Re: Mariner 1 / FORTRAN bug

[robin.vowels@gmail.com]

On Friday, August 9, 2019 at 3:26:30 AM UTC+10, Jeffrey R. Carter wrote:
> On 8/8/19 5:40 PM, r......@gmail.com wrote:
> > 
> > DO 15 I = 1.100
> > 
> > when what should have been written was:
> > 
> > DO 15 I = 1,100
> > 
> > but somehow a dot had replaced the comma. Because Fortran ignores spaces, this was seen by the compiler as:
> > 
> > DO15I = 1.100
> > which is a perfectly valid assignment to a variable called DO15I and not at all what was intended."
> 
> Some people on here may not be aware that FORTRAN did not require declaration of 
> variables,

It still doesn't.

> so the above would create a new variable DO15I. I don't know what 
> flavor of FORTRAN was used in Mariner 1,

Marina 1 (1962) preceded FORTRAN 66.

> but FORTRAN 66 added the IMPLICIT 
> statement, which declared the type used for undeclared variables. Adding 
> IMPLICIT LOGICAL A-Z at the beginning of your code helped to catch these kinds 
> of things. FORTRAN 77 added IMPLICIT NONE, which required you to declare all 
> variables.

IMPLICIT NONE is an optional statement. If is is omitted,
variables do not need to be declared.

> Sometime in the late 1970s-early 1980s people moved away from punched 
> cards in ALL UPPER CASE and started calling the language Fortran.
> 
> Given the kind of testing such software would have undergone, I somehow doubt 
> that an absence of looping would have gone unnoticed,

If you read further on past the referenced article, you will see that
the looping DID go unnoticed. [try google]

> so I suspect this story is 
> apocryphal.

It wasn't.

> While it probably didn't cause the failure of a space probe, people 
> did get bitten by this language design flaw, which is an example of a 
> single-character error (added, omitted, or changed) that results in valid
> code.

Single-character errors are still possible, whatever the language.

> This is something that competent language designers go to great lengths to 
> avoid. Note that C and most of its descendants (most of the commonly used 
> languages today) have this flaw.