FORTRAN compiler warning: obsolete arithmetic IF statement

问题

I have a gfortran error:

Warning: Obsolete: arithmetic IF statement at (1) 

What does this mean? In the source (old source):

66 s12 = max(epsilon, s1 + s2)
c Then execution will go to label 13. Will this stop any further problems?
if (s12 - 1.0) 13, 13, 12
13 z = s1 / s12

回答1:

Arithmetic if is a peculiar feature of FORTRAN

it works as follows.

 IF (expr) label1, label2, label3

If the value of the expression is

less than 0, jump to label1 
equal to 0, jump to label2
greater than 0, jump to label3

In newer FORTRAN standards this feature is obsolete

In your code you can replace it with

      IF (s12 - 1.0 .gt. 0 ) GOTO 12
13    z = s1 / s12

回答2:

Check here:

http://www.ibiblio.org/pub/languages/fortran/ch1-5.html

"The Arithmetic IF is considered harmful."

Your statement,

if (s12 - 1.0) 13, 13, 12 is an Arithmetic IF, and is considered bad programming.

回答3:

Quoth Wikipedia:

"..the Fortran statement defines three different branches depending on whether the result of an expression was negative, zero, or positive, in said order..."

"..was finally labeled obsolescent in Fortran 90."

来源：https://stackoverflow.com/questions/10758935/fortran-compiler-warning-obsolete-arithmetic-if-statement