The problem with strings in Ada is that they just don’t play nicely. As we have seen in previous posts, strings in Ada are either fixed, or unbounded. Now creating an array of strings in other languages isn’t that hard, nor is it in Ada – some things are just beyond its scope. Now by an array of strings, I mean a structure that could hold something like a dictionary of words. So how do we go about this?

The first example below creates a type, lexicon, with is a 5 element array of strings, 3 characters in length (so 5 three letter words). The variable words is then an instantiation of lexicon, which is assigned five three letter words. It’s not really possible to use the term string without the constraints (1..3). This is because of the fact that strings in Ada are fixed length – and it means all the strings assigned to words will have to be of length 3.

type lexicon is array(1..5) of string(1..3);
words : lexicon := ("gum","sin","cry","lug","fly");

It is also not possible to make the constraint larger, e.g. (1..10), and use words or variable length, e.g. “gum”, “housefly”, “star”. This will cause a constraint error (the word star would have to be specified as “star “, with 6 spaces filling out the word). The alternative is to unbounded strings, but they come with their own baggage. Consider the following code, changing the string to an unbounded_string.

type lexicon is array(1..5) of unbounded_string;
words : lexicon := ("gum","sin","cry","lug","star");

Compile this, and it will produce a series of errors of the form:

file.adb:10:24: expected private type "Ada.Strings.Unbounded.Unbounded_String"
file.adb:10:24: found a string type

This means that when it tried to assign the words, it considered “gum” to be a string, NOT an unbounded_string. The declaration of lexicon works fine if reading directly from a file, but not predefined strings. The fix? Not pretty, because the strings have to be converted to unbound strings using the function to_unbounded_string().

words : lexicon :=(to_unbounded_string("gum"),to_unbounded_string("sin"),to_unbounded_string("cry"),to_unbounded_string("lug"),to_unbounded_string("star"));

Messy right? There is one way of cleaning up this code and that is providing an alias for the function name to_unbounded_string(). Here is how that is done, renaming the function to tub().

function tub(Source : String) return unbounded_string renames ada.strings.unbounded.to_unbounded_string;

Then the code below it becomes:

type lexicon is array(1..5) of unbounded_string;
words : lexicon := (tub("gum"),tub("sin"),tub("cry"),tub("lug"),tub("star"));

Not perfect, but then that is Ada and strings for you.

The problem with strings in Ada is that they just don’t play nicely. As we have seen in previous posts, strings in Ada are either fixed, or unbounded. Now creating an array of strings in other languages isn’t that hard, nor is it in Ada – some things are just beyond its scope. Now by an array of strings, I mean a structure that could hold something like a dictionary of words. So how do we go about this?

The first example below creates a type, lexicon, with is a 5 element array of strings, 3 characters in length (so 5 three letter words). The variable words is then an instantiation of lexicon, which is assigned five three letter words. It’s not really possible to use the term string without the constraints (1..3). This is because of the fact that strings in Ada are fixed length – and it means all the strings assigned to words will have to be of length 3.

type lexicon is array(1..5) of string(1..3);
words : lexicon := ("gum","sin","cry","lug","fly");

It is also not possible to make the constraint larger, e.g. (1..10), and use words or variable length, e.g. “gum”, “housefly”, “star”. This will cause a constraint error (the word star would have to be specified as “star “, with 6 spaces filling out the word). The alternative is to unbounded strings, but they come with their own baggage. Consider the following code, changing the string to an unbounded_string.

type lexicon is array(1..5) of unbounded_string;
words : lexicon := ("gum","sin","cry","lug","star");

Compile this, and it will produce a series of errors of the form:

file.adb:10:24: expected private type "Ada.Strings.Unbounded.Unbounded_String"
file.adb:10:24: found a string type

This means that when it tried to assign the words, it considered “gum” to be a string, NOT an unbounded_string. The declaration of lexicon works fine if reading directly from a file, but not predefined strings. The fix? Not pretty, because the strings have to be converted to unbound strings using the function to_unbounded_string().

words : lexicon :=(to_unbounded_string("gum"),to_unbounded_string("sin"),to_unbounded_string("cry"),to_unbounded_string("lug"),to_unbounded_string("star"));

Messy right? There is one way of cleaning up this code and that is providing an alias for the function name to_unbounded_string(). Here is how that is done, renaming the function to tub().

function tub(Source : String) return unbounded_string renames ada.strings.unbounded.to_unbounded_string;

Then the code below it becomes:

type lexicon is array(1..5) of unbounded_string;
words : lexicon := (tub("gum"),tub("sin"),tub("cry"),tub("lug"),tub("star"));

Not perfect, but then that is Ada and strings for you.