Re: How to print this function?

[Sergei Steshenko]

----- Original Message -----
> From: marco atzeri <address@hidden>
> To: address@hidden
> Cc: 
> Sent: Tuesday, May 22, 2012 10:27 PM
> Subject: Re: How to print this function?
> 
> On 5/22/2012 8:26 PM, Sergei Steshenko wrote:
>> 
>> 
>>  Though not related to your original question, 0.1 may be a periodic 
> fraction in your _binary_ CPU, so you better write
>> 
>> 
>>  t = 0.1 * [-10:1:10];
>> 
> 
> irrilevant in this case
> 
> octave:4> [-1:0.1:1;]-0.1*[-10:1:10;] <eps
> ans =
> 
>   Columns 1 through 20:
> 
>     1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1 
> 1   1   1
> 
>   Column 21:
> 
>     1
> 
> regards
> Marco
> 


And what ? You are going to perform relevance on case per case basis ?

And you are absolutely sure you won't forget to check relevance if you switch 
to 'long double' type ?

But you didn't get my point in the first place, so I'll try to explain it in 
more detail using "C".


In "C" you can write

/* variant 1 */
double a[21];

int i;

unsigned step = 1;

for(i = -10; i <= 10; i++)
  {
  a[i + 10] = 0.1 * (double)i;
  }


/* variant 2 */
double a[21];

double step = 0.1;

double d;

unsigned i = 0;
for(d = -1.0; d <= 1; d += step)
  {
  a[i++] = d;
  }


Variant 2 is _wrong_. This is because you actually can't know without case per 
case analysis whether 'step' is slightly above or slightly below the expected 
value ('0.1' in this case).

Because of this you can _not_ be sure how many times the loop body will be 
executed, so in "C" you can also have an index overrun.

This is a "C" FAQ.

Enter "using floating point variables in loops" into your favorite web search 
engine and read, for example, this:

https://www.securecoding.cert.org/confluence/display/seccode/FLP30-C.+Do+not+use+floating+point+variables+as+loop+counters
.

And actually this is a very basic thing in numerical methods.

Unfortunately, modern generation of developers is mostly illiterate regarding 
issues of numeric accuracy.

Regards,
  Sergei.

P.S. Kindergarten .......................