Float problem

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02-27-2008
+Azazel+

Float problem

Hey guys! I think I must be going crazy...

Code:

int main() { float test = 48 / 512; printf("%f", test); return (0); }

gives me 0.0000. Why? How can I get the answer?
02-27-2008
Banana Man

48 / 512 is less than 0. Both 48 and 512 are integers so the whole division is done on integers and the precision is lopped off leaving you with 0. Change one or both of the numbers to float and it'll work.

Code:

int main() { float test = 48.0f / 512.0f; printf("%f", test); return (0); }
02-27-2008
esbo

Quote:

Originally Posted by Banana Man

48 / 512 is less than 0. Both 48 and 512 are integers so the whole division is done on integers and the precision is lopped off leaving you with 0. Change one or both of the numbers to float and it'll work.

Code:

int main() { float test = 48.0f / 512.0f; printf("%f", test); return (0); }

You don't need to add the f at the end of the value, a decimal point is enough to make
it work. Infact if you miss out the decimal point it won't compile on my machine!!

Code:

int main() { float test = 48f/512f; printf("%f", test); return (0); }

Code:

invalid sufix 'f' on integer constant,

So I am not sure what the f does,
02-27-2008
robwhit

1.0 is a double. 1.0f is a float.
02-27-2008
esbo

Quote:

Originally Posted by robwhit

1.0 is a double. 1.0f is a float.

The result is a float though - well it works anyway, so what difference does it make?
02-27-2008
Elysia

Quote:

Originally Posted by esbo

You don't need to add the f at the end of the value, a decimal point is enough to make
it work. Infact if you miss out the decimal point it won't compile on my machine!!

Leaving it out will do integer division, hence giving you 0. That is no error or warning.
If you add decimals, you get a double, but some compilers won't warn about that.

Quote:

So I am not sure what the f does,

So again, as others stated, it makes the number a float. It needs to be 1.0f, not 1f.

And for once, you used "int main" and "return 0". Congratulations! Must have been hard on you.
02-27-2008
esbo

Quote:

Originally Posted by Elysia

Leaving it out will do integer division, hence giving you 0. That is no error or warning.

Not if you add a decimal point it won't.

Quote:

Originally Posted by Elysia

If you add decimals, you get a double, but some compilers won't warn about that.
So again, as others stated, it makes the number a float. It needs to be 1.0f, not 1f.

It only make it a float if it has a decimal point, otherwise it just gives you a compiler error.

Quote:

Originally Posted by Elysia

And for once, you used "int main" and "return 0". Congratulations! Must have been hard on you.

Not really, I am quite good at cut and paste.
02-27-2008
Elysia

Quote:

Originally Posted by esbo

Not if you add a decimal point it won't.

But that was beyond the point. The point was that if you didn't do it, it would be integer division.

Quote:

It only make it a float if it has a decimal point, otherwise it just gives you a compiler error.

Of course. A float is a floating point number, after all.

Quote:

Not really, I am quite good at cut and paste.

Yes, but you could have removed them, to better suit your "tastes."
02-27-2008
esbo

Quote:

Originally Posted by Elysia

But that was beyond the point. The point was that if you didn't do it, it would be integer division.

Of course. A float is a floating point number, after all.

Yes, but you could have removed them, to better suit your "tastes."

The f changes a floating point number to a float, it won't change an integer to a float.
A double is a floating point number.
My 'taste' is just to copy what is already there, I won't change something if it compiles
and works.
I don't really see the point in having an f there at all, it seems rather pointless.
02-27-2008
VirtualAce

Quote:

The f changes a floating point number to a float, it won't change an integer to a float.
A double is a floating point number.
My 'taste' is just to copy what is already there, I won't change something if it compiles
and works.
I don't really see the point in having an f there at all, it seems rather pointless.

Because it is a very bad habit to get into. If you wish to use float then specify the f otherwise do not specify the f. It also informs other programmers that you are using float's and not doubles. There is a big difference between a double and a float. Just because 'it works' does not mean it's correct.

This:

Code:

float x = 4f;

This means nothing to me except that the person who programmed it either was very tired that day or doesn't understand floating point values. Did they want 4.0f or is it a typo?

This:

Code:

float x = 4.0f; double y = 4.0;

Means that x is a float and y is a double.

So you ask why use the f? Here is an example:

Code:

float x = some_angle * 180.0 / 3.14159f

What do you suppose happens in this assignment?
02-27-2008
esbo

Quote:

Originally Posted by Bubba

Because it is a very bad habit to get into. If you wish to use float then specify the f otherwise do not specify the f. It also informs other programmers that you are using float's and not doubles. There is a big difference between a double and a float. Just because 'it works' does not mean it's correct.

This:

Code:

float x = 4f;

This means nothing to me except that the person who programmed it either was very tired that day or doesn't understand floating point values. Did they want 4.0f or is it a typo?

This:

Code:

float x = 4.0f; double y = 4.0;

Means that x is a float and y is a double.

So you ask why use the f? Here is an example:

Code:

float x = some_angle * 180.0 / 3.14159f

What do you suppose happens in this assignment?

I don't care, I would not have used the f so it's not really my problem.
What do you think happens?

Also

Code:

float x = 4.0; double y = 4.0;

Means that x is a float and y is a double.
02-27-2008
whiteflags

>> Means that x is a float and y is a double.

Maybe, but it's an issue in precision: in Bubba's example, we're using a floating-point constant. Had he left off the trailing f, we would have done double floating point division, and the answer would have been rounded to fit in a float variable.

As you should know, the quality in answers is quite different when finer precision matters. So, it's at least fairly important to avoid conversion issues while you're using constants like pi, or some other number.
02-27-2008
esbo

Quote:

Originally Posted by citizen

>> Means that x is a float and y is a double.

Maybe, but it's an issue in precision: in Bubba's example, we're using a floating-point constant. Had he left off the trailing f, we would have done double floating point division, and the answer would have been rounded to fit in a float variable.

And exactly what is wrong with that?

Quote:

Originally Posted by citizen

As you should know, the quality in answers is quite different when finer precision matters. So, it's at least fairly important to avoid conversion issues while you're using constants like pi, or some other number.

I am not really sure what you are saying here, could you be more specific about what point
it is your are trying to make?
02-27-2008
esbo

Also I have been getting into bad habits doing this.
In 'real life' I would never write:-

Code:

float x = 4.0; double y = 4.0;

I would write:-

Code:

float x = 4; double y = 4;

Means that x is a float, y is a double

And never in a million years (I hope) would I write

Code:

float x = 4.0f; double y = 4.0;
02-27-2008
whiteflags

float x = 4;
double y = 4;

Nothing wrong with promotion (as long as it is compatible). But when you convert to a lesser data type, you might lose your data.

>> And exactly what is wrong with [rounding]?
>> what point it is your are trying to make?

It is completely reasonable to avoid possible loss of data.
02-27-2008
esbo

Quote:

Originally Posted by citizen

float x = 4;
double y = 4;

Nothing wrong with promotion (as long as it is compatible). But when you convert to a lesser data type, you might lose your data.

>> And exactly what is wrong with [rounding]?
>> what point it is your are trying to make?

It is completely reasonable to avoid possible loss of data.

Again not too clear.
Anyway, for starters, having looked up the matter in my (rather old I might add) book on
C it states that division is never performed on a float, at least not without converting it
to a double first so putting an f there would seem to be a waste of yours, and possible
the compilers time.
And even if it did do the division on floats why go for a less accurate result?
02-27-2008
whiteflags

Because again, you're cramming the result into a smaller variable with a great deal less precision. There is only so much rounding you can do until the value is just completely wrong. It depends what's important, etc.

I mean if you think the compiler cares you assign something really precise, you're wrong:

Code:

#include <stdio.h> /** my calculators answer for pi: much larger than any float or double **/ #define PI_DB 3.1415926535897932384626433832795 #define PI_FL 3.1415926535897932384626433832795f int main () { double pi = PI_FL; printf( "PI_FL = %1.6f\n", PI_FL ); printf( "PI_DB = %1.15g\n", PI_DB ); printf( "pi (promoted): = %1.10g\n", pi ); return 0; }

Rounding obviously happened (out of necessity in this case) but you shouldn't make it a complete surprise: you might decide, for a time, to treat doubles like floats, and floats like doubles, and all that rounding takes it's toll on the overall precision of your final answer [/Regis]. You can't just extend pricision back to what it once was after you've messed with it.

And evidently it's not easy to keep track of: Plenty of people aren't aware that point-anything is double by the compiler. That's why people can be finicky about decimal numbers, because they have to be when it matters. Fortunately, you don't see much of these issues creeping into financial software, though it might be easy for accountants to lay blame. :D
02-27-2008
VirtualAce

Quote:

And never in a million years (I hope) would I write

Code:
float x = 4.0f;
double y = 4.0;

And yet this is correct and all of the examples you (esbo) have shown are incorrect. Why do you think it is wrong to specify floats as 1.0f and doubles as 1.0? Second, where did you learn this as being right:

Code:

float x = 4; double y = 4;

And how does that differ from:

Code:

int x = 4; unsigned int x = 4 unsigned long x = 4; __int64 x = 4;

The last example shows integral data types being assigned integral values. The first two examples show floating point data types (float and double) being assigned integral values. Sure the compiler will convert but this will bite you someday if you continue to rely on the compiler for the conversion. It not only obfuscates the code, forces a conversion where perhaps none is needed if all vars are already using the same type, but also shows a bit of a lack of understanding about floating point data types. Take this sample:

Sample 1

Code:

float x = 4; float y = 6; float total = x + y;

Sample 2

Code:

float x = 4.0; float y = 6; float total = x + y;

Sample 3

Code:

float x = 4.0f; float y = 6.0f; float total = x + y;

Sample 4

Code:

double x = 4.0; double y = 6.0; double total = x + y;

Samples 1 and 2 are sloppy and inconsistent. There are times when variable types are not identical and casting is necessary but forcing a cast where none is needed is a bad practice.

If you write any type of modern 3D graphics code using OpenGL or Direct3D and you do all your calculations in double precision and then stuff that into a float.....what you get on screen will be very different from what you expect. If you fail to maintain your data types or toss them around sloppily your code will certainly crash and bring down the entire app. It only takes several matrix multiplies to accumulate a large floating point error even when data types are consistent much less when a lot of casting is taking place.
02-28-2008
iMalc

I tend to specify floats using 1.f for example. Do people even know that is also valid syntax?
02-28-2008
VirtualAce

Quote:

I tend to specify floats using 1.f for example. Do people even know that is also valid syntax?

Nope. Interesting. I don't think I'll switch though.
02-28-2008
esbo

Well aside from all the other stuff in this thread I can honestly say I can never
even recall seeing the 'f' notation used apart from on this site.
Is it a relatively new 'development'?

Maybe I just got out whilst the going, was good so to speak :)

Or at least slightly better anyway.....
02-28-2008
laserlight

Quote:

Well aside from all the other stuff in this thread I can honestly say I can never
even recall seeing the 'f' notation used apart from on this site.
Is it a relatively new 'development'?

No, it's pretty old.
02-28-2008
esbo

Quote:

Originally Posted by laserlight

No, it's pretty old.

Well I don't think I will ever use it, I have never had a problem with floats so far
and I can't see me ever having to use it in the future, it is surplus to requirements
as far as I am concerned.
I think it might have been introduced in ANSI C, those standards commities loved
introducing loads of stuff you never needed to use. Bit like the RS232 connector,
25 pins for serial communications - what a joke, I doubt anyone every used more then
5.
02-29-2008
Elysia

Well, you aren't forced to use floats, but if you do, use .f and even if you don't use it, even if you just give advance to someone, tell them to use .f of they use floats.