QString::number() 'f' format without trailing zeros

Question

I want to convert number to QString with 3 significant digits.

QString::number(myNumber,'f',3);

does the job but remains trailing zeros. How to use it without them.

Also I've tried 'g' and which shouldn't remain those zeros:

QString::number(myNumber,'g',3);

but for example 472.76 is converted to 473. That surprised me. Why is it so with 'g' option?

However I am interested in 'f' format. So the main question is how to do it with 'f' without trailing zeros?

Input -> Desired output

472.76 -> 472.76

0.0766861 -> 0.077

180.00001 -> 180

I've added some. I don't want trailing zeros. And the maximum number of digits after decimal point is 3. — krzych, Sep 17 '12 at 12:39

Chris · Accepted Answer · 2012-09-17T21:08:56.740

15

I'm almost embarrassed to post this but it works:

QString toString( qreal num )
{
    QString str = QString::number( num, 'f', 3 );

    str.remove( QRegExp("0+$") ); // Remove any number of trailing 0's
    str.remove( QRegExp("\\.$") ); // If the last character is just a '.' then remove it

    return str;
}

If you're really concerned about the performance using this method you may want to come up with a different solution.

edited Sep 17 '12 at 21:08

answered Sep 17 '12 at 20:44

Chris

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It's wrong. Second conversion to number takes double with default 'g', 6. So for big number it will write them as exponential. Tested with 1000000000.2023 and returns 1e+09. – krzych Sep 17 '12 at 20:50
I changed my answer to use an alternate solution, which is equally unpretty but seems to work. – Chris Sep 17 '12 at 20:59
Can you add what thas RegExp do as clarification? I've also thought about parsing and removing those zeros manually. It's pitty that such elementar thing cannot be done with framework. – krzych Sep 17 '12 at 21:04
Sure I added a couple of comments – Chris Sep 17 '12 at 21:11
1

His solution doesn't account for all of the cases that the original question presents. – Chris Jul 24 '14 at 19:29
Embarrasing for Qt. This seems to be a bug. With 'e/E' precision is equivalent to significants, with 'f' its equivalent to digits after comma. – ManuelSchneid3r Mar 05 '16 at 15:11
7

You can use only one regExp: str.remove( QRegExp("\\.?0+$") ); – Rinat Aug 07 '16 at 11:39
Wrong. 200 return 2. – l2m2 Sep 17 '20 at 07:36
1

@l2m2 200 won't occur because of the 'f' argument with 3 decimal precision. – sleepysilverdoor Jul 25 '21 at 14:26
@sleepysilverdoor you're right. thanks for reply. – l2m2 Oct 29 '21 at 05:40
@Rinat That's beautiful, thank you. – i know nothing Jul 05 '23 at 08:42
The "?" is not needed: str.remove( QRegExp("\\.0+$") ); – Rinat Jul 18 '23 at 08:19

score 4 · Answer 2 · answered Sep 17 '12 at 13:17

4

QString::number(myNumber,'g',3);

Will limit the conversion to the significant digits. You'll most always have 3 numbers.

472.76 => 472
4.7276 => 4.72

Try using the formatted printing functions like QString::sprintf. This should give you more control.

QString numStr;
numStr.sprintf("f.3f", myNumber);

answered Sep 17 '12 at 13:17

Aesthete

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Missing % in numStr.sprintf("%f.3f", myNumber); – Kent Kostelac Jan 30 '19 at 09:59

Moritz H. · Answer 3 · 2021-01-19T10:06:57.017

If you insist to have the precision after the decimal point you have to use 'f'. Here is an option to remove the trailing zeors with better performance than the Regex using only QString builtins:

QString str = QString::number(myNumber,'f',3);
while(str.back() =='0')
{
    str.chop(1);
}
if(str.back() =='.')
{
    str.chop(1);
}

This works because the f option guarantees to put out the defined digits. It's about 50% faster than C sprintf and 40% faster than QString::sprintf from the other answer.

score 0 · Answer 4 · answered Sep 17 '12 at 13:22

0

QString str;
str.setNum(myNumber, 'f', 3);

This will do what you need, I tested it. Strangely, "number" behaves differently than "setNum".

answered Sep 17 '12 at 13:22

bmkorkut

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It remains zeros. For example 472.76000 is converted to 472.760 and should be 472.76. Works for me the same as number. – krzych Sep 17 '12 at 13:34

score 0 · Answer 5 · answered Sep 23 '20 at 11:27

How about this. It is maybe more performant than the variant with regular expressions. The mynumber() function takes a new format char 'h', which does the job.

QString mynumber(qreal p_number, char p_format, int p_precision)
{
    if(p_format=='h')
    {
        //like 'f' but removes trailing zeros and the '.' evtl
        QString res=QString::number(p_number,'f',p_precision);
        int countTrailingToRemove=0;
        for(QString::const_reverse_iterator it=res.crbegin();it!=res.crend();it++)
        {
            if(*it=='0') countTrailingToRemove++;
            else
            {
                if(*it=='.') countTrailingToRemove++;
                break;
            }
        }

        return res.mid(0,res.length()-countTrailingToRemove);
    }
    return QString::number(p_number,p_format,p_precision);

}

QString::number() 'f' format without trailing zeros

5 Answers5