Factorial

[DonW]

I'm just starting in C, and I'm trying to use the factorial function and print the result from entering a nonnegative integer. Anybody got some ideas. By the way the error message says: Lvalue required. Thanks DW

#include<stdio.h>

long factorial(long number); /* function prototype */

void main(void)

{

int x; /* Integer number to be input by user*/

printf("Enter integer\n"); /* Prompt */

scanf("%d",&x); /* Prompt */

factorial=1;

printf("%d\n", factorial);

}

[prog-bman]

Hi don first off please use code tags. Second your calling a function that you havn't declared just prototyped. let me show you how it would be done.

#include<stdio.h>

long factorial(long number); /* function prototype */

int main(void)/*void main is wrong don't use it unless you know you need to use it!*/
{
  int x; /* Integer number to be input by user*/
  printf("Enter integer\n"); /* Prompt */
  scanf("%d",&x); /* Prompt */
  int getFactorial=factorial(x);/*Setting a variable to the return value of factorial*/
  printf("%d\n", getFactorial);
}

long factorial(long number)
{
  /*factorial code I'll leave for you to do :)*/
  return number;
}

[okinrus]

Declarations usually mean prototypes. The error is the result of the line "factorial = 1." The identifier "factorial" may only be used as an r-value, which is an expression on the right side of an assignment, in constrast with an l-value, which is the expression on the left side of an assignment.

[DonW]

I have changed the code a small amount, it compiles, and runs, it requests an integer, but prints out only a 0......Hmmmmm


#include<stdio.h>

int factorial;

void main(void)

{

int x; /* Integer number to be input by user*/

printf("Enter integer\n"); /* Prompt */

scanf("%d",&x); /* Prompt */


printf("\nThe factorial is %d", factorial);

}

[itsme86]

You're not ever setting your factorial variable to anything...where's your algorithm for finding the factorial? Calling a variable "factorial" isn't a hint to the C compiler that it should generate factorials for you.

[Nessarose]

Perhaps this would make more sense if you had a similar example:

#include <stdio.h>

/* the sum function calculates the sum of two integers x and y */
int sum(int x, int y);

int main(void) {
     int x = 10;
     int y = 5;
     int answer = sum(x, y);
     printf("Answer: %d\n", answer);

     return 0;
}

int sum(int x, int y) {
    return x + y;
}

Instead of factorial, I'm calculating the sum of two integers. As you can see, it's not enough to just call and declare the function sum, you must have the code written out that will calculate the sum of the two integers and return it's value back to the function that called it (main()). So in your case, you need to write the code for the factorial function.

[Shiro]

With this line of code

int factorial;

you declare a variable of type int. This variable can contain some value. In your program, this line of code

printf("\nThe factorial is %d", factorial);

does nothing but printing the actual value of variable factorial. To really calculate the factorial of some number, you must implement the function which prototype was

long factorial (long number);

and call that function in your main() function. Here factorial is the name of the function and number is the argument passed to the function. The long before number tells that this variable is of type long and the long before factorial tells that the function returns a value of type long.

Note that C does not have a factorial function itself, you will need to implement it by yourself.

Regards,
Shiro

[DonW]

My new code. it works maybe not the best, but it works. Het all of you whom commented, much kudos and love, great ideas and made me get down and dirty....


#include <stdio.h>

double fact(int n);

int main(void)

{
int current;

printf("Enter a positive integer [to terminate enter non-positive] > ");
scanf("%d", &current);
while (current >= 0)

{
printf("The factorial of %d is %.f\n", current, fact(current));
printf("Enter a positive integer [to terminate enter non-positive] > ");
scanf("%d", &current);
}

}

/* n is a positive integer. The function returns its factorial */

double fact(int n)

{
int lcv; /* loop control variable */
double p; /* set to the product of the first lcv positive integers */

for(p=1, lcv=2; lcv <= n; p=p*lcv, lcv++);
return p;

}

[Nessarose]

Good job. You could also have written it as a recursive function:

int fact(int n)
{
    if (n <= 1)
        return 1;
    return n * fact(n - 1);
}

[Prelude]

>You could also have written it as a recursive function
Don't forget to add the requisite warning that this is a bad use of recursion because it doesn't simplify the problem enough to outweigh the disadvantages of recursion.

[Sang-drax]

If you plan to use factorials for other than educational purposes, use lookup tables.

The factorial functions grows very fast, so the lookup table will be very small.