finding elements in array

I have an array

vk = [0 0.025 0.05 0.075 0.1 0.125 0.15 0.175].

I am writing this line

op= vk(vk>= 0.05 & vk<= 0.175)

The result which I am getting is

op =

0.0750 0.1000 0.1250 0.1500 0.1750

which is wrong, I should get

0.05 0.0750 0.1000 0.1250 0.1500 0.1750

does anyone know what I am doing wrong?

Also how I can get the indices number of the array vk and of op?

[Meg Noah]

It worked for me:

vk = [0	0.025	0.05 	0.075	0.1   	0.125 	0.15 	0.175];
op = vk(vk>= 0.05 & vk<= 0.175)

>> vk = [0 0.025 0.05 0.075 0.1 0.125 0.15 0.175];

op= vk(vk>= 0.05 & vk<= 0.175)

op =

Columns 1 through 4

0.05 0.075 0.1 0.125

Columns 5 through 6

0.15 0.175

[Chris Dan]

no, its not working on this

Can you tell why?

[Stephen23]

"no, its not working on this ... Can you tell why?"

Can you explain why you think that it is not working?

[Chris Dan]

the value 0.05 is not included in "op", it should be included as well, what change can be done?

[Stephen23]

Possibly you have issues with floating point error. Print the exact number values:

https://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact-exact-version-of-num2str

and you might find that the values are not what you think they are. See also:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

This is worth reading as well:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

http://faculty.tarleton.edu/agapie/documents/cs_343_arch/papers/1991_Goldberg_FloatingPoint.pdf

[Chris Dan]

I kind of got it,

op = vk(vk>= 0.05-0.00000000001 & vk<= 0.175)

subtracting a very small number, it is getting me the correct answer in my case...

Answer 1

[Meg Noah]

To get the indices to the vk array

vk = [0	0.025	0.05 	0.075	0.1   	0.125 	0.15 	0.175];
[idx] = find(vk>= 0.05 & vk<= 0.175);
op = vk(idx)

[Stephen23]

find is not required, logical indexing is simpler and more efficient:

idx = vk>=0.05 & vk<=0.175;
op  = vk(idx)

[Chris Dan]

@Meg Noah It is not giving the index of the first value

[Meg Noah]

@Stephen Cobeldick

The OP specifically asks: Also how I can get the indices number of the array vk and of op? And that is why I have a 'find' command.

@Hamzah Khan

When I run this code to find the indices of the vk meeting that condition, this is what is displayed:

>> idx = find(vk>=0.05 & vk<=0.175)

idx =

     3     4     5     6     7     8

Remember: matlab arrays begin indexing at position 1. There is no 0th element.

     >> vk(0)
Array indices must be positive integers or logical values.

The value of element 1 is 0. Since it does not meet the condition, the index 1 is not returned on the list of indices that meet the condition.

The value of element 2 is 0.025. Since it does not meet the condition, the index 2 is not returned on the list of indices that meet the condition.

The value of element 3 is 0.025. It is the first value that meets the condition, and index 3 is the first value returned on the list of indices that meet that condition.

If your problem is floating point, then maybe try recasting it. Is it being set to a double precision array initially?

vk = double(vk);

[Chris Dan]

I kind of made this loop to get the indexes of vk

vk = [0 0.025 0.05 0.075 0.1 0.125 0.15 0.175];

ind= size(vk,2);

for i = 1:1:ind

index(i) = i

end

Because first element is zero so I wasnt getting the index of that one