Use of Variable Format Expressions

Variable format expressions (a Compaq Fortran extension) is a numeric expression enclosed in angle brackets (< >) that can be used in a FORMAT statement. Variable format expressions (VFEs) are almost as flexible as run-time formatting, but they are more efficient because the compiler can eliminate run-time parsing of the I/O format. Only a small amount of processing and the actual data transfer are required during run time.

On the other hand, run-time formatting can impair performance significantly. For example, in the following statements, S₁ is more efficient than S₂ because the formatting is done once at compile time, not at run time:

  S1        WRITE (6,400) (A(I), I=1,N)
      400   FORMAT (1X, <N> F5.2)
                         .
                         .
                         .
  S2        WRITE (CHFMT,500) '(1X,',N,'F5.2)'
      500   FORMAT (A,I3,A)
            WRITE (6,FMT=CHFMT) (A(I), I=1,N)