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arctica
Joined: 10 Sep 2006 Posts: 105 Location: United Kingdom
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Posted: Wed Jan 17, 2024 2:55 pm Post subject: Find values satsfying condition in array |
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Hello
Is there a way to find the indices in array where a condition is satisfied and then extract those values. The equivalent in Matlab/Scilab:
values = x(find(x > 4.7)) where x is an array of real or integer values.
Pretty sure it must revolve around the use of findloc, but an example would be useful.
Thanks
Lester |
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arctica
Joined: 10 Sep 2006 Posts: 105 Location: United Kingdom
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Posted: Wed Jan 17, 2024 4:22 pm Post subject: |
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This will work, but cannot figure out how the same can be done via findloc:
Code: |
program FindValuesGreaterThan2
implicit none
integer, parameter :: n = 10
real :: testArray(n)
integer :: indices(n)
real, dimension(:), allocatable :: newArray
integer :: count, i
! Initialize the test array with some values
testArray = [1.0, 3.0, 2.0, 5.0, 0.0, 4.0, 2.5, 6.0, 1.5, 3.5]
! Find indices where values are greater than 2
indices = 0 ! Initialize indices array with zeros
count = 0 ! Initialize count
do i = 1, n
if (testArray(i) > 2.0) then
count = count + 1
indices(count) = i
end if
end do
! Allocate memory for the new array
allocate(newArray(count))
! Extract values greater than 2 to the new array
do i = 1, count
newArray(i) = testArray(indices(i))
end do
! Print the original and new arrays
print *, 'Original array:', testArray
print *, 'Values greater than 2:', newArray
! Deallocate the memory
deallocate(newArray)
end program FindValuesGreaterThan2
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mecej4
Joined: 31 Oct 2006 Posts: 1898
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Posted: Wed Jan 17, 2024 6:10 pm Post subject: |
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There are a number of intrinsic functions that operate on arrays, such as PACK, MERGE, etc., whose purpose is to perform operations of the type sought.
Code: | program FindValuesGreaterThan2
implicit none
integer, parameter :: n = 10
real :: testArray(n)
integer :: indices(n)
integer :: i, nsel
integer, allocatable :: selIdx(:)
! Initialize the test array with some values
testArray = [1.0, 3.0, 2.0, 5.0, 0.0, 4.0, 2.5, 6.0, 1.5, 3.5]
indices = [(i,i=1,n)]
! Select and list elements > 4.7
selIdx = pack(indices,testArray > 4.7)
nsel = size(selIdx)
print '(1x,2i3,F4.1)',(i,selIdx(i),testArray(selIdx(i)),i=1,nsel)
end program FindValuesGreaterThan2 |
Last edited by mecej4 on Fri Jan 19, 2024 2:51 am; edited 3 times in total |
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arctica
Joined: 10 Sep 2006 Posts: 105 Location: United Kingdom
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Posted: Thu Jan 18, 2024 3:24 pm Post subject: |
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Many thanks for the suggestion and syntax example. |
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JohnCampbell
Joined: 16 Feb 2006 Posts: 2593 Location: Sydney
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Posted: Fri Jan 19, 2024 1:17 am Post subject: |
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mecej4,
Did you compile your example before posting ?
looks like you didn't, so not bad for no checking.
Here is my (tested) example showing mecej4's new F90 way and mine and (Eddie's?) old way using DO loops.
Code: | program FindValuesGreaterThan2
integer, parameter :: n = 10
real :: testArray(n)
testArray = [1.0, 3.0, 2.0, 5.0, 0.0, 4.0, 2.5, 6.0, 1.5, 3.5]
call old_DO_way ( testArray,n )
call new_pack_way ( testArray,n )
end program FindValuesGreaterThan2
subroutine new_pack_way ( testArray,n )
implicit none
integer :: n
real :: testArray(n)
integer :: indices(n)
integer :: nsel, i
integer, allocatable :: selIdx(:)
! Initialize the test array index
indices = [(i,i=1,n)]
! Select and list elements > 4.7
selIdx = pack(indices,testArray > 4.7)
nsel = size(selIdx)
print '(1x,2i3,F4.1)',(i,selIdx(i),testArray(selIdx(i)),i=1,nsel)
end subroutine new_pack_way
SUBROUTINE old_DO_way ( testArray,n )
implicit none
integer :: n
real :: testArray(n)
integer :: nsel, i
integer :: selIdx(n)
! generate a list of values
nsel = 0
do i = 1,n
if ( testArray(i) <= 4.7 ) cycle
nsel = nsel+1
selIdx(nsel) = i
end do
print '(1x,2i3,F4.1)',(i,selIdx(i),testArray(selIdx(i)),i=1,nsel)
end SUBROUTINE old_DO_way |
I still prefer to see DO loops to show what is happening.
After these comments, I hope my code doesn't need correcting ! |
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mecej4
Joined: 31 Oct 2006 Posts: 1898
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