While loop until value falls within a range

New to Julia
1

I’d like to be able to randomly pick a species that has a trophic level within a certain range.

num_animals = 40
trophic_position = rand(num_animals)

function target_species_check_change()
    sp = rand((num_animal),1)
    while (trophic_position[sp] < 0.3) && (trophic_position[sp] > 0.4)
        sp = rand((num_animal),1)
    end
    return sp
end

target_species = target_species_check_change()

I would just want the number of the species (1:40) to be returned which corresponds to a trophic level between 0.3 and 0.4

2

The syntax to pick a random integer within a range is

sp = rand(1:num_animals)

If you have other questions please ask.

3

Thank you for your reply, I have updated by code below. I am unable to generate a target_species that has a trophic_position between 0.3 and 0.4.

num_animals2 = 40
trophic_position2 = rand(num_animals2)

function target_species_check_change2()
    sp = rand((1:num_animals2),1)[1]
    while (trophic_position2[sp] < 0.3) && (trophic_position2[sp] > 0.4)
        sp = rand((1:num_animals2),1)[1]
    end
    return sp
end

target_species = target_species_check_change()
trophic_position[target_species]
4

You have to use || instead of &&, or the while loop will never execute (since there is no number that is smaller than 0.3 and larger than 0.4).

Note that you can also replace rand((1:num_animals2),1)[1] by rand(1:num_animals2). Right now you are allocating an unnecessary 1-element array every time.

5

Thank you for your reply. I though || means or from here.

I understand why && would not work from your explanation. There is probably a less confusing way to write my function.

6

It does mean “or”, which is what you want here.

7

I believe the following code does what you want more cleanly. I replaced your while loop with elementwise broadcasting indicated by ..

num_animals=40
trophic_position=rand(num_animals) #Create trophic position data for each animal
trophic_position_within_range=(trophic_position.>0.3) .& (trophic_position.<0.4) #Vector of true/false
animals_within_range=(1:num_animals)[trophic_position_within_range] #Vector of animals where trophic_position_within_range is true
random_animal_in_range=rand(animals_within_range) #Selects a random animal from the previous vector

or as a function:

function select_animal(num_animals,lower_bound,upper_bound)
    trophic_position=rand(num_animals)
    trophic_position_within_range=(trophic_position.>lower_bound) .& (trophic_position.<upper_bound)
    animals_within_range=(1:num_animals)[trophic_position_within_range]
    random_animal_in_range=rand(animals_within_range)
end

chosen_animal=select_animal(40,0.3,0.4)