conditional statements on matrices

Hi, I have two matrices like below:

I want to search when the 2nd column equals 0 on either matrix and make a new matrix that shows the 0 values along with its paired column 1 values. I also want to tag it to know from which matrix it came from.

Thank you! I am new to conditional statements.

Answer 1

[Voss]

Where M1 and M2 are your matrices:

idx1 = M1(:,2) == 0;
idx2 = M2(:,2) == 0;

tags = repelem([1; 2],[nnz(idx1) nnz(idx2)]);

result = [[M1(idx1,:); M2(idx2,:)] tags]

[puccapearl]

Voss, you've done it again. I appreciate the simple solution and was over thinking it by thinking I had to use conditional statements/loops.

Many thanks!

[Voss]

You're welcome!

[puccapearl]

Hi Voss, I'm getting a larger matrix than desired for some reason. The matrix length of 0s for M1 is 47,582 and M2 433,660 so combined should have a 909,552 by 3 matrix but I'm getting 141,648 by 3 matrix.

Any suggestions?

[Voss]

Save M1 and M2 in a mat file and upload it here using the paperclip button.

[Voss]

load M1
load M2
whos M*

  Name           Size              Bytes  Class     Attributes

  M1        525600x2             8409600  double              
  M2        525600x2             8409600  double              

I'm not sure why you say "the matrix length of 0s for M1 is 47,582 [475,892?] and M2 433,660". How are you determining that?

I get 49,708 zeros in the second column of M1 and 91,940 zeros in the second column of M2:

idx1 = M1(:,2) == 0;
idx2 = M2(:,2) == 0;
nnz(idx1)

ans = 49708

nnz(idx2)

ans = 91940

49708+91940 = 141648, exactly the size of the result you get.

[puccapearl]

thats interesting, if I do

x = nnz(idx2);

from your code I also get 91940 but if I do

x = nnz(M2(:,2));

I get 433660

** I am guessing what I did was incorrect/invalid?

[Voss]

You're basically counting the opposite of what you want. (Notice that 433660 (your count) + 91940 (my count) = 525600 (the total number of elements in M(:,2).)

This

nnz(M2(:,2))

gives you the number of non-zero elements in column 2 of M2. (nnz means number of non-zero elements.)

But you want to count the number of zero elements, which is

idx2 = M2(:,2) == 0;
nnz(idx2)

or

nnz(M2(:,2) == 0)

because idx2 (or M2(:,2) == 0) is a logical vector that is true where M(:,2) is zero and false where M(:,2) is non-zero. In a logical array, false is considered zero and true is non-zero. So nnz(idx2) is the number of non-zero elements in idx2, which is the number of elements where idx2 is true, which is the number of elements where M(:,2) is zero.

[puccapearl]

oh my gosh! *face palm*

Thank you! I'm sorry for the run around!

[Voss]

No problem! You're welcome!