Selection Sort Prefix - Modern Fortran Programming

A selection-style nested loop can order a small active prefix by swapping lower values forward.
Program

Play the program to sort a different number of active values.
                            item_count
                            item_count = 3 (default)item_count = 2item_count = 4
                        

                            selection_sort_prefix.f90
                        
Replay: real traced execution (multi-file project)

                            program selection_sort_prefix_demo
    implicit none
    integer :: values(4)
    integer :: item_count
    integer :: i
    integer :: j
    integer :: temp

    values = [9, 4, 7, 2]
    item_count = 3
    do i = 1, item_count - 1
        do j = i + 1, item_count
            if (values(j) < values(i)) then
                temp = values(i)
                values(i) = values(j)
                values(j) = temp
            end if
        end do
    end do
    print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)
end program selection_sort_prefix_demo
program selection_sort_prefix_demo
    implicit none
    integer :: values(4)
    integer :: item_count
    integer :: i
    integer :: j
    integer :: temp

    values = [9, 4, 7, 2]
    item_count = 2
    do i = 1, item_count - 1
        do j = i + 1, item_count
            if (values(j) < values(i)) then
                temp = values(i)
                values(i) = values(j)
                values(j) = temp
            end if
        end do
    end do
    print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)
end program selection_sort_prefix_demo
program selection_sort_prefix_demo
    implicit none
    integer :: values(4)
    integer :: item_count
    integer :: i
    integer :: j
    integer :: temp

    values = [9, 4, 7, 2]
    item_count = 4
    do i = 1, item_count - 1
        do j = i + 1, item_count
            if (values(j) < values(i)) then
                temp = values(i)
                values(i) = values(j)
                values(j) = temp
            end if
        end do
    end do
    print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)
end program selection_sort_prefix_demo

                        
1values ← [9, 4, 7, 2]⋯9values = [9, 4, 7, 2]10item_count = 3⋯values this step[9, 4, 7, 2]values
2item_count ← 3⋯9values = [9, 4, 7, 2]10item_count = 311do i = 1, item_count - 1⋯values this step3item_count
3i ← 1⋯10item_count = 311do i = 1, item_count - 112    do j = i + 1, item_count⋯values this step1i
4j ← 2⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step2j
5if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(2) < values(1)
6temp ← 9⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step9temp9values(1)
7values ← [4, 4, 7, 2]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[4, 4, 7, 2]values
8values ← [4, 9, 7, 2]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[4, 9, 7, 2]values
9j ← 3⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step3j
10if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.false.values(3) < values(1)
11i ← 2⋯10item_count = 311do i = 1, item_count - 112    do j = i + 1, item_count⋯values this step2i
12j ← 3⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step3j
13if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(3) < values(2)
14temp ← 9⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step9temp9values(2)
15values ← [4, 7, 7, 2]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[4, 7, 7, 2]values
16values ← [4, 7, 9, 2]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[4, 7, 9, 2]values
17print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)⋯19    end do20    print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)21end program selection_sort_prefix_demooutput3 4 7 9values this step3item_count[4, 7, 9, 2]values
1values ← [9, 4, 7, 2]⋯9values = [9, 4, 7, 2]10item_count = 2⋯values this step[9, 4, 7, 2]values
2item_count ← 2⋯9values = [9, 4, 7, 2]10item_count = 211do i = 1, item_count - 1⋯values this step2item_count
3i ← 1⋯10item_count = 211do i = 1, item_count - 112    do j = i + 1, item_count⋯values this step1i
4j ← 2⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step2j
5if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(2) < values(1)
6temp ← 9⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step9temp9values(1)
7values ← [4, 4, 7, 2]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[4, 4, 7, 2]values
8values ← [4, 9, 7, 2]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[4, 9, 7, 2]values
9print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)⋯19    end do20    print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)21end program selection_sort_prefix_demooutput2 4 9values this step2item_count[4, 9, 7, 2]values
1values ← [9, 4, 7, 2]⋯9values = [9, 4, 7, 2]10item_count = 4⋯values this step[9, 4, 7, 2]values
2item_count ← 4⋯9values = [9, 4, 7, 2]10item_count = 411do i = 1, item_count - 1⋯values this step4item_count
3i ← 1⋯10item_count = 411do i = 1, item_count - 112    do j = i + 1, item_count⋯values this step1i
4j ← 2⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step2j
5if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(2) < values(1)
6temp ← 9⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step9temp9values(1)
7values ← [4, 4, 7, 2]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[4, 4, 7, 2]values
8values ← [4, 9, 7, 2]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[4, 9, 7, 2]values
9j ← 3⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step3j
10if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.false.values(3) < values(1)
11j ← 4⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step4j
12if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(4) < values(1)
13temp ← 4⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step4temp4values(1)
14values ← [2, 9, 7, 2]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[2, 9, 7, 2]values
15values ← [2, 9, 7, 4]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[2, 9, 7, 4]values
16i ← 2⋯10item_count = 411do i = 1, item_count - 112    do j = i + 1, item_count⋯values this step2i
17j ← 3⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step3j
18if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(3) < values(2)
19temp ← 9⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step9temp9values(2)
20values ← [2, 7, 7, 4]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[2, 7, 7, 4]values
21values ← [2, 7, 9, 4]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[2, 7, 9, 4]values
22j ← 4⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step4j
23if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(4) < values(2)
24temp ← 7⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step7temp7values(2)
25values ← [2, 4, 9, 4]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[2, 4, 9, 4]values
26values ← [2, 4, 9, 7]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[2, 4, 9, 7]values
27i ← 3⋯10item_count = 411do i = 1, item_count - 112    do j = i + 1, item_count⋯values this step3i
28j ← 4⋯11do i = 1, item_count - 112    do j = i + 1, item_count13        if (values(j) < values(i)) then⋯values this step4j
29if (values(j) < values(i)) then⋯12do j = i + 1, item_count13    if (values(j) < values(i)) then14        temp = values(i)⋯values this step.true.values(4) < values(3)
30temp ← 9⋯13if (values(j) < values(i)) then14    temp = values(i)15    values(i) = values(j)⋯values this step9temp9values(3)
31values ← [2, 4, 7, 7]⋯14temp = values(i)15values(i) = values(j)16values(j) = temp⋯values this step[2, 4, 7, 7]values
32values ← [2, 4, 7, 9]⋯15    values(i) = values(j)16    values(j) = temp17end if⋯values this step[2, 4, 7, 9]values
33print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)⋯19    end do20    print '(I0, *(1X, I0))', item_count, (values(i), i = 1, item_count)21end program selection_sort_prefix_demooutput4 2 4 7 9values this step4item_count[2, 4, 7, 9]values
active prefix `item_count` chooses how much of the array is sorted.
nested scan Each outer position compares against later active positions.
swap `temp` holds one value while two array positions trade places.