Hello, everyone!

  I'm a new D fan, still learning the language and this is my first post.

  I came up with a strange behavior, using D 2.089 for windows.

  Considering the program "error.d":

import std.stdio;
import std.conv;

void main() {
        uint c(double v) {
                return (37.05*100.0).to!uint;
        }

        double f;
        uint c2(double v) {
                return (37.05*f).to!uint;
        }
        f=100.0;

        writeln("All should be 3705, right? -- ", 37.05*100, "; ", 37.05*100.0, "; ", (37.05*100).to!uint, "; ",c(37.05),"; ",c2(37.05));
}

  I expect all values to be 3705, yet running it with '/c/D/dmd-2.089.0/windows/bin/dmd.exe -run error.d' gives me the output:

All should be 3705, right? -- 3705; 3705; 3705; 3705; 3704

  I also played with several compilation options, all yelding the same results:

dmd -boundscheck=on -m64 -lowmem -of="error" "error".d && ./error
dmd -boundscheck=off -m64 -lowmem -of="error" "error".d && ./error
dmd -boundscheck=on -m32 -lowmem -of="error" "error".d && ./error
dmd -release -O -mcpu=native -boundscheck=on -m64 -lowmem -of="error" "error".d && ./error
dmd -release -O -mcpu=native -boundscheck=on -m64 -of="error" "error".d && ./error
dmd -of="error" "error".d && ./error

  What am I doing wrong?

On Monday, 18 November 2019 at 21:08:39 UTC, Luiz Silveira wrote:
>         double f;

If this is changed to `real f;`, you get the desired result.

real is also the type used for the comparison code:
https://dlang.org/spec/float.html#fp_const_folding

On Monday, 18 November 2019 at 21:14:37 UTC, mipri wrote:
>>         double f;
>
> If this is changed to `real f;`, you get the desired result.
>
> real is also the type used for the comparison code:
> https://dlang.org/spec/float.html#fp_const_folding

Yes, it worked as expected.

Thank you,
Luiz.

On 11/18/19 4:08 PM, Luiz Silveira wrote:
> Hello, everyone!
> 
>    I'm a new D fan, still learning the language and this is my first post.
> 
>    I came up with a strange behavior, using D 2.089 for windows.
> 
>    Considering the program "error.d":
> 
> import std.stdio;
> import std.conv;
> 
> void main() {
>          uint c(double v) {
>                  return (37.05*100.0).to!uint;
>          }
> 
>          double f;
>          uint c2(double v) {
>                  return (37.05*f).to!uint;
>          }
>          f=100.0;
> 
>          writeln("All should be 3705, right? -- ", 37.05*100, "; ", 37.05*100.0, "; ", (37.05*100).to!uint, "; ",c(37.05),"; ",c2(37.05));
> }
> 
>    I expect all values to be 3705, yet running it with '/c/D/dmd-2.089.0/windows/bin/dmd.exe -run error.d' gives me the output:
> 
> All should be 3705, right? -- 3705; 3705; 3705; 3705; 3704
> 
>    I also played with several compilation options, all yelding the same results:
> 
> dmd -boundscheck=on -m64 -lowmem -of="error" "error".d && ./error
> dmd -boundscheck=off -m64 -lowmem -of="error" "error".d && ./error
> dmd -boundscheck=on -m32 -lowmem -of="error" "error".d && ./error
> dmd -release -O -mcpu=native -boundscheck=on -m64 -lowmem -of="error" "error".d && ./error
> dmd -release -O -mcpu=native -boundscheck=on -m64 -of="error" "error".d && ./error
> dmd -of="error" "error".d && ./error
> 
>    What am I doing wrong?

You are expecting floating point to behave as if it is stored as a decimal number. It's not.

An interesting thing is that 5/100 (or 1/20) is not accurately representable in floating point. So you get an approximation in some cases (actually in all cases), but happens to print the right thing.

What you may want to do is use std.math.ceil.

-Steve

On 11/18/19 4:54 PM, Steven Schveighoffer wrote:
> What you may want to do is use std.math.ceil.

Sorry, what you want is std.math.round.

-Steve

On Monday, 18 November 2019 at 21:54:53 UTC, Steven Schveighoffer wrote:
> You are expecting floating point to behave as if it is stored as a decimal number. It's not.

I was actually asking why 'c' and 'c2' functions behave differently. After mipri's answer I learned that the default floating point representation for literals in D is 'real', not 'double', as in C++.

Changing the type to real resolves the different behavior I was observing.

Just out of curiosity: in C++ a similar code also issues 3704 -- but the same value is returned by 'c' and 'c2', hence my doubt. Using, in C++, 'long double' and suffixing all floating point literals with 'l' gave me the same results as my "fixed" D code.

But, yes, thanks for clarifying about math.round.