C Program to find combinations

[Toysh]

I am a beginner trying to write a program for my intro C class. I am trying to write a program that calculates the combination of numbers given a specific output by the user. The number has to be between 1-10 to be valid. I am not allowed to use the built in library that calculates a combination. I am getting stuck at the part where i need to calculate (n-k)! I see that my problem is after my loops the numbers the user inputs for n and k are now zero. I am not sure if I should have written this a different way or how I should go about getting the original entered digits back so that I can take and use them in calculating (n-k)! Any help would be very much appreciated.

int main()        /* Main declaration with arguments           */
{

int n,
    nFact = 1,
    k,
    kFact = 1,
    n_k = ( n - k ),
    nkFact = 1,
    combination;
 
 
   
do {
 printf("\nEnter the number of items in the list (n): ");
 scanf("%d", &n);
       if ( n < 1 || n > 10 ) /* is n between 1 - 10 */
          printf("\n?Invalid input: Number must be between 1 and 10\n");
       else  
          break;
} while ( n > 1 || n < 10 );
 
 
 
  /* As long as n>=1 program will loop the following n*n-1 */
  while (n>=1)
  {
       nFact = ( nFact * n );
       n = ( n - 1 );
  }    
  printf("n! = %d", nFact);
 

 
do {
 printf("\nEnter the number of items to choose (k): ");
 scanf("%d", &k);    
       if ( k < 1 || k > 10 )  /* is k between 1 - 10 */
          printf("\n?Invalid input: Number must be between 1 and 10\n");
       else  
          break;
} while ( k > 1 || n < 10 );





  while (k>=1)
  {
       kFact = ( kFact * k );
       k = ( k - 1 );
  }    
  printf("k! = %d", kFact);




  while (n_k>=1)
  {
        nkFact = ( nkFact * n_k );
        n_k = ( n_k - 1 );
  }    
printf("\n(n-k)! = %d", nkFact);


printf("\n");
system("PAUSE");
return(0);                                  
}

[Adak]

I am a beginner trying to write a program for my intro C class. I am trying to write a program that calculates the combination of numbers given a specific output by the user. The number has to be between 1-10 to be valid. I am not allowed to use the built in library that calculates a combination. I am getting stuck at the part where i need to calculate (n-k)! I see that my problem is after my loops the numbers the user inputs for n and k are now zero. I am not sure if I should have written this a different way or how I should go about getting the original entered digits back so that I can take and use them in calculating (n-k)! Any help would be very much appreciated.

int main()        /* Main declaration with arguments           */
{

int n,
    nFact = 1,
    k,
    kFact = 1,
    n_k = ( n - k ),
    nkFact = 1,
    combination;
   
do {
 printf("\nEnter the number of items in the list (n): ");
 scanf("%d", &n);
       if ( n < 1 || n > 10 ) /* is n between 1 - 10 */
          printf("\n?Invalid input: Number must be between 1 and 10\n");
       else  
          break;
} while ( n > 1 || n < 10 );
//the while test is goofed. If n < 1 || n > 10, THEN you want to loop back.
 
  /* As long as n>=1 program will loop the following n*n-1 */
  while (n>=1)
  {
       nFact = ( nFact * n );
       n = ( n - 1 );
  }    
  printf("n! = %d", nFact);
 
do {
 printf("\nEnter the number of items to choose (k): ");
 scanf("%d", &k);    
       if ( k < 1 || k > 10 )  /* is k between 1 - 10 */
          printf("\n?Invalid input: Number must be between 1 and 10\n");
       else  
          break;
} while ( k > 1 || n < 10 );

  while (k>=1)
  {
       kFact = ( kFact * k );
       k = ( k - 1 );
  }    
  printf("k! = %d", kFact);

  while (n_k>=1)
  {
        nkFact = ( nkFact * n_k );
        n_k = ( n_k - 1 );
  }    
printf("\n(n-k)! = %d", nkFact);

printf("\n");
system("PAUSE");
return(0);                                  
}

It looks like you're solving for the number of permutations, rather than combinations (which are far fewer). Can you confirm which it is?

Giving your variables better names than: nfact, kfact, nkfact, k, etc., would be very helpful.

Always highlight your program, and click on the # symbol, so your code, is punctuated to look like code, on this (and any other code), forum.

[Toysh]

Sorry about that. I need to solve the equation:

combinations = n! / ((n-k)!*k!)

I am trying to break it down into its parts so I am first trying to solve for:

n!
k!
(n-k)!

the last is where I am getting stuck. I am not sure how to have n and k be equal to their original values instead of 0. Sorry for the confusion I am still very new, let me know if I am approaching this problem incorrectly.

[Toysh]

#include <stdio.h>           /* Standard library for C input/output                */
#include <stdlib.h>        /* Standard library definitions                       */
#define TERMINAL -9        /* This is a macro that is defined globally           */
                           /* it is used to tell the program that the user is    */
                           /* done with their input.                             */

int main()        /* Main declaration with arguments           */
{

int userInput1,
    userInput2,
    nFact = 1,
    kFact = 1,
    n_k,
    nkFact = 1,
    combination;
 
 
   
do {
 printf("\nEnter the number of items in the list (n): ");
 scanf("%d", &userInput1);
       if ( userInput1 < 1 || userInput1 > 10 ) /* is n between 1 - 10 */
          printf("\n?Invalid input: Number must be between 1 and 10\n");
       else  
          break;
} while ( userInput1 < 1 || userInput1 > 10 );
 
 
 
  /* As long as n>=1 program will loop the following n*n-1 */
 
 int n; 
  for ( n = userInput1; n > 1; n--) {
      nFact = ( nFact * n );
  }    
 
  printf("n! = %d", nFact);
 
 
do {
 printf("\nEnter the number of items to choose (k): ");
 scanf("%d", &userInput2);    
       if ( userInput2 < 1 || userInput2 > 10 )  /* is k between 1 - 10 */
          printf("\n?Invalid input: Number must be between 1 and 10\n");
       else  
          break;
} while ( userInput2 < 1 || userInput2 > 10 );


int k;
  for ( k = userInput2; k > 1; k--) {
      kFact = ( kFact * k );
  }    

 
  printf("k! = %d", kFact);
printf("n = %d k = %d", userInput1, userInput2);

n_k = userInput1 - userInput2;

  while (n_k>=1)
  {
        nkFact = ( nkFact * n_k );
        n_k = ( n_k - 1 );
  }     

printf("\n(n-k)! = %d", nkFact);


combination = nFact / ( (nkFact) * kFact );


printf("\nNumber of combinations: %d", combination);




printf("\n");
system("PAUSE");
return(0);                                  
}

I got it to work, I went with a for statement instead. Its looks sloppy but I am going to clean it up a bit.

[ArkM]

Better use double type to calculate factorials. Don't take the risk of integer overflow w/o any signals and with very strange results...

[hideout1]

Your original code was fine. There was only one mistake and you corrected it in your second code.

If you look carefully at both, in the first you initialized n_k = (n-k)
but in the second you did the same thing but after you took the user input of n and k. This made your code work as you didn't have values for n and k when you ran your code until you manually entered them.

So if you had take n = 5 and k = 2, then it would have worked fine.

Also you should be careful that k cannot be greater than n as negative factorials don't exist.

You can do this by doing k<1 || k>n

[peter_budo]

You are 2 years late. Thread closed