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I am new to Octave, so I was reading documentation and I found sub2ind function. I started to test it, but sometimes it works weird or I just don't understand how it must work.

So this is how subscripts must be converted to linear indices. (Example from documentation)

[(1,1), (1,2), (1,3)]     [1, 4, 7]
[(2,1), (2,2), (2,3)] ==> [2, 5, 8]
[(3,1), (3,2), (3,3)]     [3, 6, 9]

And this is another example from documentation

s1 = [2, 2];
s2 = [1, 3];
ind = sub2ind ([3, 3], s1, s2)
    ⇒ ind =  2   8

So if we look at the first example the (2, 2) == 5, but second example says [2, 2] == 2. The (1, 3) has different results too. Practically It works as the second example shows.

If I try to use this function with only 1 pair it return the same pair

sub2ind([3, 3], [2, 2])
# ans = [2, 2]

In this test I can't see any relation between input and output

sub2ind([3, 3], [2, 2], [3, 3])
# ans = [8, 8]

Function works this strange(maybe not) way only when it gets 1 pair or when one of pairs is pair kind [x, x](two same values).

But otherwise it works fine, so this test returns that it should:

sub2ind([3, 3], [2, 1], [1, 3])
# ans = [2, 7]

Also it works fine when this variant is used sub2ind (dims, i, j). How does the function works?

Zeritonik
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1 Answers1

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You misunderstand the input format.

Change

s1 = [2, 2];
s2 = [1, 3];
ind = sub2ind ([3, 3], s1, s2)
    ⇒ ind =  2   8

to this:

row = [2, 2];  % x1 and x2
col = [1, 3];  % y1 and y2
ind = sub2ind ([3, 3], row, col)
    ⇒ ind =  2   8

You have two inputs that you convert to linear indices: [x1, y1] = [2, 1] = 2 and [x2 y2] = [2, 3] = 8.

This:

sub2ind([3, 3], [2, 2])
# ans = [2, 2]

appears to be equivalent to:

sub2ind([3, 3], [2, 2], [1, 1])

even though it's not in the documentation.

Stewie Griffin
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  • Thank you! You are right. So stupid mistake from my side. – Zeritonik Aug 15 '22 at 21:51
  • it appears neither matlab nor octave documentation addresses trailing dimensions when the number of inputs are less than the array dimension. both programs assume any unspecified trailing dimensions are unary, as your example shows. (you can keep appending as many [1,1] as you want, and the answer stays the same) – Nick J Aug 16 '22 at 15:42