Concepts of Programming Languages

6.5 Array Types 259

for the subtype, except assignment of values outside the specified range. For

example, in

Day1 : Days;

Day2 : Weekdays;

Day2 := Day1;

the assignment is legal unless the value of Day1 is Sat or Sun.

The compiler must generate range-checking code for every assignment to

a subrange variable. While types are checked for compatibility at compile time,

subranges require run-time range checking.

One of the most common uses of user-defined ordinal types is for the

indices of arrays, as will be discussed in Section 6.5. They can also be used for

loop variables. In fact, subranges of ordinal types are the only way the range of

Ada for loop variables can be specified.

6.4.2.2 Evaluation

Subrange types enhance readability by making it clear to readers that variables

of subtypes can store only certain ranges of values. Reliability is increased with

subrange types, because assigning a value to a subrange variable that is outside

the specified range is detected as an error, either by the compiler (in the case of

the assigned value being a literal value) or by the run-time system (in the case

of a variable or expression). It is odd that no contemporary language except

Ada has subrange types.

6.4.3 Implementation of User-Defined Ordinal Types

As discussed earlier, enumeration types are usually implemented as integers.

Without restrictions on ranges of values and operations, this provides no

increase in reliability.

Subrange types are implemented in exactly the same way as their parent

types, except that range checks must be implicitly included by the compiler in

every assignment of a variable or expression to a subrange variable. This step

increases code size and execution time, but is usually considered well worth the

cost. Also, a good optimizing compiler can optimize away some of the checking.

6.5 Array Types

An array is a homogeneous aggregate of data elements in which an individual

element is identified by its position in the aggregate, relative to the first ele-

ment. The individual data elements of an array are of the same type. References

to individual array elements are specified using subscript expressions. If any of

the subscript expressions in a reference include variables, then the reference