I ran the following simple code in C++ and got unexpected results:
float f = 139.4;
cout << f;
Output:
139.399994;
if( f == 139.4)
cout << "Expected result";
else
cout << "Unexpected result";
Output:
Unexpected reult
I ran the following equivalent code in VB but got the correct results.
Dim f As Single
f = 139.4
Print f
If f = 139.4 Then
Print "Expected result"
Else
Print "Unexpected result"
End If
Doesn't this look bad on C++'s resume?
Jun 27 '08
13  3619 
On May 13, 2:04*am, Lionel B <m...@privacy.n etwrote:
On Tue, 13 May 2008 08:55:23 +0000, Lionel B wrote:
On Mon, 12 May 2008 16:02:54 -0700, Greg Herlihy wrote:
On May 12, 3:08*pm, Lionel B <m...@privacy.n etwrote:
>On my system (compiler GCC 4.1.2) with
>* float f = 139.4;
>and compiled with:
>* g++ -std=c++98 -pedantic -Wall -Wextra
>I get no warnings and the output is 'Expected result'. Should the
compiler warn if there's an implicit conversion? Or only if it results
in loss of precision?
The g++ compiler can issue a warning if you want one:
* *g++ -Wshorten-64-to-32
cc1plus: error: unrecognized command line option "-Wshorten-64-to-32"
GCC 4.3.0 on x86_64
Anyway, that doesn't sound like a floating-point warning...
This does it: -Wconversion. With
* float f = 139.4;
warning: conversion to ‘float’ alters ‘double’ constant value
On my machine, using gcc (version 4.2.1 (Apple Inc. build 5559)) -
Wconversion reports nothing.
Whereas -Wshorten-64-to-32 reports:
warning: implicit conversion shortens 64-bit value into a 32-bit
value
Greg
On Tue, 13 May 2008 02:50:53 -0700, Greg Herlihy wrote:
On May 13, 2:04Â*am, Lionel B <m...@privacy.n etwrote:
>On Tue, 13 May 2008 08:55:23 +0000, Lionel B wrote:
On Mon, 12 May 2008 16:02:54 -0700, Greg Herlihy wrote:
>On May 12, 3:08Â*pm, Lionel B <m...@privacy.n etwrote:
>>On my system (compiler GCC 4.1.2) with
>>Â* float f = 139.4;
>>and compiled with:
>>Â* g++ -std=c++98 -pedantic -Wall -Wextra
>>I get no warnings and the output is 'Expected result'. Should the
compiler warn if there's an implicit conversion? Or only if it
results in loss of precision?
>The g++ compiler can issue a warning if you want one:
>Â* Â*g++ -Wshorten-64-to-32
cc1plus: error: unrecognized command line option "-Wshorten-64-to-32"
GCC 4.3.0 on x86_64
Anyway, that doesn't sound like a floating-point warning...
This does it: -Wconversion. With
Â* float f = 139.4;
warning: conversion to ‘float’ alters ‘double’ constant value
On my machine, using gcc (version 4.2.1 (Apple Inc. build 5559)) -
Wconversion reports nothing.
Whereas -Wshorten-64-to-32 reports:
warning: implicit conversion shortens 64-bit value into a 32-bit
value
Sure, I see that -Wshorten-64-to-32 is a Mac thing - it's all going to be
highly machine-specific. I'm surprised -Wconversion doesn't report
anything, though.
--
Lionel B
bintom wrote:
float f = 139.4;
if( f == 139.4)
You are converting a 64-bit floating point number into a 32-bit one,
and then comparing the result to a 64-bit one. All bets are off.
The difference with some other languages is that C++ doesn't try to
second-guess what you are really trying to do, and it does what and only
what you asked it to do.
That the equality comparison is giving false is a result of the
underlying hardware and the floating point format used, not a flaw in
C++. The underlying hardware is dropping some bits from this value in
question when you are making the 64->32 bit conversion. When you try to
compare it to the original 64-bit value, the result will be different.
A big thanks to Erik and Juha for showing me where I was going wrong.
You saved my day.
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