem, but it is worth bearing such things in mind.)
So here's your function, completely rewritten:
int is_factorial(int n)
{
int fact = 1;
int k = 1;
while (k < 13 && fact <= n) {
fact *= k;
if (n == fact) return k;
k++;
}
return 0;
}
It returns 0 when the n isn't a factorial, otherwise it returns the number that n is a factorial of. (This information is used anyway, so why not provide it? The caller can choose whether to use this infor or just use it as truth value.)
While we're at it, let's adjust the main function so that the program checks for bad input and prints out the extra information we return:
int main(void)
{
int n;
printf("Enter a number: ");
if (scanf("%d", &n) < 1) {
printf("Illegal input!\n");
} else {
int m = is_factorial(n);
if (m) {
printf("%d is the factorial of %d!\n", n, m);
} else {
printf("%d is not a factorial!\n", n);
}
}
return 0;
}
The things to take away here are that you should use the compiler warning to tell you about simple errors, that you should avoid global and static variables for closed problems like these and that loops are often simpler than recursion.