Menu
Search

Forums

Forum Index

Thread overview
Confused about something in the D book relating to precision
Dec 05, 2022
thebluepandabear
Dec 05, 2022
Ali Çehreli
Dec 05, 2022
thebluepandabear
December 05, 2022
Gravatar of thebluepandabearPosted by thebluepandabearReply
thebluepandabear
Gravatar of thebluepandabear


Reply

Hello guys,

(Noob question.)

I would appreciate some help.

I am reading Ali's book on D language, and I am up to page 127 -- talking about format specifiers.

He says the following about the '%e' (exponent) specifier:

"e: A floating point argument is printed according to the following rules.

  • a single digit before the decimal mark
  • a decimal mark if precision is nonzero"

I am not understanding why Ali said there is a decimal mark if precision is nonzero?

How can a number have zero precision? I thought all numbers have a precision of greater than 0. I am confused what this means :/

Then he says:

"the required digits after the decimal mark, the number of which is determined
by precision (default precision is 6)"

Well double has a precision of 15, and when I print the following there aren't 15 digits after the decimal mark:

	double value = 123.456789;
	writeln("precision: ", double.dig);
	writefln("with e: %e", value);

Output:

precision: 15
with e: 1.234568e+02

I feel like this section was explained poorly and it's confusing.

Help would be appreciated.

December 04, 2022
Gravatar of Ali ÇehreliPosted by Ali Çehreli
in reply to thebluepandabear		Reply
Ali Çehreli
Gravatar of Ali Çehreli
Posted in reply to thebluepandabear


Reply
On 12/4/22 18:57, thebluepandabear wrote:

> I am not understanding why Ali said there is a decimal mark if precision
> is nonzero?
>
> How can a number have zero precision?

That "precision" is referring to how many digits are printed after the decimal mark in the formatted output.

> "the required digits after the decimal mark, the number of which is
> determined
> by precision (default precision is 6)"

So, if we print with %e, we get 6 digits:

    enum f = 1.23456789;
    writefln!"%e"(f);

Prints

1.234568e+00

There are 6 digits after the decimal point.

Now 3 digits of precision:

    writefln!"%.3e"(f);

Prints

1.235e+00

Now 0 precision, where the decimal point will disappear:

    writefln!"%.0e"(f);

Prints

1e+00

> Well double has a precision of 15

Different meanings for the same word...

> I feel like this section was explained poorly and it's confusing.

I have to agree. Nobody really knows these by heart. Once you know what's available, you just come back and pick what you need for that occasion.

Ali
December 05, 2022
Gravatar of thebluepandabearPosted by thebluepandabear
in reply to Ali Çehreli		Reply
thebluepandabear
Gravatar of thebluepandabear
Posted in reply to Ali Çehreli


Reply
> I have to agree. Nobody really knows these by heart. Once you know what's available, you just come back and pick what you need for that occasion.
>
> Ali

Thanks for your effort :-) It helped clear things up.