# Position of an element in a vector

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F.O on 18 Oct 2017
Commented: Star Strider on 18 Oct 2017
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 .

Cam Salzberger on 18 Oct 2017
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
Star Strider on 18 Oct 2017
@Stephen — Thank you!

Star Strider on 18 Oct 2017
Edited: Star Strider on 18 Oct 2017
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
##### 2 CommentsShowHide 1 older comment
Star Strider on 18 Oct 2017
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.

KL on 18 Oct 2017
Edited: KL on 18 Oct 2017
[row,col,v] = find(z==-2)

Jan on 18 Oct 2017
Remember the old FAQ:
Why does
any((0:0.1:1) == 0.3)

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