Re: how to use Form()

Hi Zhu,

On Wed, 2006-05-10 at 14:32 +0800, Zhu FengRong wrote:
> Hi,
>
> I can use the code to make a plot:
> int i=3,j=4;
> spt->Draw(Form("count[%d][%d]:time/6/24+1",i,j)
> but how to add the Option like this:
> count[i][j]>0
> in Form() function ?

Note, that `Form' uses a circular buffer, and return a pointer to that buffer. That means, that subsequent calls to Form will overwrite the buffer and previous results. Therefor, unless you know that the operation is atomic, you should copy the result of form to some store. E.g.,

        TString var(Form("count[%d][%d]:time/6/24+1", i, j));
        TString cut(Form("count[%d][%d]>0", i, j));
        spt->Draw(var.Data(), cut.Data());
        

If you don't do this, but do the naiive

        spt->Draw(Form("count[%d][%d]:time/6/24+1", i, j), 
              Form("count[%d][%d]>0", i, j)); 

One of these calls to Form could overwrite the other call (note, in which order the arguments are evaluated is unspecified). The reason is, that Form looks something like

   const char* Form(const char* f, ...)
   {

     static char[1024] buf;
     va_list ap;
     va_begin(ap,f);
     vsprintf(buf, f, ap);
     va_end(ap);
     return buf;
   }
        

Yours,

-- 
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