Sparse Lookup - Modern Fortran Programming

A sparse lookup scans stored keys and returns zero when the requested key is absent.
Program

Play the program to query a present or missing sparse key.
                            query_key
                            query_key = 20 (default)query_key = 30query_key = 40
                        

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

                            program sparse_lookup_demo
    implicit none
    integer :: keys(3)
    integer :: values(3)
    integer :: query_key
    integer :: i
    integer :: result

    keys = [10, 20, 30]
    values = [5, 8, 13]
    query_key = 20
    result = 0
    do i = 1, 3
        if (keys(i) == query_key) result = values(i)
    end do
    print '(I0, 1X, I0)', query_key, result
end program sparse_lookup_demo
program sparse_lookup_demo
    implicit none
    integer :: keys(3)
    integer :: values(3)
    integer :: query_key
    integer :: i
    integer :: result

    keys = [10, 20, 30]
    values = [5, 8, 13]
    query_key = 30
    result = 0
    do i = 1, 3
        if (keys(i) == query_key) result = values(i)
    end do
    print '(I0, 1X, I0)', query_key, result
end program sparse_lookup_demo
program sparse_lookup_demo
    implicit none
    integer :: keys(3)
    integer :: values(3)
    integer :: query_key
    integer :: i
    integer :: result

    keys = [10, 20, 30]
    values = [5, 8, 13]
    query_key = 40
    result = 0
    do i = 1, 3
        if (keys(i) == query_key) result = values(i)
    end do
    print '(I0, 1X, I0)', query_key, result
end program sparse_lookup_demo

                        
1keys ← [10, 20, 30]⋯9keys = [10, 20, 30]10values = [5, 8, 13]⋯values this step[10, 20, 30]keys
2values ← [5, 8, 13]⋯9keys = [10, 20, 30]10values = [5, 8, 13]11query_key = 20⋯values this step[5, 8, 13]values
3query_key ← 20⋯10values = [5, 8, 13]11query_key = 2012result = 0⋯values this step20query_key
4result ← 0⋯11query_key = 2012result = 013do i = 1, 3⋯values this step0result
5i ← 1⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step1i
6if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(1) == query_key
7i ← 2⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step2i
8result ← 8⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step8result.true.keys(2) == query_key
9i ← 3⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step3i
10if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(3) == query_key
11print '(I0, 1X, I0)', query_key, result⋯15    end do16    print '(I0, 1X, I0)', query_key, result17end program sparse_lookup_demooutput20 8values this step20query_key8result
1keys ← [10, 20, 30]⋯9keys = [10, 20, 30]10values = [5, 8, 13]⋯values this step[10, 20, 30]keys
2values ← [5, 8, 13]⋯9keys = [10, 20, 30]10values = [5, 8, 13]11query_key = 30⋯values this step[5, 8, 13]values
3query_key ← 30⋯10values = [5, 8, 13]11query_key = 3012result = 0⋯values this step30query_key
4result ← 0⋯11query_key = 3012result = 013do i = 1, 3⋯values this step0result
5i ← 1⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step1i
6if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(1) == query_key
7i ← 2⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step2i
8if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(2) == query_key
9i ← 3⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step3i
10result ← 13⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step13result.true.keys(3) == query_key
11print '(I0, 1X, I0)', query_key, result⋯15    end do16    print '(I0, 1X, I0)', query_key, result17end program sparse_lookup_demooutput30 13values this step30query_key13result
1keys ← [10, 20, 30]⋯9keys = [10, 20, 30]10values = [5, 8, 13]⋯values this step[10, 20, 30]keys
2values ← [5, 8, 13]⋯9keys = [10, 20, 30]10values = [5, 8, 13]11query_key = 40⋯values this step[5, 8, 13]values
3query_key ← 40⋯10values = [5, 8, 13]11query_key = 4012result = 0⋯values this step40query_key
4result ← 0⋯11query_key = 4012result = 013do i = 1, 3⋯values this step0result
5i ← 1⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step1i
6if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(1) == query_key
7i ← 2⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step2i
8if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(2) == query_key
9i ← 3⋯12result = 013do i = 1, 314    if (keys(i) == query_key) result = values(i)⋯values this step3i
10if (keys(i) == query_key) result = values(i)⋯13do i = 1, 314    if (keys(i) == query_key) result = values(i)15end do⋯values this step.false.keys(3) == query_key
11print '(I0, 1X, I0)', query_key, result⋯15    end do16    print '(I0, 1X, I0)', query_key, result17end program sparse_lookup_demooutput40 0values this step40query_key0result
stored keys `keys` identifies only positions that have stored values.
zero default `result` starts at zero, which represents a missing sparse value.
lookup scan A matching key copies the stored value into `result`.