RE: [AD] Small change in file.c

[Shawn Hargreaves <shargreaves@xxxxxxxxxx>]

Title: RE: [AD] Small change in file.c

Peter Wang writes:
> > why doesn't this code in file.c (line: 342) use a for loop?
> ...
> > Is there a good reason a while loop is used?
>
> Personal preference I guess.  I'd use a `while' loop there too.

for loops are a very easily abused feature of the C language,
because you can do almost any type of looping construct with
them, and can also put other code which has nothing at all to
do with the loop inside what looks like the loop control
section. This can produce some very strange statements with
all sorts of odd side-effects, and can be very confusing to
read as when people see a for statement they tend to think
"iterate over a fixed number of items or values".

My personal rule of good coding style is to limit for loops
to a small set of simple, idiomatic types which any C coder
will instantly recognise, such as:

    // iterate over a range of values
    for (i=a; i<b; i++) {
        ...
    }

    // iterate through a linked list
    for (ptr=head; ptr; ptr=ptr->next) {
        ...
    }

    // iterate through an STL container
    for (ContainerType::iterator it = container.begin(); it != container.end(); it++) {
        ..
    }

Any more unusual types of loop (for instance using <= instead
of a < condition, or decrementing rather than incrementing
the pointer, or with multiple exit conditions), are risky
as someone in a hurry will tend to assume they are one of
the standard versions. In those cases I prefer to write it
as a while loop, which don't have the same limited number of
idiomatic uses, so people are more likely to look carefully
and notice that something odd is going on. By the same
principle, I think it is bad style to use a while loop for
one of these idiomatic cases. Experienced C coders will
recognise:

    for (i=0; i<n; i++) {
        ...
    }

more quickly than:

    i = 0;
    while (i < n) {
        ...
        i++;
    }

And where possible, it's good to rearrange loops to fit into
one of the idiomatic styles. The use of < rather than <= is
almost universal as a terminator (because C arrays are zero
based), hence if you want to process everything up to and
including the end value, you could write:

    for (i=0; i<=n; i++) {
        ...
    }

but someone reading that might assume it was a mistake, and
that you are accidentally processing one element past the end
of your array. If you instead write this as:

    for (i=0; i<n+1; i++) {
        ...
    }

you make it much more obvious that you really do mean to
include that extra one value, so everyone will know this
isn't a mistake, and will be in less danger of 'fixing'
it for you :-)

        Shawn.