Steven D'Aprano wrote:
On Fri, 16 Apr 2010 08:48:03 -0700, Aahz wrote:
In article <4bb92850$0$8827$c3e8...@news.astraweb.com>, Steven D'Aprano
<st...@remove-this-cybersource.com.au> wrote:
Nevertheless, it is a common intuition that the list comp variable
should *not* be exposed outside of the list comp, and that the for-loop
variable should. Perhaps it makes no sense, but it is very common --
I've never heard of anyone being surprised that the for-loop variable is
exposed, but I've seen many people surprised by the fact that list-comps
do expose their loop variable.
I've definitely seen people surprised by the for-loop behavior.
What programming languages were they used to (if any)?
I don't know of any language that creates a new scope for loop variables,
but perhaps that's just my ignorance...
It's not clear whether a language like C or C++ has "loop variables."
It just has variables with varying scope depending on where they're
declared. And you can add extra braces with the sole purpose being to
introduce new variable scoping.
But two things that changed as C evolved were where you could introduce
new variables, and the lifetime of variables introduced in the loop
control structure, rather than inside the braces. The first change was
in C++ from the start, but I think the second change was also an
evolution in C++.
1) In original C, all declarations in a given scope had to occur before
any executable code began. For example, the following was illegal:
int a=12, b=42;
myfunc(a, b);
int c = 9; /* illegal */
2) In original C, and I think in C++, the lifetime of i lasted long
after the loop ended.
for (int i=0; i< limit; ++i)
{
z += i;
}
i is still valid after this curly brace
In C99, and at least in later C++, the scope of i ends with the curly,
as though there were another invisible pair of braces:
{
for (int i=0; i< limit; ++i)
{
z += i;
}}
i is no longer valid here
Because C and C++ have explicit declarations, people who need the loop
variable after the loop is done can simply declare the loop variable
before the for statement.
DaveA
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