Arrays

Array Reduce

Count With a Mask

Whole-array comparisons return a logical mask. count returns how many positions are .true..

Program

Play the program to count scores at or above 80.

array_reduce.f90
Replay: real traced execution (multi-file project)
program array_reduce
    implicit none
    integer :: scores(5)
    integer :: above
    scores = [65, 82, 91, 74, 88]
    above = count(scores >= 80)
    print '(I0)', above
end program array_reduce

  1. scores ← [65, 82, 91, 74, 88]

    4integer :: above5scores = [65, 82, 91, 74, 88]6above = count(scores >= 80)
    values this step[65, 82, 91, 74, 88]scores

  2. above ← 3

    5scores = [65, 82, 91, 74, 88]6above = count(scores >= 80)7print '(I0)', above
    values this step3above[F, T, T, F, T]scores >= 80

  3. print '(I0)', above

    6    above = count(scores >= 80)7    print '(I0)', above8end program array_reduce
    output3
    values this step3above

Count Passing Scores

  1. scores starts as [65, 82, 91, 74, 88].
  2. scores >= 80 checks every score.
  3. Scores at least 80 become .true. in the mask.
  4. count(...) returns the number of .true. entries: 3. | Score | >= 80? | | --- | --- | | 65 | .false. | | 82 | .true. | | 91 | .true. | | 74 | .false. | | 88 | .true. |
logical mask `scores >= 80` produces a logical array.
count `count(mask)` returns the number of `.true.` positions.
whole array op Comparisons apply to every element at once.

Exercise: array_reduce.f90

Compare an array of scores to a threshold, count the true mask entries, and print the count