Why doesn't this program work as expected?

[Alex Buell]

#include <vector>
#include <algorithm>
#include <iostream>

using namespace std;

template <typename T>
class LessThan
{
private:
T comp;
public:
LessThan(T val) : comp(val) {}
void setComp(const T& val) { comp = val; }
T getComp(void) { return comp; } const
bool operator()(cons t T& value) { return value < comp; }
};

int main()
{
double darr[10] = { 234.4, 11.1, 22.2, 500.0, 50.0, 11.1, 12.1,
600.6, 99.9, 0.0 }; vector<double> v2(darr, darr + 10);

double dal = 100.0;
LessThan<double > lt2(dal);

int count1 = count_if(v2.beg in(), v2.begin(), lt2);
cout << "There are " << count1 << " values less than " << dal
<< endl;

return 0;
}

Expected output should have been "There are 7 values less than 100",
but it actually comes up with: "There are 0 values less than 100". Why?

I'm using GCC 3.4.4 on Linux

Thanks
--

http://www.munted.org.uk

"Honestly, what can I possibly say to get you into my bed?" - Anon.

[Mark P]

Re: Why doesn't this program work as expected?

Ready to kick yourself?

Alex Buell wrote:[color=blue]
> #include <vector>
> #include <algorithm>
> #include <iostream>
>
> using namespace std;
>
> template <typename T>
> class LessThan
> {
> private:
> T comp;
> public:
> LessThan(T val) : comp(val) {}
> void setComp(const T& val) { comp = val; }
> T getComp(void) { return comp; } const
> bool operator()(cons t T& value) { return value < comp; }
> };
>
> int main()
> {
> double darr[10] = { 234.4, 11.1, 22.2, 500.0, 50.0, 11.1, 12.1,
> 600.6, 99.9, 0.0 }; vector<double> v2(darr, darr + 10);
>
> double dal = 100.0;
> LessThan<double > lt2(dal);
>
> int count1 = count_if(v2.beg in(), v2.begin(), lt2);[/color]

Hmm, begin() to begin()... :)

-Mark
[color=blue]
> cout << "There are " << count1 << " values less than " << dal
> << endl;
>
> return 0;
> }
>
> Expected output should have been "There are 7 values less than 100",
> but it actually comes up with: "There are 0 values less than 100". Why?
>
> I'm using GCC 3.4.4 on Linux
>
> Thanks[/color]

[Alex Buell]

Re: Why doesn't this program work as expected?

On Thu, 23 Feb 2006 23:17:00 GMT Mark P
<usenet@fall200 5REMOVE.fastmai lCAPS.fm> waved a wand and this message
magically appeared:
[color=blue][color=green]
> > int count1 = count_if(v2.beg in(), v2.begin(), lt2);[/color]
>
> Hmm, begin() to begin()... :)[/color]

Doh! No wonder it didn't work as expected! lol, thanks.

--

http://www.munted.org.uk

"Honestly, what can I possibly say to get you into my bed?" - Anon.

[Alf P. Steinbach]

Re: Why doesn't this program work as expected?

* Alex Buell:[color=blue]
> #include <vector>
> #include <algorithm>
> #include <iostream>
>
> using namespace std;
>
> template <typename T>
> class LessThan[/color]

Are you aware that the standard library provides std::less and
std::bind2nd, which together gives you this functionality?
[color=blue]
> {
> private:
> T comp;
> public:
> LessThan(T val) : comp(val) {}
> void setComp(const T& val) { comp = val; }
> T getComp(void) { return comp; } const[/color]

Note that that 'const' actually applies to the 'bool' on the line
below... 'getComp' should be declared as 'const'. Also, 'comp' would
IMO be a better name (I don't like 'computeSin' & friends...), and even
better, som other name than 'comp', e.g. just 'value'.
[color=blue]
> bool operator()(cons t T& value) { return value < comp; }[/color]

Should ideally be declared as 'const'.
[color=blue]
> };[/color]

[color=blue]
>
> int main()
> {
> double darr[10] = { 234.4, 11.1, 22.2, 500.0, 50.0, 11.1, 12.1,
> 600.6, 99.9, 0.0 }; vector<double> v2(darr, darr + 10);[/color]

The array 'darr' should ideally be declared 'const'.

Ditto for the vector 'v2', unless you plan on modifying the values.

[color=blue]
> double dal = 100.0;
> LessThan<double > lt2(dal);
>
> int count1 = count_if(v2.beg in(), v2.begin(), lt2);
> cout << "There are " << count1 << " values less than " << dal
> << endl;
>
> return 0;
> }
>
> Expected output should have been "There are 7 values less than 100",
> but it actually comes up with: "There are 0 values less than 100". Why?[/color]

Perhaps you meant to use begin() and end() instead of begin() and begin()?


--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Alex Buell]

Re: Why doesn't this program work as expected?

On Fri, 24 Feb 2006 00:22:25 +0100 "Alf P. Steinbach" <alfps@start.no >
waved a wand and this message magically appeared:
[color=blue][color=green]
> >
> > int main()
> > {
> > double darr[10] = { 234.4, 11.1, 22.2, 500.0, 50.0, 11.1, 12.1,
> > 600.6, 99.9, 0.0 }; vector<double> v2(darr, darr + 10);[/color]
>
> The array 'darr' should ideally be declared 'const'.
>
> Ditto for the vector 'v2', unless you plan on modifying the values.[/color]

Is there a good reason to declare them as const? Would it result in
better optimisations?

--

http://www.munted.org.uk

"Honestly, what can I possibly say to get you into my bed?" - Anon.

[Alf P. Steinbach]

Re: Why doesn't this program work as expected?

* Alex Buell:[color=blue]
> On Fri, 24 Feb 2006 00:22:25 +0100 "Alf P. Steinbach" <alfps@start.no >
> waved a wand and this message magically appeared:
>[color=green][color=darkred]
>>> int main()
>>> {
>>> double darr[10] = { 234.4, 11.1, 22.2, 500.0, 50.0, 11.1, 12.1,
>>> 600.6, 99.9, 0.0 }; vector<double> v2(darr, darr + 10);[/color]
>> The array 'darr' should ideally be declared 'const'.
>>
>> Ditto for the vector 'v2', unless you plan on modifying the values.[/color]
>
> Is there a good reason to declare them as const?[/color]

Yes.

[color=blue]
> Would it result in better optimisations?[/color]

Probably not.

The habit of declaring things 'const', whenever possible, results in a
much better chance of a correct program.

If the program doesn't need to be correct you can make it arbitrarily
fast... ;-)


--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Alex Buell]

Re: Why doesn't this program work as expected?

On Fri, 24 Feb 2006 00:35:21 +0100 "Alf P. Steinbach" <alfps@start.no >
waved a wand and this message magically appeared:
[color=blue][color=green]
> > Is there a good reason to declare them as const?[/color]
>
> Yes.
>[color=green]
> > Would it result in better optimisations?[/color]
>
> Probably not.
>
> The habit of declaring things 'const', whenever possible, results in a
> much better chance of a correct program.[/color]

In case someone else comes along and tries to modify the values, no
doubt, yes?
[color=blue]
> If the program doesn't need to be correct you can make it arbitrarily
> fast... ;-)[/color]

lol, that's true.

--

http://www.munted.org.uk

"Honestly, what can I possibly say to get you into my bed?" - Anon.