Find index of same values in array

Hi all,

i have to find the index of the same value in an array,see the following example

a=[1 2 3 1]

i want b=[1 4] as output..how can i do this? A solution using find is this

• u=unique(a)
• n=histc(a,u)
• find(a==u(n>1))

but if in the a array there isn't 2 or more same value i get this error

??? Error using ==> eq Matrix dimensions must agree.

Thank you for your support :-)

[Walter Roberson]

What if there are multiple repeated values?

Answer 1

[Andrei Bobrov]

find(ismember(a,u(n>1)))

ADD

a=[nan nan 12 3 5 9 8 7 12 NaN NaN 0 0 2 1 4 4 4]
u=unique(a(~isnan(a)));
n=histc(a,u);
d = u(n > 1);
out =find(ismember(a,d(end)))

[roberto verrelli]

Thank you very much Andrei,it works!! But now i have the problem issued by Walter.. if i have

• a=[10 9 9 10]

i have to take only the indexes of the higher value in a,in this example 10 is the max value so the output that i would have is b=[1 4] How take care of this?

[Andrei Bobrov]

see ADD part in this answer

[roberto verrelli]

Thank you Andrei! I have still a problem,sometimes the a matrix is like this

• a=[12 12 NaN NaN]

in this case n=histc(a,u) returns n=[0 0 0] so it's not working..i can replace NaN with zeros but if then i have the a matrix like this

• a=[12 0 0 0] it's not working beacuse out =find(ismember(a,d(end))) will be
• out=[2 3 4]

and not

• out=[1]

[Andrei Bobrov]

corrected

[roberto verrelli]

Now it's perfect!Thank you very much Andrei ;-)

Answer 2

[Pramod Kumar Jangir]

Hi Andrei,

your code only wokrs for the consecutive equal values. Could you please modify it for random same values in an array?

Pramod