Re: Voyager (2nd Edition)

From: [at]} <skulski{>
Date: Wed, 17 Mar 2004 20:23:08 -0500 (EST)

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"Rex Couture" <rex{([at]})nowhere.xy

> The assertion is that [in FORTRAN]
> DO 20 I
100
> could be mistyped as
> DO20I
100
> and interpreted by the compiler as an assignment statement instead of the
> intended DO loop. While such an error is possible, it is worth pointing
> out that it would require a specific combination of three typing errors,
> any one of which should be noticed by the programmer.

Actually, only one error. FORTRAN would skip all whitespace.
The text "DO 20 I
100" would have been interpreted as "DO20I = 1.100"
The compiler would concatenate "DO 20 I" and interpret it as a name
of a variable.

I have heard this story several times, the first time it would have
been a lunar probe that crashed. I am not sure if the story is not
apocryphal. Would be interesting to know the original source.
I vaguely remember seeing this story in a well-known book on
computer programming, forgot which.

> In addition, the error is easily avoided with the statement
> IMPLICIT NONE, which has been available and known to FORTRAN
> programmers and encouraged for at least 20 years. The IMPLICIT
> NONE statement requires that all variables be explicitly declared,
> thereby catching many such typing errors.

IMPLICIT NONE was officialy introduced in FORTRAN-77, though I seem
to remember seeing it in CDC FORTRAN earlier. The abovementioned
story refers to FORTRAN IV, I think. But since we do not know the
source, the story may not be true.

Regards,

Wojtek

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Received on Thu Mar 18 2004 - 02:23:08 UTC