Fortran IF statement without logical expression

Question

I am converting an old code in Fortran which I think is a 77. there a couple of usage of IF in the following way that I can't realize what it does:

    IF(x1-x2) 12, 13, 14

12  WRITE(*,*) x1
    .
    .
    .
13  y=...
    .
    .
14  DO x=x1,x2
    IF(x-x2) 33, 34, 40
    .
    .
    .

The code complies and runs fine and it produces the results. Has anyone encountered such a usage of IF?

See also http://stackoverflow.com/q/6789946/3157076. – francescalus Sep 15 '14 at 23:25 — francescalus, Sep 15 '14 at 23:25
Been here several times. – Vladimir F Героям слава Sep 16 '14 at 10:47 — Vladimir F Героям слава, Sep 16 '14 at 10:47

score 5 · Accepted Answer · answered Sep 15 '14 at 22:36

5

That's an arithmetic IF. It branches to the first, second, or third label depending on whether the expression is negative, zero, or positive.

In very old versions of Fortran (or FORTRAN), it was the only form of IF statement that was available.

It's probably a little odd to see it used in a version as recent as Fortran 77, which had a more modern logical IF. The feature was declared obsolescent in Fortran 90, but I wouldn't be surprised if more modern Fortran compilers still support it.

answered Sep 15 '14 at 22:36

Keith Thompson

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Oh ok,so the order is 1st,2nd,3rd for negative,zero,positive. thanks – user3780714 Sep 15 '14 at 22:38
gfortran still support it, yes. I'm using it with some legacy code in Fortran 77. – siritinga Sep 16 '14 at 06:55
1

In 1984 u was telling my students using it was bad practice and it is probably going to become obsolete soon. Welcome to to the world of real computing. – Jakub Sep 16 '14 at 08:45
ah yes i remember it well from my first program in 73 – pm100 Jun 22 '18 at 17:35

Fortran IF statement without logical expression

1 Answers1