10. Assignment

Assignment is how a running program stores a computed value into a variable. The form is the same for every type — variable = expression — but the conversion rules differ by type (§10).

Arithmetic assignment

      v = e

The expression e is evaluated, then converted to the type of v before being stored (§10.1, Table 4). The conversions that matter:

• Real/double → integer truncates toward zero (the fractional part is dropped):

        I = 3.9          ! I = 3
        I = -3.9         ! I = -3   (toward zero, not -4)
  

• Integer → real/double widens exactly; → complex sets the real part, imaginary 0.

• Assigning a value already of v's type stores it unchanged.

Note the asymmetry with Chapter 6: there, mixing types in an expression converts up; here, the final store converts to the target's type, which may convert down (real → integer) and lose the fraction.

Logical assignment

      LOGICAL FLAG
      FLAG = X .GT. 0.0        ! the relational yields a logical, stored in FLAG

v and e are both logical.

Character assignment

v and e are character; they may differ in length (§10.4). The value is fitted to v's declared length: a shorter value is blank-padded on the right, a longer one is truncated.

      CHARACTER*5 S
      S = 'HI'           ! S = 'HI   '   (padded)
      S = 'HELLO!'       ! S = 'HELLO'   (truncated to 5)

Assigning to a substring changes only those character positions; the rest of the string is untouched (Chapter 5):

      S = 'ABCDE'
      S(2:4) = 'XY'      ! S = 'AXY E'  (RHS fitted to the 3-char slice)

The ASSIGN statement

ASSIGN is a special, narrow form that stores a statement label (not a value) into an integer variable, for use by an assigned GO TO or as a run-time format identifier (§10.3):

      ASSIGN 100 TO LBL
      GO TO LBL                ! jumps to the statement labelled 100
      ...
  100 CONTINUE

While a variable holds an assigned label it must not be used as an ordinary integer. ASSIGN and the assigned GO TO are little used in new code; you will meet them in older programs (Chapter 11).

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forterp notes. Assignment conversions follow Table 4 exactly, including the truncate-toward- zero rule and the character pad/truncate rule. Two edges handled per the audit: assigning to a PARAMETER named constant is a hard error (Chapter 8), and a character substring target outside 1 ≤ e1 ≤ e2 ≤ len is unspecified by default / an error under bounds_check (Chapter 5).