Position of an element in a vector

Question

F.O 2017-10-18

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评论： Star Strider 2017-10-18

Hi,I want to find the position not value of an element of a vector that meet equal a number, I have this vector z=[0:-0.01:-16] and want to find the position when z=-2 .

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Answer 1

Cam Salzberger 2017-10-18

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Hello Farhan,

You can use find to get the index when an expression is true, and "==" to get that logical array. Something like this should work:

idx = find(z == -2);

Note, however, that this will only do an absolute comparison. If you wanted to find with some kind of tolerance, you could do:

idx = find(abs(z+2) < 0.001);

Also note that if you are then going to use this index for indexing into the array, it's usually more efficient to just use the logical array to index:

z(z == -2) = NaN; % Or something

Hope this helps!

-Cam

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F.O 2017-10-18

I am sorry I had a mistake in the step which is 0.1 not 0.01 thanks for the answer and it make sense

Star Strider 2017-10-18

@Stephen — Thank you!

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Answer 2

Star Strider 2017-10-18

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编辑：Star Strider 2017-10-18

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The ismember (or perhaps preferably ismembertol) function is your friend here.

EDIT — The ‘-2’ index will be 21 if the step in ‘z’ is ‘-0.1’ rather than ‘-0.01’:

z=[0:-0.1:-16];
[~,idx] = ismembertol(-2, z, 1E-8)
idx =
    21

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F.O 2017-10-18

Thanks it works but it is difficult to understand how it works

Star Strider 2017-10-18

The ismembertol function checks to see whether the first argument (here -2) is a member of the set in the second argument (here ‘z’), and returns the index/indices in the second argument that match. Since floating-point calculations (including those involved in the colon operator calculations) can produce values that are not exactly -2 (in this instance), the third ‘tolerance’ argument gives a range of values around -2 that would meet the criterion. The find and ismember functions match the condition exactly, while ismembertol matches within a tolerance.

See: Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? (link) for a full explanation of the reason ismembertol may be the preferred function, if there is a reason to suspect that no element of ‘z’ is exactly equal to -2.

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