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 Gaijinco 03-26-2006 04:31 PM

Problem with recursion

I'm trying to do a function that returns a string which is a string the
characters in inverse order of the one given as a parameter. It
compiles but it crashes! Can anyone help me?

string inverse(string s, int pos, int size)
{
if(pos==size-1)
return s;
else
{
int idx = s.size()-1;
char character = s.at(idx);
s.erase(idx);
s.insert(s.begin()+pos,character);
inverse(s,pos+1,size);
}
}

 Maxim Yegorushkin 03-26-2006 04:51 PM

Re: Problem with recursion

Gaijinco wrote:
> I'm trying to do a function that returns a string which is a string the
> characters in inverse order of the one given as a parameter. It
> compiles but it crashes! Can anyone help me?
>
> string inverse(string s, int pos, int size)
> {
> if(pos==size-1)

Better:

if(pos >= size)
return s;

> return s;
> else
> {
> int idx = s.size()-1;
> char character = s.at(idx);
> s.erase(idx);
> s.insert(s.begin()+pos,character);
> inverse(s,pos+1,size);

This probably should be:

inverse(s, pos + 1, size - 2);

> }
> }

 davidrubin@warpmail.net 03-27-2006 05:29 AM

Re: Problem with recursion

Gaijinco wrote:
> I'm trying to do a function that returns a string which is a string the
> characters in inverse order of the one given as a parameter. It
> compiles but it crashes! Can anyone help me?
>
> string inverse(string s, int pos, int size)

Your implementation make far too many copies, and the contract is
unclear. If I had to guess, I'd say it returns a string which is a copy
of the characters s[pos]..s[pos+size] reversed. Is this what you mean?

 Gaijinco 03-27-2006 08:48 PM

Re: Problem with recursion

Yeah, if you input carlos it would output solrac but there is something
quite odd, this works fine:

void inverse(string s, int pos, int size)
{
if(pos==size-1)
{
cout << s;
return s;
}
else
{
int idx = s.size()-1;
char character = s.at(idx);
s.erase(idx);
s.insert(s.begin()+pos,character);
inverse(s,pos+1,size);
}
}

But if I try to return the string, then the program crashes. I'm not
sure why is that!

 Howard 03-27-2006 11:54 PM

Re: Problem with recursion

"Gaijinco" <gaijinco@gmail.com> wrote in message
> Yeah, if you input carlos it would output solrac but there is something
> quite odd, this works fine:
>
> void inverse(string s, int pos, int size)
> {
> if(pos==size-1)
> {
> cout << s;
> return s;
> }
> else
> {
> int idx = s.size()-1;
> char character = s.at(idx);
> s.erase(idx);
> s.insert(s.begin()+pos,character);
> inverse(s,pos+1,size);
> }
> }
>
> But if I try to return the string, then the program crashes. I'm not
> sure why is that!
>

You defined the function as void, but then have a "return s" statement in
the first part.

If it is supposed to return a string, then define it to return a string.
And then you also need a return statement for the else condition. (Probably
"return inverse(s,pos+1,size):"? I haven't tested the thought.)

If it is not supposed to return a string, then it should be operating on s
directly, and never returning a value. In that case, make s a string&
instead, and change "return s;" to just "return;".

-Howard

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