问题

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?

回答1:

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.

来源：https://stackoverflow.com/questions/36189651/can-characters-be-used-as-indices

标签

arrays