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Help please, strange behavior

i'm sorry i posted this in other groupes, and i didn't see it, and as
this group is most actif, i repost it here, and sorry for
mutliposting:

Hi,
i'm using VC6, i have this declaration:

typedef struct tagTLimite {
double Debut;
double Fin;
}Limites;

typedef struct TagElemTab {
double NivY;
bool Existe;
std::vector<Lim itesPtLimites;
}ElemTabCont;

void myfunc( )
{
......
ElemTabCont * ContNonOrd;//contour non ordone
long nbNiveau = (yhi-ylo)/stepY +1 ;
ContNonOrd = new ElemTabCont [nbNiveau];
......
}
i don't know why,nbNiveau is equal to (yhi-ylo)/stepY, it didn't
+1????and even if i put:
ContNonOrd = new ElemTabCont [nbNiveau+1]; the size of ContNonOrd =
nbNiveau????

Feb 14 '07
31 1663
On 14 fév, 15:42, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
Simply_Red wrote:
when i just put long nbNiveau = (yhi-ylo)/stepY. i get nbNiveau =
160 not 159
i was wrong it's 159...........
and if i use float, is there pbs with the precision???

Of course. Just like if you use 'double', only worse.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
So i got to use:

float FRound(double f, BYTE b)
{
char sz[20];
char szFmt[5] = { '%','.','0'+b,' f', 0 };
sprintf(sz, szFmt, f);
return atof(sz);
}

and put :
long nbNiveau = FRound ((y1-y2)/y3+1,0);

Feb 14 '07 #21
On 14 fév, 15:43, Rolf Magnus <ramag...@t-online.dewrote:
Well, that's not the case. In C++, when converting a floating point value
into an integer, the fractional part is always truncated.
now i know it, but what i found strange is that, if i put the same
instruction in watch window(debug), i get the result that i need. (yhi-
ylo)/stepy+1 became 161, in watch window and nbNiveau = 160

Feb 14 '07 #22
Simply_Red wrote:
On 14 fév, 15:42, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
>Simply_Red wrote:
>>>when i just put long nbNiveau = (yhi-ylo)/stepY. i get nbNiveau =
160 not 159
>>i was wrong it's 159...........
>>and if i use float, is there pbs with the precision???

Of course. Just like if you use 'double', only worse.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

So i got to use:

float FRound(double f, BYTE b)
{
char sz[20];
char szFmt[5] = { '%','.','0'+b,' f', 0 };
sprintf(sz, szFmt, f);
return atof(sz);
}

and put :
long nbNiveau = FRound ((y1-y2)/y3+1,0);
Ugh!... Please don't do that. For any 'f' greater than 1e10
it will overrun the buffer ('sz'), which is how security holes
get created/exploited.

What is your goal here? Is (y1-y2) very close to N * y3 or is
it arbitrary? If it's close, you can always try

long nbNiveau = (y1-y2)/y3 + 1; // your formula
if ((nbNiveau - 1) * y3 < (y1-y2)) // verify!
++nbNiveau; // bump it up!

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
Feb 14 '07 #23
Simply_Red wrote:
On 14 fév, 15:43, Rolf Magnus <ramag...@t-online.dewrote:
>Well, that's not the case. In C++, when converting a floating point
value into an integer, the fractional part is always truncated.
now i know it, but what i found strange is that, if i put the same
instruction in watch window(debug), i get the result that i need.
(yhi- ylo)/stepy+1 became 161, in watch window and nbNiveau = 160
Apparently your "watch window" rounds it before outputting. Try

printf("%f", (yhi-ylo)/stepy+1)

and you're likely to see 161 as well. Conversion to text rounds
the value (and lies to you).

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
Feb 14 '07 #24
Simply_Red wrote:

i know this, but my pb is that i need
It's best to post in straight English with strange abreviations like
"pb". I assume you mean "problem", but that's not a standard
abreviation in English, except as the chemical name of lead.


Brian
Feb 14 '07 #25
Default User wrote:
Simply_Red wrote:

>i know this, but my pb is that i need

It's best to post in straight English with strange abreviations like
"pb". I assume you mean "problem", but that's not a standard
abreviation in English, except as the chemical name of lead.
V.E.R.A. offes "Pipeline Burst".
Feb 15 '07 #26
Rolf Magnus wrote:
Default User wrote:
Simply_Red wrote:

i know this, but my pb is that i need
It's best to post in straight English with strange abreviations like
"pb". I assume you mean "problem", but that's not a standard
abreviation in English, except as the chemical name of lead.

V.E.R.A. offes "Pipeline Burst".
And we all know how painful that can be.


Brian
Feb 15 '07 #27
On 14 fév, 16:09, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
Ugh!... Please don't do that. For any 'f' greater than 1e10
it will overrun the buffer ('sz'), which is how security holes
get created/exploited.
I don't understand what is le10???
>
What is your goal here? Is (y1-y2) very close to N * y3 or is
it arbitrary? If it's close, you can always try
long nbNiveau = (y1-y2)/y3 + 1; // your formula
if ((nbNiveau - 1) * y3 < (y1-y2)) // verify!
++nbNiveau; // bump it up!
Y1,Y2 and Y3 aren't arbitrary and i know that (y1-y2)/y3 = N
(integer).
and if i use:

long LRound(double f)
{
char sz[20];
char szFmt[5] = { '%','.','0','f' , 0 };
sprintf(sz, szFmt, f);
return atol(sz);
}

Feb 15 '07 #28
On 14 fév, 17:13, "Default User" <defaultuse...@ yahoo.comwrote:
Simply_Red wrote:
i know this, but my pb is that i need

It's best to post in straight English with strange abreviations like
"pb". I assume you mean "problem", but that's not a standard
abreviation in English, except as the chemical name of lead.

Brian
Sorry, this abreviation is used in french.

Feb 15 '07 #29
Simply_Red wrote:
On 14 fév, 17:13, "Default User" <defaultuse...@ yahoo.comwrote:
>Simply_Red wrote:
>>i know this, but my pb is that i need

It's best to post in straight English with strange abreviations like
"pb". I assume you mean "problem", but that's not a standard
abreviation in English, except as the chemical name of lead.

Brian

Sorry, this abreviation is used in french.
Good to know. c.l.c++ is an English-speaking newsgroup, though.
Feb 15 '07 #30

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