Strange problem with size_t | | |
For some time now I have forced myself to use size_t instead of int
when I'm using a loop to go thru an array.
But today I found something odd:
long hex2long(const string& s)
{
string alpha="0123456789ABCDEF";
const long base=16;
long power=1, n=1;
for(int i=s.size()-1; i>=0; --i)
{
n+=power*alpha.find(s[i]);
power*=base;
}
return n;
}
This works, but if I make the change the int for size_t, the program
behavies on a very unspected way.
What's wrong with that code? When I should definitely use size_t insted
of int?
Thanks.
| | | | re: Strange problem with size_t
Gaijinco wrote: Quote:
For some time now I have forced myself to use size_t instead of int
when I'm using a loop to go thru an array.
>
But today I found something odd:
>
long hex2long(const string& s)
{
string alpha="0123456789ABCDEF";
const long base=16;
long power=1, n=1;
for(int i=s.size()-1; i>=0; --i)
{
n+=power*alpha.find(s[i]);
power*=base;
}
return n;
}
>
This works, but if I make the change the int for size_t, the program
behavies on a very unspected way.
>
Which is?
--
Ian Collins. | | | | re: Strange problem with size_t
It prints an infinte amount of character until it crashes but I
supposed the exact behavior must be machine and compiler-dependant so I
did'nt want to get into details.
What's wrong with using size_t?
| | | | re: Strange problem with size_t
Gaijinco wrote: Quote:
For some time now I have forced myself to use size_t instead of int
when I'm using a loop to go thru an array.
>
But today I found something odd:
>
long hex2long(const string& s)
{
string alpha="0123456789ABCDEF";
const long base=16;
long power=1, n=1;
for(int i=s.size()-1; i>=0; --i)
size_t is unsigned, so i>=0 will never fail if i is a size_t.
--
Ian Collins. | | | | re: Strange problem with size_t
Gaijinco wrote: Quote:
For some time now I have forced myself to use size_t instead of int
when I'm using a loop to go thru an array.
>
But today I found something odd:
>
long hex2long(const string& s)
{
string alpha="0123456789ABCDEF";
const long base=16;
long power=1, n=1;
for(int i=s.size()-1; i>=0; --i)
{
n+=power*alpha.find(s[i]);
power*=base;
}
return n;
}
>
This works, but if I make the change the int for size_t, the program
behavies on a very unspected way.
>
What's wrong with that code? When I should definitely use size_t insted
of int?
>
Thanks.
>
size_t is unsigned, so it will never be less than zero; hence, the
expression (i>=0) is always true. Counting down with unsigned variables
can be tricky.
You could do something like:
....
const size_t size = s.size();
for(size_t i=size-1; i<size; --i)
{
....
--
Clark S. Cox III clarkcox3@gmail.com | | | | re: Strange problem with size_t
Gaijinco wrote: Quote:
For some time now I have forced myself to use size_t instead of int
when I'm using a loop to go thru an array.
>
But today I found something odd:
>
long hex2long(const string& s)
{
string alpha="0123456789ABCDEF";
const long base=16;
long power=1, n=1;
for(int i=s.size()-1; i>=0; --i)
{
n+=power*alpha.find(s[i]);
power*=base;
}
return n;
}
>
This works, but if I make the change the int for size_t, the program
behavies on a very unspected way.
size_t is an unsigned type. The idioms for counting down are therefore
different:
for(size_t i=s.size(); i-- 0; )
{
n+=power*alpha.find(s[i]);
power*=base;
}
Best
Kai-Uwe Bux | | | | re: Strange problem with size_t
Kai-Uwe Bux posted: Quote:
for(size_t i=s.size(); i-- 0; )
{
n+=power*alpha.find(s[i]);
power*=base;
}
My own personal favourite is:
for(size_t i = WHATEVER - 1; (size_t)-1 != i; --i)
(Cast is used to suppress compiler warning)
Or another one would be:
for(size_t i = WHATEVER -1; ~i; --i)
The latter example will break if integer promotion takes place (but I don't
think it should for a size_t).
--
Frederick Gotham | | | | re: Strange problem with size_t
Frederick Gotham wrote: Quote:
Kai-Uwe Bux posted:
> Quote:
> for(size_t i=s.size(); i-- 0; )
> {
> n+=power*alpha.find(s[i]);
> power*=base;
> }
>
>
My own personal favourite is:
>
for(size_t i = WHATEVER - 1; (size_t)-1 != i; --i)
>
(Cast is used to suppress compiler warning)
>
Or another one would be:
>
for(size_t i = WHATEVER -1; ~i; --i)
>
The latter example will break if integer promotion takes place (but I
don't think it should for a size_t).
Maybe this is a good opportunity to point out a little detail in the code I
provided: the use of "i-- 0" as opposed to "i-- != 0". I tend to prefer
reusable code. The version
for ( i = upper_bound; i-- 0; ) {
some code;
}
will work for signed and unsigned types. Thus, I could even use it when the
type of i is given by a template parameter.
Best
Kai-Uwe Bux | | | | re: Strange problem with size_t
"Kai-Uwe Bux" <jkherciueh@gmx.netwrote in message
news:eccok1$2k2$1@murdoch.acc.Virginia.EDU... Quote:
Maybe this is a good opportunity to point out a little detail in the code
I Quote:
provided: the use of "i-- 0" as opposed to "i-- != 0". I tend to prefer
reusable code. The version
>
for ( i = upper_bound; i-- 0; ) {
some code;
}
>
will work for signed and unsigned types. Thus, I could even use it when
the Quote:
type of i is given by a template parameter.
I don't see how "i-- != 0" fails to have this property. (Indeed, for
unsigned types it is exactly the same.) | | | | re: Strange problem with size_t
Philip Potter wrote: Quote:
"Kai-Uwe Bux" <jkherciueh@gmx.netwrote in message
news:eccok1$2k2$1@murdoch.acc.Virginia.EDU... Quote:
>Maybe this is a good opportunity to point out a little detail in the code
I Quote:
>provided: the use of "i-- 0" as opposed to "i-- != 0". I tend to
>prefer reusable code. The version
>>
> for ( i = upper_bound; i-- 0; ) {
> some code;
> }
>>
>will work for signed and unsigned types. Thus, I could even use it when
the Quote:
>type of i is given by a template parameter.
>
I don't see how "i-- != 0" fails to have this property. (Indeed, for
unsigned types it is exactly the same.)
If upper_bound happens to be negative, the != 0 version enters an infinite
loop, whereas the 0 version just does nothing.
Best
Kai-Uwe Bux | | | | re: Strange problem with size_t
In article <4ksc0uFdj9l1U2@individual.net>,
Ian Collins <ian-news@hotmail.comwrote: Quote:
>Gaijinco wrote: Quote:
>For some time now I have forced myself to use size_t instead of int
>when I'm using a loop to go thru an array.
>>
>But today I found something odd:
>>
>long hex2long(const string& s)
>{
> string alpha="0123456789ABCDEF";
> const long base=16;
> long power=1, n=1;
> for(int i=s.size()-1; i>=0; --i)
> {
> n+=power*alpha.find(s[i]);
> power*=base;
> }
> return n;
>}
>>
>This works, but if I make the change the int for size_t, the program
>behavies on a very unspected way.
>>
>Which is?
I'll wager a quick quess that the problem is in some case size yields 0
therfore size() - 1 is negative, but size_t is unsigned.
--
Greg Comeau / 20 years of Comeauity! Intel Mac Port now in alpha!
Comeau C/C++ ONLINE == http://www.comeaucomputing.com/tryitout
World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90.
Comeau C/C++ with Dinkumware's Libraries... Have you tried it? | | | | re: Strange problem with size_t
"Greg Comeau" <comeau@panix.comwrote... Quote:
In article <4ksc0uFdj9l1U2@individual.net>,
Ian Collins <ian-news@hotmail.comwrote: Quote:
>>Gaijinco wrote: Quote:
>>For some time now I have forced myself to use size_t instead of int
>>when I'm using a loop to go thru an array.
>>>
>>But today I found something odd:
>>>
>>long hex2long(const string& s)
>>{
>> string alpha="0123456789ABCDEF";
>> const long base=16;
>> long power=1, n=1;
>> for(int i=s.size()-1; i>=0; --i)
>> {
>> n+=power*alpha.find(s[i]);
>> power*=base;
>> }
>> return n;
>>}
>>>
>>This works, but if I make the change the int for size_t, the program
>>behavies on a very unspected way.
>>>
>>Which is?
>
I'll wager a quick quess that the problem is in some case size yields 0
therfore size() - 1 is negative, but size_t is unsigned.
"In some case"? In *all cases* the function is bound to run infinitely
if 'i' is 'size_t' because the condition in the 'for' statement (i>=0) is
simply always true (all unsigned values are greater than, or equal, zero).
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask | | | | re: Strange problem with size_t
Greg Comeau wrote: Quote:
In article <4ksc0uFdj9l1U2@individual.net>,
Ian Collins <ian-news@hotmail.comwrote:
> Quote:
>>Gaijinco wrote:
>> Quote:
>>>For some time now I have forced myself to use size_t instead of int
>>>when I'm using a loop to go thru an array.
>>>
>>>But today I found something odd:
>>>
>>>long hex2long(const string& s)
>>>{
>> string alpha="0123456789ABCDEF";
>> const long base=16;
>> long power=1, n=1;
>> for(int i=s.size()-1; i>=0; --i)
>> {
>> n+=power*alpha.find(s[i]);
>> power*=base;
>> }
>> return n;
>>>}
>>>
>>>This works, but if I make the change the int for size_t, the program
>>>behavies on a very unspected way.
>>>
>>
>>Which is?
>
>
I'll wager a quick quess that the problem is in some case size yields 0
therfore size() - 1 is negative, but size_t is unsigned.
I was prompting the OP the explain "behavies on a very unspected way".
Posters often make vague statements like "didn't work" where an exact
description would help to identify the problem.
--
Ian Collins. | | | | re: Strange problem with size_t
In article <ecd7or$9cn$1@news.datemas.de>,
Victor Bazarov <v.Abazarov@comAcast.netwrote: Quote:
>"Greg Comeau" <comeau@panix.comwrote... Quote:
>In article <4ksc0uFdj9l1U2@individual.net>,
>Ian Collins <ian-news@hotmail.comwrote: Quote:
>>>Gaijinco wrote:
>>>For some time now I have forced myself to use size_t instead of int
>>>when I'm using a loop to go thru an array.
>>>>
>>>But today I found something odd:
>>>>
>>>long hex2long(const string& s)
>>>{
>>> string alpha="0123456789ABCDEF";
>>> const long base=16;
>>> long power=1, n=1;
>>> for(int i=s.size()-1; i>=0; --i)
>>> {
>>> n+=power*alpha.find(s[i]);
>>> power*=base;
>>> }
>>> return n;
>>>}
>>>>
>>>This works, but if I make the change the int for size_t, the program
>>>behavies on a very unspected way.
>>>>
>>>Which is?
>>
>I'll wager a quick quess that the problem is in some case size yields 0
>therfore size() - 1 is negative, but size_t is unsigned.
>
>"In some case"? In *all cases* the function is bound to run infinitely
>if 'i' is 'size_t' because the condition in the 'for' statement (i>=0) is
>simply always true (all unsigned values are greater than, or equal, zero).
Indeed. I wasn't trying to say otherwise, but -- clearly poorly -- why
size_t is not always text replaceable for int.
--
Greg Comeau / 20 years of Comeauity! Intel Mac Port now in alpha!
Comeau C/C++ ONLINE == http://www.comeaucomputing.com/tryitout
World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90.
Comeau C/C++ with Dinkumware's Libraries... Have you tried it? |  |
| | |  /bytes/about
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