Finding the index of spesific element in a matrix?

Lets say we have matrix:

a=[ 2, 3, 5

3, 4, 7

4, 4, 4] ;

I want to know index of a(4) which is the bold 3. This is just an example. I want to learn how to find index of a(n).

Answer 1

[madhan ravi]

Assuming you want to index of the last match in the matrix:

Index=find(a==3,1,'last')

https://in.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html - to know more

[IBM watson]

Actually this is not what I want to find. Maybe i asked it wrong. I will try to explain:

Lets say we can't see the elements of the matrix and we want to know the index of nth element in this matrix. (n is a number)

So in the given example we don't know a(4)=3 but we want to know index of the a(4)

I hope I could make it clear.

[madhan ravi]

Perhaps you mean:

a(4) % ? To access the fourth element?

[IBM watson]

Yes, but then I get 3. But how could I know if the 3 is the last or first or something else. I can't find its index if I can't see the whole matrix. Lets say we have a very big matrix so How could I possibly know that the 3 is last or something else.

[madhan ravi]

You don‘t have to see "through your eyes to know the last index where the actual value is 3" , you just use find() as shown in my answer with the option 'last' to know the index.

[IBM watson]

    a=[0 3 4 1 1 0 3 4 1 1 
       3 0 5 1 1 3 0 5 1 1 
       4 5 0 1 1 3 0 5 1 1 
       2 2 2 2 1 3 0 5 1 1 
       1 1 1 1 1 3 0 5 1 1 
       3 0 5 1 1 3 0 5 1 1 
       4 5 0 1 1 3 0 5 1 1 
       2 2 2 2 1 3 0 5 1 1 
       1 1 1 1 1 3 0 5 1 1
       1 1 1 1 1 1 1 1 1 1 ]

Lets say I want to find the index of a(19).

Will that work?

[madhan ravi]

Do you know what does index mean? Why a(19) ? what are you trying to imply? Do you have any logic (pattern) behind it?

[IBM watson]

specifying its row and columns? In my code I pick two random elements then I swap their locations. The matrix that I work on is very big and has lots of repitative elements. And instead of generating two random numbers for each element( for row and column) I tried to select elements like this :

generate 2 random numbers ---> 19 and 3 are generated for example

then swap a(19) and a(3)

For doing that swap, I am trying to find their indexes.

[madhan ravi]

According to what I understand see if this does what you want:

idx=randperm(numel(a),2);
a(idx)=a(flip(idx))

If the above doesn’t do what you want then:

    a=[ 2, 3, 5 
        3, 4, 7
        4, 4, 4] ;

With this example , how you expect your desired result to look like. Show it explicitly.

[IBM watson]

Lets say generated random numbers are 2 and 7.

Now I should swap a(2) and a(7)

-> a_new=[ 2, 3, 3

5, 4, 7

4, 4, 4]

[madhan ravi]

idx=randperm(numel(a),2); % generate 2 random numbers
a(idx)=a(flip(idx))

To make you a understand the above:

    a=1:5;
    a([3,2]=a([2,3]) % the second element becomes the third element and vice versa

[IBM watson]

That's it! Thak you so much.

But I can't find the index of them right? as columns and rows. Just curious.

[madhan ravi]

No problem, by the way the original question you asked is totally irrelevant to your latter comment. Please don’t do this in the future riddling the answers.

Look into ind2sub() function to convert the linear indices. To understand what this statement means , you need to read the link which I shared thoroughly and practice. And finally have a look at ind2sub().

[IBM watson]

Sure, I didn't do it on purpose. Just could't explain clearly what I want. Thanks again.

Answer 2

[KALYAN ACHARJYA]

I want to know index of a(4) which is the bold 3

here ......................... this 4^ represent the index, I guessing you are looking for following one.

idx=find(a==3);

One dimentional case array case

>> a=[2,3,5,3,4,7,4,4,4];
>> idx=find(a==7)
idx =
     6
>> idx=find(a==3)
idx =
     2     4

But Matrices have 2 dimention so indices represents (x,y), where answer will be little different

>> a=magic(3)

a =

     8     1     6
     3     5     7
     4     9     2

>> [r c]=find(a==3)

r =

2

c =

1

[madhan ravi]

Lets get in depth here this would return all the indices which have the element 3.

[KALYAN ACHARJYA]

Yes, we can acess the particular element with 'first' or 'last' as you mentioned below

[mohamed essabir]

why c =1 ?

[Stephen Cobeldick]

"why c =1 ? "

Because the value 3 is in the 1st column.

[Ulika Naidoo]

Thank you!