Function that pulls a name from a list at random but doesn't repeat

Question

Matt Rulli 2018-4-14

0
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https://ww2.mathworks.cn/matlabcentral/answers/394931-function-that-pulls-a-name-from-a-list-at-random-but-doesn-t-repeat

编辑： David Fletcher 2018-4-14

I'm working on a two-part assignment that requires a function to pick a random index from a list of names. The function itself needs to check if the index has been pulled before so that it doesn't repeat any names. This is what I have so far,

function name = pick_name(list)
  persistent ind_picked
    n_picked = length(ind_picked);
    n_list = length(list);
    if isempty(n_picked) 
        ind = randi(n_list);
        ind_picked = ind;
        name = list(ind);
    else 
        while found == find(ind_picked==ind)
            ind = randi(n_list);    
            ind_picked = [ind_picked,ind];
        end
        name = list(ind);
    end

What am I doing wrong??

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David Fletcher 2018-4-14

编辑：David Fletcher 2018-4-14

All those variables that you are carefully declaring in the if condition on the first call of the function will go out of scope when the function exits (the condition on the if statement is slightly iffy as well - shouldn't it be if n_picked==0). On the second call because n_picked is persistent and no longer has a length of 0all those variables declared in the if condition won't execute and so ind won't exist when you are trying to use it in the while condition of your else statement (where is found defined btw). It might be worth detailing what your 'prescribed steps' are exactly. It will always be harder to pick a number that doesn't exist in a list of numbers that have already been picked rather than remove a number from a list of numbers that haven't already been picked.

Matt Rulli 2018-4-14

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Answer 1

David Fletcher 2018-4-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/394931-function-that-pulls-a-name-from-a-list-at-random-but-doesn-t-repeat#answer_315145

编辑：David Fletcher 2018-4-14

listNames={'a','b','c','d','e','f','g','n','h'}
for iter=1:9
    name=pick_name(listNames)
end
function name = pick_name(list)
    persistent ind_picked
    candidate=randi(length(list));
    if isempty(ind_picked)
        %Initialize persistent variable with random index
        ind_picked=candidate;
    else
        %look for int that is not already in the list
        while any(candidate==ind_picked)
            candidate=randi(length(list));
        end
        ind_picked=[ind_picked candidate];
    end
    name=list{candidate};
end

Some things you may want to consider and make improvements: something bad happens if you call the function and all items in the list have already been picked. Maybe there should also be a way to 'reset' the function so it forgets it's internal list of picked indexes. Personally, I would also clearly state why this is a bad approach to the problem, and that the while loop could be eliminated, and the code much simplified by instead maintaining a list of indexes that had not been picked.