Re: [Dev-C++] Number backwards?

[Fabien V. <fv...@ne...>]

Hi.

Palindrom numbers are numbers which you can "read" from the begining or the
end.

11
151
1551
15251
152251

and so on....


----- Original Message -----
From: "Bolek Vrany" <vr...@pa...>
To: <dev...@li...>
Sent: Saturday, April 21, 2001 2:50 PM
Subject: Re: [Dev-C++] Number backwards?


> Hi,
>
> what palindrom numbers are?
>
> > Hello,
> >     To use a number backward first you need to separare the digits of
> > the no.
> >
> >    To do this say you have nNo=5234
> >    To separate thousands digit ie  5   thousnad_digit = nNo/1000;
> >    To get the hundred digit ie 2          hunreds_digit =
> > (nNo%1000)/100;
> >           (  In above nNo%1000 will give 234  and then 234/100 will
> > give 2 )
> >    To get tens digit ie 3                     tens_digit =
> > (nNo%100)/10;
> >           ( In above nNo%100 will give 34 and then 34/10 will give 3 )
> >    To get the units digit ie 4                 unit_digit = (nNo%10);
> >           ( In Above nNo%10 Will give 4 )
> >
> >                  % is Called MODULUS OPERATOR and it give remainder of
> > a division.
> >
> >    Now you have separated all the digits of original no now to revert
> > the no. simply change the units digit to thousands digit,
> > tens digit to hundreds digit, hundreds digit to tens digit, thousands
> > digit to unit digit.  The nos get reversed.
> >
> >       RevNo =  unit_digit*1000 + tens_digit*100 + hundreds_digit*10 +
> > thousand_digit ;
> >
> >         (  This above expression is equal to   RevNo = 4*1000 + 3*100
> > + 2*10 + 5 ;   therefore RevNo=4325 )
>
> Sorry for talking about my function again. I am not sure what Jesper
> Ekberg needs to do - if he needs to convert 00012345 to 54321000 then
> the above is probably the best algorithm - provided you know the maximum
> number of digits. However, if he needs to convert 00012345 to just
> 54321, then he needs to find the number of digits first. That's why it
> is better to do it this way:
>
> unsigned backwards( unsigned n)
> // Reverse order of digits (in decadic)
> {
>   unsigned result = 0;
>
>   while (n > 0) {
>     result = 10*result + n % 10;
>     n = n / 10;
>   }
>   return result;
> }
>
> Sorry for not explaining it in previous mails. result is simply the
> result. 10*result shifts the result one digit left. n % 10 is the last
> digit of n and n = n/10 shifts n one digit right. So assuming n=12345 at
> the beginning it does the following:
> result=0, n=12345
> first iteration:
>   result = 10*0 + 5 // == 5
>   n = 1234;
> second it.:
>   result = 10*5 + 4; // == 54
>   n = 123;
> third it.:
>   result = 10*54 + 3; // == 543
>   n = 12;
> etc.
>
> This way he needs not know the number of digits.
>
> !!! WARNING: (I should have realized it immediately) There is a problem
> you will always have - all algorithms will lead to integer overflow in
> some cases. Assume max. unsigned is 65535 and you want to convert 12347.
> The correct result is 74321, which outside the range of unsigned. The
> correct version of my function is
>
> #include <stdexcept>
> #include <limits>
>
> unsigned backwards( unsigned n)
> // Reverse order of digits (in decadic)
> {
>   unsigned result = 0, max = numeric_limits<unsigned>.max;
>
>   while (n > 0) {
>     if ((result > max / 10) ||
>         // avoid overflow in multiplying 10*result
>         (n % 10 > max - 10*result))
>         // avoid overflow in adding n % 10
>        throw overflow_error( "Overflow in function backwards");
>     result = 10*result + n % 10;
>     n = n / 10;
>   }
>   return result;
> }
>
> Or you must either restrict the range of n or the type of result must be
> at least one digit wider than the type of n. Might be double result and
> unsigned (long) n.
> --
> ============================================================
>                            Ing. Boleslav Vrany
>                            Stanice techniku DDM hl. m. Prahy
>                            Pod Juliskou 2a
>                            Praha 6 - Dejvice
>                            16000
>                            Czech Republic
>
>                            tel. (420 2) 243 10 240
>
>
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