QString::number(14.24,'g',3) returns 14.3 ?

Seb Tur

I used QString::number(doubleNumber, 'g' , 3) to get String representing the significant decimals up to 3.
It works well in 99.9999% cases but for 14.25 instead of giving 14.25 it returns 14.3
Why is that and what to use instead?

I know I could use QString::number(doubleNumber'f',3) and manually remove trailing zeros but why ?

JonB

@Seb-Tur
I don't know about your cases where you say it "works", but where does g format say it produces 3 significant decimals? It does not. See https://doc.qt.io/qt-6/qlocale.html#toString-8

'g' use 'e' or 'f' format, whichever is more concise maximum number of significant digits (trailing zeroes are omitted)

The precision is "maximum number of significant digits".

Paul Colby

I used QString::number(doubleNumber, 'g' , 3) to get String representing the significant decimals up to 3.

The g format doesn't give 3 "significant decimals", it gives 3 "significant digits" - this includes digits both before (excluding leading 0's) and after the decimal. Have a read of https://en.wikipedia.org/wiki/Significant_figures for more info.

It works well in 99.9999% cases but for 14.25 instead of giving 14.25 it returns 14.3

14.3 is correct. The three significant digits there are 1, 4, and .3.

I know I could use QString::number(doubleNumber'f',3) ... Why is that

The reason the f format gives more decimals in your example, is that the f format interprets the precision parameter as a number of decimals, whereas g interprets it as a number of significant digits (which, as I mentioned above, includes digits before the decimal too).

This is documented in the table under QLocale::toString(), which QString::number() links to (and @JonB referenced above). See the "Meaining of precision column:

and what to use instead?

If you specifically want "up to 3 decimals", as opposed to "exactly 3 decimals" (f) or "up to 3 significant digits" (g), then I suppose using f then trimming trailing 0 chars is one way to go.

Just one of many ways to do it:

    const double doubleNumber = 14.25;
    qDebug() << QString::number(doubleNumber,'f',3);
    qDebug() << QString::number(doubleNumber,'g',3);
    qDebug() << QString::number(doubleNumber,'f',3).remove(QRegularExpression(QStringLiteral("0+$")));

Outputs:

"14.250"
"14.3"
"14.25"

Cheers.