Showing numbers in decimal not scientific notation

tomy

@VRonin said in Showing numbers in decimal not scientific notation:

@tomy No, don't use QTextStream you don't need something like stringstream to pass numbers to string.
QString ss;
ss = result_box -> locale().toString(expression(),'f');
result_box -> setText(ss);

I used it. 2 + 3 = 5.000000 :( :(

VRonin

what is result_box and what is inside expression()?

tomy

@VRonin

what is result_box

It's a lineEdit.

and what is inside expression()?

It returns only a double value. Consider something simple like:

double My_First_Calculator::expression()
{
   double d1, d2;    // these d1, d2 are gotten from input E.g. d1 = 2, d2 = 3.5
  if(_ch == '+') return d1+d2;  // _ch is a previously defined varible 
 else if (_ch == '-') return d1-d2;  // and so on
}

tomy

I simplified the code as follows. This, too, has exactly that problem:

test.h

#ifndef TEST_H
#define TEST_H
#include <QDialog>

class QLineEdit;
class QPushButton;

class test : public QDialog
{
    Q_OBJECT
public:
    test(QWidget* parent = 0);

private slots:
    void expression();

private:
    QLineEdit* result_box;
    QPushButton* equal;
    QPushButton* quit;
};

#endif // TEST_H

test.cpp

#include <QtWidgets>
#include "test.h"

test::test(QWidget* parent) : QDialog(parent)
{
  result_box = new QLineEdit;
  equal = new QPushButton(tr("="));
  quit = new QPushButton(tr("Close"));


  connect(quit, SIGNAL(clicked(bool)), this, SLOT(close()));
  connect(equal,SIGNAL(clicked(bool)), this, SLOT(expression()));

  QHBoxLayout* layout = new QHBoxLayout;
  layout -> addWidget(result_box);
  layout -> addWidget(equal);
  layout -> addWidget(quit);

  setLayout(layout);
}

//******************

void test::expression()
{
    QString ss;
    double d = 1000000;
    QTextStream (&ss) << d;
    result_box -> setText(ss);
}

And main.cpp

#include <QApplication>
#include "test.h"

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    test t;
    t.show();

   return app.exec();
}

Just run it and click on the = button.

tomy

I think I should write a code for it like this:

if( d is like an int number)
    result_box -> setText(QString::number(d , 'f', 0));
else if ( d is a double number with n numbers after point)
   result_box -> setText(QString::number(d , 'f', n));

koahnig

@tomy
Maybe you should have a look to the docs as well. E.g. here

In case of integer assignment that would be:

int i = 10;
result_box -> setText ( QString::number( i ) );

kshegunov

@tomy said in Showing numbers in decimal not scientific notation:

Don't be that sure. :)

Yes! There's a typo in the code ... :)

ss = qFuzzyCompare(expression(),
                 static_cast<qint64>(expression()) ? //< Missing a )
                 QString::number(static_cast<qint64>(expression())) :
                 QString::number(expression(), 'f')); //< Extra )

What you want is to have the if with qFuzzyCompare, not with the static cast. It should rather read like this:

ss = qFuzzyCompare(expression(), static_cast<qint64>(expression())) ?
                 QString::number(static_cast<qint64>(expression())) :
                 QString::number(expression(), 'f');

If you wish you can of course use the usual if-else construct, not the ternary operator, so the last snippet'd be equivalent to:

if (qFuzzyCompare(expression(), static_cast<qint64>(expression()))
    ss = QString::number(static_cast<qint64>(expression()));
else
    ss = QString::number(expression(), 'f');

tomy

This post is deleted!

tomy

@kshegunov

What you want is to have the if with qFuzzyCompare, not with the static cast. It should rather read like this:
ss = qFuzzyCompare(expression(), static_cast<qint64>(expression())) ?
                 QString::number(static_cast<qint64>(expression())) :
                 QString::number(expression(), 'f');
If you wish you can of course use the usual if-else construct, not the ternary operator, so the last snippet'd be equivalent to:
if (qFuzzyCompare(expression(), static_cast<qint64>(expression()))
    ss = QString::number(static_cast<qint64>(expression()));
else
    ss = QString::number(expression(), 'f');

Sorry, I don't know how I tested it but it doesn't work as it's expected! :(
for example:
2 + 3 = 5 OK
10 ^ 6 = 1000000 OK
2 + 1.5 = 3.500000 !!!

That is, the function QString::number(expression(), 'f'); (with the default precision 6) shows all range of precision whether it's need or not!

Maybe it's what we need:
http://doc.qt.io/qt-5/qtextstream.html#setRealNumberNotation
But how to use it? Docs doesn't offer a little example of it to show how beginners should use it. !!!!!!
(Docs are not for learners/beginners, they are for professionals — if they are professional, so they don't need Docs much => Docs are not useful)

kshegunov

What you want to do in the floating point case is not at all trivial. Here you can read on that topic if you are willing to digest the math.

The simpler but inefficient approach is to save the floating point text representation and just discard all the trailing 0 characters manually.

EDIT:
Here's (probably) the most concise way, but requires knowledge of regular expressions, which is yet again for you to read on to understand how or why it works:

if (qFuzzyCompare(expression(), static_cast<qint64>(expression()))
    ss = QString::number(static_cast<qint64>(expression()));
else  {
    ss = QString::number(expression(), 'f', 17);
    ss.replace(QRegularExpression("\\.?0+$"), "");
}

(Docs are not for learners/beginners, they are for professionals — if they are professional, so they don't need Docs much => Docs are not useful)

You got that backwards. The point of the Qt docs is they are a documentation for Qt itself, they will give no instruction into C++, its types, the types' memory layout and other such technical topics. If you are in need of that, then you need to look elsewhere, for example http://en.cppreference.com/w/

tomy

@kshegunov

The point of the Qt docs is they are a documentation for Qt itself, they will give no instruction into C++, its types, the types' memory layout and other such technical topics.

Thanks but I didn't talk about C++, but Docs.
I thought we can look at the Docs as a set of instructions useful for learners to be used to Qt, because they have been frequently suggested to new comers of Qt for reading.
Thanks also for your code.

jsulm

@tomy Qt documentation is not a set of instructions and it will never be. It is just not possible to have instructions for every use case you can imagine.
Qt documentation documents Qt API and provides some examples. For specific use cases you need to think about it, read Qt documentation, try out. This is same for all frameworks I was using so far.

kenchan

You are wasting your time trying to learn programming and c++ just by looking at the Qt docs... talk about an uphill hike :p

kshegunov

@tomy said in Showing numbers in decimal not scientific notation:

I thought we can look at the Docs as a set of instructions useful for learners to be used to Qt, because they have been frequently suggested to new comers of Qt for reading.

You can, but the implication is you have a decent knowledge of C++ before that, as Qt is a library that's written and intended to be used from (among others) C++ code. So these are two separate issues you need to address. Learning about Qt without a good fundament is (and always will be) a very, very hard thing to do. Think about it like this, you wouldn't start writing a book in a foreign language, unless you're very intimately familiar with the actual language, right?

tomy

@kshegunov
First off, I like your attitude. And I wish we all believe in Democracy. Indeed, I use a book for learning Qt. But it's sometimes possible to refer to a good resource for a topic. I know Docs are in a high position in Qt folk's perspectives, but if I can use democracy and say my opinion, I say, "I have not found them useful up to now".
It may change. I've joined Qt just recently.
Thanks for your talks.

jsulm

@tomy It is perfectly fine to tell others your opinion. But it is as well perfectly fine for others to disagree with you :-)

J.Hilk

Hi,

for your specific situation, I would suggest something like:

QString truncValue(double value, int prec)
{
    QString sReturn = QString::number(value,'f',prec);
    if(sReturn.endsWith("0")){
        while(sReturn.endsWith("0"))
            sReturn.remove(sReturn.length()-1,1);

        if(sReturn.endsWith("."))
            sReturn.remove(sReturn.length()-1,1);
    }
    
    return sReturn;
}

But, this is successively chaining a lot of string operations. The previously mentions methods are probably better.

tomy

@J.Hilk
Thank you for the code.
honestly, I solved the issue two or three days ago. When I found I can't rely on a function offered by Qt on this specific problem, I returned to my old friend, C++, and solved the issue using it.
But I appreciate your paying attention to the problem.