Which of the following is incorrect reference to the array in FORTRAN :
A (N, X)
The rule being tested. In FORTRAN an array subscript must evaluate to an integer. A subscript may be an integer constant, an integer variable, or an integer expression built from them, but it may never be a real (floating-point) quantity, because a subscript selects a discrete storage location and a fractional index has no meaning.
Step 1 — how FORTRAN decides a variable's type. Under the implicit typing rules of classical FORTRAN, a name beginning with the letters I, J, K, L, M or N is INTEGER; a name beginning with any other letter (A–H or O–Z) is REAL. This is the famous "I through N" convention, and it is exactly what the question turns on.
Step 2 — test each option.
| Reference | Subscripts | Valid? |
|---|---|---|
| A (2, 5) | two integer constants | valid |
| A (2 * 3, 6) | integer expression 2×3 = 6, and a constant | valid |
| A (N, X) | N is integer, but X begins with X, so X is REAL | invalid |
| A (LOCATION, 8) | LOCATION begins with L, so it is integer | valid |
Step 3 — read off the odd one out. Only A (N, X) uses a real subscript, so it is the incorrect array reference.
Why the other three tempt you. A (2 * 3, 6) looks suspicious because it contains an operator, but arithmetic expressions are perfectly legal subscripts as long as the result is integer — here 2×3 = 6. A (LOCATION, 8) looks suspicious because the name is long and descriptive, but the only thing that matters is its first letter, L, which makes it an integer under the implicit rule. If the array had been declared with a name starting in A–H, that would affect the type of the array's elements, not the legality of its subscripts.
Modern practice. Because implicit typing causes exactly this kind of silent bug, current FORTRAN code almost always begins with IMPLICIT NONE and declares every variable explicitly — then X would have to be declared INTEGER before it could ever be used as a subscript.
Hence, the incorrect reference is A (N, X).
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