Indexing string scalar odd behaviour. Intended or bug?

[Salah Mahmud]

Hopefully this is not a repeat; could not find references to this issue in a quick search.

In mata (Stata 14.2), when I index a string scalar, I get the following:

. mata: s = "hello"; s[1]
hello

. mata: s = "hello"; s[2]
<istmt>: 3301 subscript invalid
r(3301);

It seems like s[1] is just an alias for s. I was expecting to get the byte at position 1. Is this the intended behaviour? If so, why?
if 2 is invalid subscript as per the error message above, what is the range of valid subscripts in this case?

How do I get the byte at a specific index (other than using substr)?

Thanks,

/salah

[Joseph Coveney]

It seems like s[1] is just an alias for s. I was expecting to get the byte at position 1. Is this the intended behaviour? If so, why?

Yes, it is intended behavior. [1] is the element index of the string matrix (vector), and not a byte position of a string scalar.

if 2 is invalid subscript as per the error message above, what is the range of valid subscripts in this case?

You've created a string matrix (vector) with a single element, and so its valid subscript range is 1.

How do I get the byte at a specific index (other than using substr)?

Wouldn't you be using substr() with 1 as the third argument?

[Salah Mahmud]

Thanks Joseph,
For me, the ability to subscript a variable explicitly declared as scalar is counterintuitive except in the case of a string which in every programming language I used is either implicitly or explicitly a vector. I guess I will just have to get used to it.

[Nick Cox]

Scalars in Mata have one row and one column. See e.g. help m6_glossary

scalar
A special case of a matrix with one row and one column. A scalar may be substituted anywhere a matrix,
vector, column vector, or row vector is required, but not vice versa.

That's intuitive once it's understood. Further, all of these work with your example:

: s[1,1]
  hello

: s[1]
  hello

: s[,1]
  hello

: s[1,]
  hello

but no subscript 2 or more will work.

[Joseph Coveney]

. . . the case of a string which in every programming language I used is either implicitly or explicitly a vector.

I believe that Stata and Mata are written in such a programming language, and so string variables in them are implemented as such vectors (underneath). It's just that you access their elements (underneath) using a function, substr().