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digitalmars.D.learn - Strange double to uint conversion

reply Luiz Silveira <zertyz gmail.com> writes:
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?
Nov 18 2019
next sibling parent reply mipri <mipri minimaltype.com> writes:
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
Nov 18 2019
parent Luiz Silveira <zertyz gmail.com> writes:
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.
Nov 18 2019
prev sibling parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
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
Nov 18 2019
next sibling parent Steven Schveighoffer <schveiguy gmail.com> writes:
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
Nov 18 2019
prev sibling parent Luiz Silveira <zertyz gmail.com> writes:
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.
Nov 19 2019