Question

I’m at a beginner level in FORTRAN. Recently, I tried designing a simple program to calculate permutations and combinations. It will have a “menu” for the user to choose an option to proceed…

      !/*-------MAIN PROGRAM STARTS-------*/
      !PURPOSE: TO CALCULATE COMBINATIONS AND PERMUTATIONS
      PROGRAM COMBINATION_PERMUTATION
      IMPLICIT NONE

      !DECLARATION OF VARIABLES
      REAL N, K, FACT, COMBINATION, PERMUTATION
      INTEGER CHOICE

      !CHOICE MENU
      CALL MENU

      !USER INPUT OF CHOICE
      READ(*,*) CHOICE

      !EXECUTION OF CODE BASED ON USER'S CHOICE
      SELECT CASE(CHOICE)
        CASE(1)
            !CALLS SUBROUTINE FOR COMBINATION 
            CALL COMBINATION_CALC (N, K, FACT, COMBINATION)            
        CASE(2)
            !CALLS SUBROUTINE FOR PERMUTATION
            CALL PERMUTATION_CALC (N, K, FACT, PERMUTATION)
        CASE(3)
            !EXITS THE PROGRAM
            STOP 
        CASE DEFAULT
        !INVALID CHOICE LEADS TO MENU BACK
        WRITE(*,*) 'INVALID CHOICE! PLEASE CHOOSE AN OPTION TO CONTINUE'
        CALL MENU
      END SELECT

      END PROGRAM COMBINATION_PERMUTATION
      !/*-------MAIN PROGRAM ENDS-------*/


      !/*-------SUB PROGRAMS START-------*/
      !FUNCTION FOR FACTORIAL
      FUNCTION FACT(N)
      IMPLICIT NONE
      REAL FACT, N
      INTEGER P, I
      P=1
      DO I=1, N
        P=P*I
      END DO
      FACT=P
      RETURN
      END FUNCTION 

      !SUBROUTINE FOR MENU
      SUBROUTINE MENU
      IMPLICIT NONE
      WRITE(*,*) ' CHOOSE AN OPTION TO CONTINUE.. '
      WRITE(*,*) ' 1. COMBINATION '
      WRITE(*,*) ' 2. PERMUTATION '
      WRITE(*,*) ' 3. EXIT PROGRAM '
      RETURN
      END SUBROUTINE MENU

      !SUBROUTINE FOR COMBINATION
      SUBROUTINE COMBINATION_CALC (N, K, FACT, COMBINATION)
      IMPLICIT NONE

      REAL N, K, FACT, COMBINATION

      WRITE(*,*) ' WHAT IS THE VALUE OF N? '
      READ(*,*) N

2     WRITE(*,*) ' WHAT IS THE VALUE OF K? '
      READ(*,*) K

      IF (K<0) THEN 
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE MORE THAN 0! '
            GO TO 2
        ELSE IF (K>N)THEN
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE LESS THAN N! '
            GO TO 2
        ELSE
      END IF 

      COMBINATION=FACT(N)/(FACT(N-K)*FACT(N))
      WRITE(*,*) ' HENCE, THERE ARE ', COMBINATION, ' WAYS TO ARRANGE. '
      CALL MENU
      RETURN
      END SUBROUTINE COMBINATION_CALC

      !SUBROUTINE FOR PERMUTATION
      SUBROUTINE PERMUTATION_CALC (N, K, FACT, PERMUTATION)
      IMPLICIT NONE

      REAL N, K, FACT, PERMUTATION

      WRITE(*,*) ' WHAT IS THE VALUE OF N? '
      READ(*,*) N

3     WRITE(*,*) ' WHAT IS THE VALUE OF K? '
      READ(*,*) K

      IF (K<0) THEN 
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE MORE THAN 0! '
            GO TO 3
        ELSE IF (K>N) THEN
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE LESS THAN N! '
            GO TO 3
        ELSE
      END IF

      PERMUTATION=FACT(N)/FACT(N-K)
      WRITE(*,*) ' HENCE THERE ARE', PERMUTATION,' WAYS TO ARRANGE. '
      CALL MENU      
      RETURN
      END SUBROUTINE PERMUTATION_CALC
      !/*-------SUB PROGRAMS END-------*/

And I’m getting run-time error whenever I enter input 1 or 2 to select a case to proceed….

The error shows as follows in a pop-up window:-

---

RUN TIME ERROR

Attempt to call a routine with argument number three as real(kind=1) when a procedure was required.

COMBINATION_CALC- in file combinationpermutation.for at line 62[+0068]
main- in file combinationpermutation.for at line 23 [+00e2]

---

As far as I know, the argument number three in COMBINATION_CALC is “FACT” which is function to calculate factorial. I am not able to understand the error. Kindly guide me. Thanks in advance..

---

UPDATE

I’ve fixed the RUN-TIME ERROR using the fix given by yosukesabai… my program is now complete..but, INTEGER OVERFLOW is a new problem for me. I’ve changed REAL declaration to INTEGER(KIND=4).. When the N value and K value are more than 12… the resulting answer for combination and permutation will be large… May I know how to fix this?

Here is the new code for my program…

          !/*-------MAIN PROGRAM STARTS-------*/
      !PURPOSE: TO CALCULATE COMBINATIONS AND PERMUTATIONS
      !CREATED BY: RETHNARAJ RAMBABU
      !DATE: 28/10/2011
      PROGRAM COMBINATION_PERMUTATION
      IMPLICIT NONE

      !DECLARATION OF VARIABLES
      INTEGER CHOICE, N, K
      INTEGER(KIND=4) COMBINATION, PERMUTATION
      INTEGER, external :: FACT

      !CHOICE MENU
1     WRITE(*,*) ' CHOOSE AN OPTION TO CONTINUE.. '
      WRITE(*,*) ' 1. COMBINATION '
      WRITE(*,*) ' 2. PERMUTATION '
      WRITE(*,*) ' 3. EXIT PROGRAM '

      !USER INPUT OF CHOICE
      READ(*,*) CHOICE

      !EXECUTION OF CODE BASED ON USER'S CHOICE
      SELECT CASE(CHOICE)
        CASE(1)
            !CALLS SUBROUTINE FOR COMBINATION 
            CALL COMBINATION_CALC (N, K, FACT, COMBINATION)  
            GO TO 1          
        CASE(2)
            !CALLS SUBROUTINE FOR PERMUTATION
            CALL PERMUTATION_CALC (N, K, FACT, PERMUTATION)
            GO TO 1
        CASE(3)
            !EXITS THE PROGRAM
            STOP 
        CASE DEFAULT
        !INVALID CHOICE LEADS TO MENU BACK
        WRITE(*,*) 'INVALID CHOICE! PLEASE CHOOSE AN OPTION TO CONTINUE'
        GO TO 1
      END SELECT

      END PROGRAM COMBINATION_PERMUTATION
      !/*-------MAIN PROGRAM ENDS-------*/


      !/*-------SUB PROGRAMS START-------*/
      !FUNCTION FOR FACTORIAL
      FUNCTION FACT(N)
      IMPLICIT NONE
      INTEGER N, P, I, FACT
      P=1
      DO I=1, N
        P=P*I
      END DO
      FACT=P
      RETURN
      END FUNCTION 

      !SUBROUTINE FOR COMBINATION
      SUBROUTINE COMBINATION_CALC (N, K, FACT, COMBINATION)
      IMPLICIT NONE

      INTEGER N, K, FACT
      INTEGER(KIND=4) COMBINATION

      WRITE(*,*) ' WHAT IS THE VALUE OF N? '
      READ(*,*) N

2     WRITE(*,*) ' WHAT IS THE VALUE OF K? '
      READ(*,*) K

      IF (K<0) THEN 
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE MORE THAN 0! '
            GO TO 2
        ELSE IF (K>N)THEN
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE LESS THAN N! '
            GO TO 2
        ELSE
      END IF 

      COMBINATION=FACT(N)/(FACT(N-K)*FACT(K))
      WRITE(*,*) ' HENCE, THERE ARE ', COMBINATION, ' WAYS TO ARRANGE. '
      RETURN
      END SUBROUTINE COMBINATION_CALC

      !SUBROUTINE FOR PERMUTATION
      SUBROUTINE PERMUTATION_CALC (N, K, FACT, PERMUTATION)
      IMPLICIT NONE

      INTEGER N, K, FACT
      INTEGER(KIND=4)PERMUTATION

      WRITE(*,*) ' WHAT IS THE VALUE OF N? '
      READ(*,*) N

3     WRITE(*,*) ' WHAT IS THE VALUE OF K? '
      READ(*,*) K

      IF (K<0) THEN 
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE MORE THAN 0! '
            GO TO 3
        ELSE IF (K>N) THEN
            WRITE(*,*) ' ERROR! VALUE OF K SHOULD BE LESS THAN N! '
            GO TO 3
        ELSE
      END IF

      PERMUTATION=FACT(N)/FACT(N-K)
      WRITE(*,*) ' HENCE THERE ARE', PERMUTATION,' WAYS TO ARRANGE. '
      RETURN
      END SUBROUTINE PERMUTATION_CALC
      !/*-------SUB PROGRAMS END-------*/

Answer 1

Near the top of your code,

  PROGRAM COMBINATION_PERMUTATION
  IMPLICIT NONE

  !DECLARATION OF VARIABLES
  REAL N, K, FACT, COMBINATION, PERMUTATION
  INTEGER CHOICE

Here you said FACT is real, so fortran assumes that it is a real variable. But you meant this to be reference to function you defined in the same file. So revise like

  !DECLARATION OF VARIABLES
  REAL N, K, COMBINATION, PERMUTATION
  INTEGER CHOICE
  REAL, external :: FACT

external is fortran way of saying that it is a function you defnined somewhere outside of a program block (program, subroutine or function)

---

(ignore below here if you dont care)

I notice that there is no point of passing function reference. so you may want to stop passing function around, and just call that straight from subroutine combination_calc. in that case, n,k,combination are the three arguments for subroutine, and inside combination_calc you will have real, external :: fact.