Can characters be used as indices?

Question

Let's define, for example,

x = 10:10:2000;

As is well known, integer values can be used as indices:

>> x(9)
ans =
    90

In Matlab, characters can often be used where a number would be expected, with Matlab doing the conversion automatically. For example, since the ASCII code of 'a' is 97,

>> 'a'+1
ans =
    98

Can characters be also used as indices? Does Matlab convert them into integers?

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

6

They can be used... but careful if the index is a single colon!

Let's define

>> x = 10:10:2000;

Indexing with 'a' produces the 97-th element of x, as expected:

>> x('a')
ans =
   970

However, indexing with ':' is a special case. The string ':' acts as a : index, thus producing a column vector of all values of x. That is, x(':') is the same as x(:):

This means that the index ':' is being evaluated (x(':') acts like x(:)), whereas other character arrays used as indices are not evaluated (x('a') doesn't act like x(a)):

>> a = 1;
>> x('a')
ans =
   970

This also implies that with ':', converting to a numeric type before indexing does matter, unlike with other characters used as indices:

>> x(double('abc'))
ans =
   970   980   990
>> x('abc')
ans =
   970   980   990

>> x(double(':'))
ans =
   580
>> x(':')
ans =
          10
          20
          30
         ...
        1990
        2000

The "evaluated" behaviour of ':' used as index was already known. What's surprising is the contrast with other characters or character arrays used as indices (which are not evaluated).

The examples have used a single dimension for simplicity, but the described behaviour also applies to multidimensional indexing. The same behaviour is observed in Octave too.

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 23 '16 at 22:08

Luis Mendo

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One thing I don't get is this: how the hell did Octave know to test for this?:) Could have `subsref` been open-source in the early days? – Andras Deak -- Слава Україні Mar 23 '16 at 22:19
1

@AndrasDeak Good point! See [this comment](http://stackoverflow.com/questions/22537326/on-shape-agnostic-slicing-of-ndarrays#comment34308905_22539481) – Luis Mendo Mar 23 '16 at 22:25
Oh, very funky post, and also gives a legit reason for this odd behaviour. – Andras Deak -- Слава Україні Mar 23 '16 at 22:27
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@AndrasDeak The `':'` (string) index is mentioned [here](http://es.mathworks.com/help/matlab/matlab_oop/indexed-reference.html): _A colon used as an index is passed [...] as the colon character `':'`. Ranges specified using a colon (e.g., `2:5`) are expanded to `2 3 4 5`._ – Luis Mendo Mar 23 '16 at 22:29
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So it's *technically* not undocumented, only for all intents and purposes;) – Andras Deak -- Слава Україні Mar 23 '16 at 22:31
Nice post @LuisMendo. I'll have to file this away in the back of my mind. I'm sure it will come in handy in the future when someone wants to combine multiple variables into a *useful* datastructure. – Suever Mar 23 '16 at 23:32
@Suever Thanks! Using `':'` as a colon is useful indeed (as in the answer I linked). But for Matlab to have a specific character singled out that way is weird – Luis Mendo Mar 24 '16 at 00:14

Can characters be used as indices?

1 Answers1

They can be used... but careful if the index is a single colon!

Linked