Strange Fortran instruction

Question

Learning old Fortran77-codes full of "goto-spagetti", I met with the following instruction:

if(condition) label1, label2, label3

where labels are just three numbers. Has anybody any ideas what it means?

Answer 1

Arithmetic if. It goes to

• label1 if the expression is negative
• label2 if the expression is 0
• label3 if the expression is positive.

Caveat: If you use it with floating point expressions, note that a result that should be 0 might be "a small number close to 0" due to rounding errors. This limits the usefulness of the arithmetic if.