using find function and logical array

69 views (last 30 days)
Lorenne
Lorenne on 5 Jun 2018
Commented: Lorenne on 5 Jun 2018
If i have a matrix Z=[1 0 1;0 1 0;1 0 1];
>> find(Z==1)
ans =
1
3
5
7
9
>> find(Z(Z==1))
ans =
1
2
3
4
5
Why does the find(Z(Z==1)) produces the above output?

Accepted Answer

Birdman
Birdman on 5 Jun 2018
Edited: Birdman on 5 Jun 2018
Because
Z(Z==1)
will produce
1
1
1
1
1
which is a new column vector generated from the initial line. Then, you want to find the Z(Z==1) which is equivalent to Z(Z==1)~=0, therefore you obtain
1
2
3
4
5
which are the indices of all ones in your vector.

More Answers (2)

monika shivhare
monika shivhare on 5 Jun 2018
find(M) takes the matrix given in argument M, and returns matrix of indexes in M where value at that index in M is not zero. Z==1 gives a logical matrix of size(Z) showing 1 at index where value of Z is equal to 1.
>> Z==1
ans =
3×3 logical array
1 0 1
0 1 0
1 0 1
logical indexing of Z returns a array of elements of Z where logical value at that index is 1. So, Z(Z==1) gives array of elements of Z where (Z==1) has value 1
>> Z(Z==1)
ans =
1
1
1
1
1
Now, if we execute find(Z(Z==1)), it will return indexes of Z(Z==1) where value of Z(Z==1) is not zero. Value of Z(Z==1) is nonzero at index [1,2,3,4,5]. Hence,
>> find(Z(Z==1))
ans =
1
2
3
4
5

Walter Roberson
Walter Roberson on 5 Jun 2018
What else could it produce? Z(Z==1) is logical indexing and returns the elements of Z where Z is 1 -- a vector of values. Because you asked for 1, all the values in that vector are 1. You then find() on that vector of 1's. None of the 1's are 0, and find() returns the locations of all non-zero values... but all of the values are non-zero, so that is all of the locations in that vector that was created by Z(Z==1)

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!