How do I search through a tall array element by element?

I would like to be able to search through a tall array for a specific item. However, it seems any logical operator cannot be used with tall arrays. For example:

A = rand(1000,1);
tA = tall(A);

for i=1:1000
    if tA(i) == 0.5
        disp('i is equal to 0.5')
    end
end

This code will result in a 'conversion to logical from tall is not possible' error. So is there a way to search through a tall array without using subsets of the array, such as:

tAsubset = gather(tA(1:100));

Answer 1

[Edric Ellis]

You can make this code work by writing

if gather(tA(i) == 0.5)
    disp('i is equal to 0.5')
end

however this will be incredibly inefficient - each call to gather needs to pass over the data. You can use the == operator on the whole array to get a tall logical result.

isHalf = (tA == 0.5);

Or, bearing in mind the sound advice from @Image Analyst, you could use a tolerance (unfortunately tall arrays don't support ismembertol)

isRoughlyHalf = (tA >= 0.49 & tA <= 0.51);

The real question is what do you want to do next with all the entries that satisfy the criterion? You could gather just those using logical indexing

dataSubset = tA(isRoughlyHalf);
gather(dataSubset);

or perform some other operation on the subset of data.

[James Tursa]

"You can make this code work by writing

    if gather(tA(i) == 0.5)

You can't index into a tall array with an arbitrary index i. So the tA(i) expression you have above will not work. You can only use indexing that is e.g. of the form 1:K or end-K:end (i.e., a consecutive range at the beginning or at the end).

[Edric Ellis]

The ability to use some forms of numeric subscripts in the first dimension (including this form) was added in R2017a - but you're right, this wouldn't work in R2016b.

[James Tursa]

Yes, I was running R2016b ...

Answer 2

[Image Analyst]

You can't test floating point numbers for equality. See the FAQ: http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

You can use ismembertol().

Answer 3

[James Tursa]

tall arrays are not "in memory". As such, they cannot be used for controlling "if" and "while" statements since their values (and results of operations on their values) isn't known to the code until they are "gathered" into memory. So you cannot do what you are trying to do the way you are trying to do it. If you really want to use tall array elements this way, you will need to gather subsets into memory so that the results can be used in "if" tests and "while" loops. E.g., see this deferred evaluation link:

https://www.mathworks.com/help/matlab/import_export/deferred-evaluation-of-tall-arrays.html?s_tid=answers_rc2-1_p4_Topic

Also, you can't use arbitrary indexes with tall arrays, so there will be restrictions on how you can pull the subsets out. See the doc.