A Simple Histogram Program

Problem Statement

Suppose there are n score values x₁, x₂, ..., x_n. We want to know the number of scores in the range of less than or equal to 59, [60,69], [70,79], [80,89] and larger than or equal to 90. Then, plot a histogram. For example, if there are 2, 3, 3, 5 and 2 scores in the above specified range, your program should print a histogram like the following:

 ** (2)
 *** (3)
 *** (3)
 ***** (5)
 ** (2)

Solution

! --------------------------------------------------------------------
! PROGRAM  Histogram:
!    Given a set of scores, this program plots a histogram showing the
! number of score in the range of [0,59], [60,69], [70,79], [80,89]
! and [90,100].  In subroutine Distribute() a local array is used 
! to store these counts which is, in turn, sent to subroutine
! Plot() for plotting.
! --------------------------------------------------------------------

PROGRAM  Histogram
   IMPLICIT NONE
   INTEGER, PARAMETER         :: SIZE = 20   ! array size
   INTEGER, DIMENSION(1:SIZE) :: Score       ! array containing scores
   INTEGER                    :: ActualSize  ! the # of scores read in
   INTEGER                    :: i

   READ(*,*)  ActualSize, (Score(i), i = 1, ActualSize)
   WRITE(*,*) "Input Scores:"
   WRITE(*,*) (Score(i), i = 1, ActualSize)
   WRITE(*,*)

   CALL  Distribute(Score, ActualSize)

CONTAINS

! --------------------------------------------------------------------
! SUBROUTINE  Distribute() :
!    This subroutine receives a set of scores and count the number of
! scores in the range of [0,59], [60,69], [70,79], [80,89] and
! [90,100].  A local array is used to accumulate these counts.
! --------------------------------------------------------------------

   SUBROUTINE  Distribute(X, N)
      IMPLICIT NONE
      INTEGER, DIMENSION(1:), INTENT(IN) :: X
      INTEGER, INTENT(IN)                :: N
      INTEGER, PARAMETER                 :: Bucket_Size = 5
      INTEGER, DIMENSION(1:Bucket_Size)  :: Bucket  
      INTEGER                            :: i

      DO i = 1, Bucket_Size             ! clear the local array
         Bucket(i) = 0
      END DO

      DO i = 1, N                       ! for each score ...
         SELECT CASE (X(i))             !    determine the range it lies
            CASE (:59)
               Bucket(1) = Bucket(1) + 1
            CASE (60:69)
               Bucket(2) = Bucket(2) + 1
            CASE (70:79)
               Bucket(3) = Bucket(3) + 1
            CASE (80:89)
               Bucket(4) = Bucket(4) + 1
            CASE (90:)
               Bucket(5) = Bucket(5) + 1
         END SELECT
      END DO
      CALL  Plot(Bucket, Bucket_Size)   ! send the count to Plot()
   END SUBROUTINE  Distribute

! --------------------------------------------------------------------
! SUBROUTINE  Plot() :
!    This subroutine receives a set of count from an array and for 
! each count, displays that number of '*'s on the same line.  Thus,
! the result is a histogram.
! --------------------------------------------------------------------

   SUBROUTINE  Plot(Input, Size)
      IMPLICIT NONE
      INTEGER, DIMENSION(1:), INTENT(IN) :: Input
      INTEGER, INTENT(IN)                :: Size
      INTEGER                            :: i, j

      WRITE(*,*) "Histogram:"
      WRITE(*,*)
      DO i = 1, Size                    
         WRITE(*,*)  ('*', j=1,Input(i)), ' (', Input(i), ')'
      END DO
   END SUBROUTINE  Plot

END PROGRAM  Histogram

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Program Input and Output

If the input data consist of the following:

15
78  95  68  85  55
88  82  75  63  90
85  76  82  40  68

The output of the program is:

 Input Scores:
 78,  95,  68,  85,  55,  88,  82,  75,  63,  90,  85,  76,  82,  40,  68

 Histogram:

 ** (2)
 *** (3)
 *** (3)
 ***** (5)
 ** (2)

Discussion

The job of subroutine Distribute() is to count the number of scores in each of the five ranges (i.e., [0,59], [60,69], [70,79], [80,89] and [90,100]). Thus, it requires an array of five elements. This is declared to be a local array:
```
INTEGER, PARAMETER                 :: Bucket_Size = 5
INTEGER, DIMENSION(1:Bucket_Size)  :: Bucket  
```
If a score is in the range of [0,59], [60,69], [70,79], [80,89] and [90,100], one is added to Bucket(1), Bucket(2), Bucket(3), Bucket(4), Bucket(5). This is implemented with the SELECT CASE statement.
After the counting is completed, array Bucket() is sent to subroutine Plot() for generating a histogram. For each count, subroutine Plot() prints that number of asterisks on the same row followed the counting value.