Thread: [Dev-C++] double and float ??

Hi all,

I tried the following code
============================
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

main(){
    double a,b;
    scanf("%f",&a);
    printf("\n");
    scanf("%f",&b);
    printf("\n\n%f   %f",a,b);
    getch();
    exit(EXIT_SUCCESS);   
}
==============================
I enter any numbers and the output is
0.0000   0.00000
but, when I change the "double" with "float", the code
gives me the right answer.

Is there any explanation why I can't use "double" with
"%f" in dev-cpp ? Maybe I use old c++ manual lol. How
can I print the "double" number ?

Titi

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No, you can't use an old c++ manual (or actually C manual).
It must be a broken manual.

The runtime library must know the physical size of the variable to be able
to initialize it.

For printf() you send a value. The standard says that functions without a
prototype - or variadric functions - should have the variables upgraded
from float to double on call. This makes printf() support float and
double.

However, scanf() doesn't take a float our double. It takes a pointer to a
float or double. To perform the assign - it has to look at the
"formatting" parameter to decide what it should assign to.

%f specifies float
%lf specifies double
%Lf specifies long double (when supported)

/Per W

On Thu, 23 Feb 2006, Titi Anggono wrote:

> Hi all,
>
> I tried the following code
> ============================
> #include <stdio.h>
> #include <stdlib.h>
> #include <conio.h>
>
> main(){
>     double a,b;
>     scanf("%f",&a);
>     printf("\n");
>     scanf("%f",&b);
>     printf("\n\n%f   %f",a,b);
>     getch();
>     exit(EXIT_SUCCESS);
> }
> ==============================
> I enter any numbers and the output is
> 0.0000   0.00000
> but, when I change the "double" with "float", the code
> gives me the right answer.
>
> Is there any explanation why I can't use "double" with
> "%f" in dev-cpp ? Maybe I use old c++ manual lol. How
> can I print the "double" number ?
>
> Titi
>
> __________________________________________________
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Per Westermark escreveu:
> %Lf specifies long double (when supported)
>   

Which the MS runtime doesn't seem to support... from my experiences (I 
don't have the MS compiler, I just asked somebody else to compile some 
code and send me the program output and some assembly dumps), VC++ 
"demotes" long doubles to plain doubles in printf.

But now that you mentioned %Lf, I would like to confirm with anyone here 
who has VC++; what does happen when trying to printf a long double on 
VC++? For me, it looks like VC++ tries a "magic" conversion in variadic 
functions, but if it is true, it may break legit code (when trying to 
va_arg a long double where a plain double was put).



---
Daniel K. O.

VC++ doesn't have an long double data type. A long double variable will be
8 byte large, just as a double. In short, there will be no problem trying
to send a long double parameter to a variadic function. Both sender and
receiver will assume 8 bytes storage size.

It is only code that relies on extra precision and/or range of a long
double that will have problems. For example a numeric algorithm using a
numerically unstable method of solving an equation might diverge with
double numbers while it always have enough precision to converge on a
solution with true long doubles.

/Per W

On Fri, 24 Feb 2006, Daniel K. O. wrote:

> Per Westermark escreveu:
> > %Lf specifies long double (when supported)
> >
>
> Which the MS runtime doesn't seem to support... from my experiences (I
> don't have the MS compiler, I just asked somebody else to compile some
> code and send me the program output and some assembly dumps), VC++
> "demotes" long doubles to plain doubles in printf.
>
> But now that you mentioned %Lf, I would like to confirm with anyone here
> who has VC++; what does happen when trying to printf a long double on
> VC++? For me, it looks like VC++ tries a "magic" conversion in variadic
> functions, but if it is true, it may break legit code (when trying to
> va_arg a long double where a plain double was put).
>
>
>
> ---
> Daniel K. O.
>
>