The Industry choice

beliavsky at aol.com beliavsky at aol.com
Sun Jan 2 19:11:04 CET 2005


Roy Smith wrote:
>I think you've hit the nail on the head. In awk (and perl, and most
>shells, and IIRC, FORTRAN), using an undefined variable silently gets
>you a default value (empty string or zero). This tends to propagate
>errors and make them very difficult to track down.

You may recall correctly, but Fortran compilers have improved. The
following Fortran 90 program

integer, parameter :: n = 1
real :: x,y=2.0,z(n)
print*,"dog"
print*,x
z(n+1) = 1.0
print*,z
end

has 3 errors, all detected at compile time by the Lahey/Fujitsu Fortran
95 compiler, with the proper options:

2004-I: "xundef.f", line 2: 'y' is set but never used.
2005-W: "xundef.f", line 4: 'x' is used but never set.
2153-W: "xundef.f", line 5, column 1: Subscript out of range.

At run time, the output is

dog
The variable (x) has an undefined value.
Error occurs at or near line 4 of _MAIN__

Running Python 2.4 on the Python analog,

n = 1
y = 2.0
z = range(n)
print "dog"
print x
z[n] = 1.0
print z

one error is caught:

dog
Traceback (most recent call last):
File "xundef.py", line 5, in ?
print x
NameError: name 'x' is not defined

You will see the out-of-bounds error for z only after fixing the
undefined-x error. No warning is ever given about y, which is set but
never used. In practice, 'print "dog"' could be some operation taking
hours. Can PyChecker find all the problems in a single run, without
executing 'print "dog"'? If so, it would be great if it were integrated
with the CPython interpreter.

One reason interpreted languages like Python are recommended to
beginners is to avoid the edit/compile/debug cycle. But I think it is
faster and less frustrating to have many errors caught in one shot.




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