Databases
StackOverflow2013
Postsdbo
POOctave and Matlab "wat" matrix/vector inconsistencies
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POOctave and Matlab "wat" matrix/vector inconsistencies
    primarykey
    data
    text
    <p>I've noticed various cases in Matlab and octave where functions accept both matrices and vectors, but doesn't do the same thing with vectors as it does with matrices.</p> <p>This can be frustrating because when you input a matrix with a variable number of rows/columns, it could be interpreted as a vector and do something you don't expect when the height/width is 1 making for difficult debugging and weird conditional edge cases.</p> <p>I'll list a few I've found, but I'm curious what others people have run into</p> <p>(Note: I'm only looking for cases where code accepts matrices as valid input. Anything that raises an exception when a non-vector matrix is given as an argument doesn't count)</p> <p>1) "diag" can be used to mean diagonal of a matrix or turn a vector into a diagonal matrix</p> <p>Since the former is generally only used for square matrices this isn't so egregious in matlab, but in Octave it can be particularly painful when Octave interperets a vector beginning with a nonzero element and everything else zeros as a "diagonal matrix" ie</p> <pre><code>t=eye(3); size(diag(t(:,3))) == [3,3] size(diag(t(:,2))) == [3,3] size(diag(t(:,1))) == [1,1] </code></pre> <p>2) Indexing into a row-vector with logicals returns a row-vector</p> <p>Indexing into anything else with logicals returns a column vector</p> <pre><code>a = 1:3; b = true(1,3); size(a(b)) == [1, 3] a = [a; a]; b = [b; b]; size(a(b)) == [6, 1] </code></pre> <p>3) Indexing into a vector v with an index vector i returns a vector of the same (row/col) type as v. But if either v or i is a matrix, the return value has the same size as i.</p> <pre><code>a = 1:3; b = a'; size(a(b)) == [1, 3] b = [b,b]; size(a(b)) == [3, 2] </code></pre> <p>4) max, min, sum etc. operate on the columns of a matrix M individiually unless M is 1xn in which case they operate on M as a single row-vector</p> <pre><code>a = 1:3 size(max(a)) == [1, 1] a = [a;a] size(max(a)) == [1, 3] </code></pre> <p>max is particularly bad since it can't even take a dimension as an argument (unlike sum)</p> <p>What other such cases should I watch out for when writing octave/matlab code?</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USJonathan Leffler
    1. USdspyz
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POOctave and Matlab "wat" matrix/vector inconsistencies
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POOctave and Matlab "wat" matrix/vector inconsistencies
      1. This table or related slice is empty.
      1. VTDownMod
    3. VO
      singulars
      1. POOctave and Matlab "wat" matrix/vector inconsistencies
      1. USdspyz
      1. VTFavorite
    1. COjust to clarify: you can specify dimension for max/min: `min(rand(3),[],1)` or `max(rand(3),[],2)`
      singulars
      1. POOctave and Matlab "wat" matrix/vector inconsistencies
      1. USAmro
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload