problem with float

could somebody explain why the following code gives "invalid" as the output?

// code

float w=2.3;
if(w==2.3)
puts("valid");
else
puts("invalid");

//********

when I tried the same with double datatype, it worked!!!
It also worked when I changed the if statement to

if(w==2.3f)
puts("valid");
else
puts("invalid");

> float w=2.3;
> if(w==2.3)
Because what you have here is

if ( (double)w == 2.3 )
The conversion from float to double introduces a minute error (compared to the result of calculating 2.3 as a double from scratch), resulting in an inequality

Your other two experiments did not have an implicit cast from float to double.

Rule: NEVER compare floats/doubles for equality

See here
In particular, the David Goldberg article down the page

Salem is right on the money here. You should also try and use floats sparingly. There are a few exceptions but overall it is safe to say that if want something to run quickly on even old computers floats can take a toll on speed.

>it is safe to say that if want something to run quickly on even old computers floats can take a toll on speed.
Bzzzzt! Wrong answer. This used to be true for older computers, but now math co-processors are pretty much on every new machine. So floating point math can sometimes be faster than integer math. Forget about performance until there is a real need and just use what best suits the problem. But be aware of the problems that floating point calculations can create.

-Prelude

It may be better to avoid testing floating point values for equality and instead check against a suitably small tolerance.

http://www.eskimo.com/~scs/C-faq/q14.5.html

> Bzzzzt! Wrong answer. This used to be true for older computers
And is still true for a number of processors which are either RISC architecture, or aimed at the embedded market.

>And is still true for a number of processors which are either RISC architecture, or aimed at the embedded market.
Which is why I was careful with my wording.

So floating point math can sometimes be faster than integer math.

-Prelude

This type of programs, beter to use double and
in printf use like
printf(" %5.2lf");

>printf(" %5.2lf");
%lf is a double for scanf, with printf this flag means long double.

-Prelude

>printf(" %5.2lf");
%lf is a double for scanf, with printf this flag means long double.

-Prelude

No. With regard to printf, the l in the %lf is not necessary -- %f works for both a double and a float. For a long double, use %Lf.

[EDIT]
Dinkumware Print Conversion Specifiers
[/EDIT]

>No. With regard to printf, the l in the %lf is not necessary -- %f works for both a double and a float. For a long double, use %Lf.
That's what I said.

-Prelude