For a purpose I need to use void QTextStream::setRealNumberNotation(RealNumberNotation notation). I looked at so-called Docs for it, but there is no example for beginners! (It's here)
For example:
QString ss;
QTextStream (&ss);
// here how to use it
Here is an example of what setRealNumberNotation does and how you can use it
QString mystring;
QTextStream bla(&mystring);
bla.setRealNumberNotation(QTextStream::ScientificNotation);
bla << 0.000123;
qDebug() << mystring;
bla.setRealNumberNotation(QTextStream::FixedNotation);
mystring.clear();
bla << 0.000123;
qDebug() << mystring;
This prints out the following:
"1.230000e-04"
"0.000123"
Some more Details about the options for setRealNumberNotation. The Qt Documentation lists 3 Options
ScientificNotation # Scientific notation
FixedNotation # Fixed-point notation
SmartNotation # Scientific or fixed-point notation
SmartNotation uses the one which makes most sense like printf() %g conversion specifier. If you want to know how printf decides what makes most sense read the printf documentation
Related
According to the documentation QString's toUpper() and toLower() member functions convert in the C-locale. Does it mean that only Posix Portable Character Set characters (Latin A-Z/a-z) are converted and any international unicode characters are left as-is?
We can easily test this with the simple test program here:
#include <QString>
#include <iostream>
int main()
{
QString s("БΣTest3φب");
std::cout << "String: " << s.toStdString() << std::endl;
std::cout << "Lower: " << s.toLower().toStdString() << std::endl;
std::cout << "Upper: " << s.toUpper().toStdString() << std::endl;
return 0;
}
Which returns:
09:48:48: Starting /home/tzig/test/test ...
String: БΣTest3φب
Lower: бσtest3φب
Upper: БΣTEST3Φب
09:48:49: /home/tzig/test/test exited with code 0
So we know that:
Cyrillic letters work fine
Latin letters work fine
Numbers are untouched
Greek letters work fine
Arabic letters don't have uppercase/lowercase so they are untouched
Feel free to test other Unicode characters with the test program.
Using console, I want to write the desired user's input into a file via getline function inside the wFile function and then read it. I face with logical error during Runtime; whatever I as user write doesn't type into the output terminal and it doesn't succeed more steps. Apparently fwrite function with this feature in the libraries exists, but I want to write my own code differently this way. I think I must have neglected a point. Here's the code:
#include <QCoreApplication>
#include <QDebug>
#include <QFile>
#include <QString>
#include <QTextStream>
#include <String>
#include <cstdlib>
using namespace std;
void wFile(QString Filename)
{
QFile mFile(Filename);
QTextStream str(&mFile);
qDebug() << "what do you want to write in the desired file: ";
istream& getline (istream& is, string& str);
if (!mFile.open(QFile::WriteOnly | QFile::Text))
{
qDebug() << "could not open the file";
return;
}
mFile.flush();
mFile.close();
}
void read (QString Filename){
QFile nFile(Filename);
if(!nFile.open(QFile::ReadOnly | QFile::Text))
{
qDebug() << "could not open file for reading";
return;
}
QTextStream in(&nFile);
QString nText = in.readAll();
qDebug() << nText;
nFile.close();
}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString nFilename ="P:/DocumentArminV.txt";
wFile(nFilename);
read(nFilename);
return a.exec();
}
Spoiler alarm: At the very end of this answer, there is a very simple recommendation for a fix.
What OP did
Concerning istream& getline (istream& is, string& str); in wFile(QString Filename):
This declares function getline() in function wFile().
This is a valid declaration concerning C++.
Concerning the sample code, I missed the respective headers. IMHO,
#include <istream> and
#include <string>
are required to make this compiling.
However, it is possible that the existing #includes include them indirectly. So, OP's code may even compile without them.
Declaring functions, which are not used as well as re-declaring functions which are already declared is somehow useless but not wrong.
To demonstrate this, I made a small sample:
#include <cstdio>
#include <istream>
#include <string>
void func()
{
puts("in func()\n");
std::istream& getline(std::istream&, std::string&);
// Even duplicated prototyping is accepted without complaints:
std::istream& getline(std::istream&, std::string&);
}
int main ()
{
func();
return 0;
}
compiles and runs perfectly.
Output:
in func()
Live Demo on coliru
What OP (probably) wanted
Using console, I want to write the desired user's input into a file via getline function inside the wFile function and then read it.
This sounds a bit confusing to me. std::getline(std::cin, ) can be used to read user input from console. May be, it's a bit bad wording only.
Assuming, the OP wanted to read input from console, obviously, declaring a function is not sufficient – it must be called to become effective:
#include <iostream>
void func()
{
std::cout << "Enter file name: ";
std::string fileName; std::getline(std::cin, fileName);
std::cout << "Got file name '" << fileName << "'\n");
}
int main ()
{
func();
return 0;
}
Output:
Enter file name: test.txt↵
Got file name 'test.txt'
Live Demo on coliru
C++ std vs. Qt
Qt is undoubtly built on top of the C++ std library. However, though it's possible it is not recommended to mix both APIs when it can be prevented (or there aren't specific reasons to do so).
Both, Qt and C++ std, are a possibility to write portable software.
Qt covers a lot of things which are provided in the std library as well but a lot of other things additionally which are not or not yet part of std. In some cases, the Qt is a bit less generic but more convenient though this is my personal opinion. IMHO, the following explains how I came to this:
std::string vs. QString
std::string stores a sequence of chars. The meaning of chars when exposed as glyph (e.g. printing on console or displaying in a window) depends on the encoding which is used in this exposing. There are lot of encodings which interprete the numbers in the chars in distinct ways.
Example:
std::string text = "\xc3\xbc";
Decoded/displayed with
Windows-1252: ü
UTF-8: ü
Based on character type of std::string, it is not possible to determine the encoding. Hence, an additional hint must be provided to decode this properly.
(AFAIK, it is similar for std::wstring and wchar_t.)
QString stores a sequence of Unicode characters. So, one universal encoding was chosen by design to mitigate the "encoding hell".
As long as the program operates on QString, no encoding issues should be expected. The same is true when combining QString with other functions of Qt. However, it becomes a bit more complicated when "leaving the Qt universe" – e.g. storing contents of a std::string to QString.
This is the point where the programmer has to provide the additional hint for the encoding of the contents in std::string. QString provides a lot of from...() and to...() methods which can be used to re-encode contents but the application programmer is still responsible to chose the right one.
Assuming that the intended contents of text should have been the ü (i.e. UTF-8 encoding), this can be converted to QString (and back) by:
// std::string (UTF-8) -> QString
std::string text = "\xc3\xbc";
QString qText = QString::fromUtf8(text.c_str());
// QString -> std::string (UTF-8)
std::string text2 = qText.toUtf8();
This has to be considered when input from std::cin shall be passed to QString:
std::cout << "Enter file name: ";
std::string input; std::getline(std::cin, input);
QString qFileName = QString::fromLocal8Bit(input);
And even now, the code contains a little flaw – the locale of std::cin might have changed with std::ios::imbue(). I must admit that I cannot say much more about this. (In daily work, I try to prevent this topic at all e.g. by not relying on Console input which I consider especially critical on Windows – the OS on which we usually deploy to customers.)
Instead, a last note about OP's code:
How to fix it
Remembering my above recommendation (not to mix std and Qt if not necessary), this can be done in Qt exclusively:
QTextStream qtin(stdin);
qtin.readline();
I must admit that I never did it by myself but found this in the Qt forum: Re: stdin reading.
I'm trying to get into the fascinating world of Common Lisp embedded in C++. My problem is that I can't manage to read and print from c++ a string returned by a lisp function defined in ECL.
In C++ I have this function to run arbitrary Lisp expressions:
cl_object lisp(const std::string & call) {
return cl_safe_eval(c_string_to_object(call.c_str()), Cnil, Cnil);
}
I can do it with a number in this way:
ECL:
(defun return-a-number () 5.2)
read and print in C++:
auto x = ecl_to_float(lisp("(return-a-number)"));
std::cout << "The number is " << x << std::endl;
Everything is set and works fine, but I don't know to do it with a string instead of a number. This is what I have tried:
ECL:
(defun return-a-string () "Hello")
C++:
cl_object y = lisp("(return-a-string)");
std::cout << "A string: " << y << std::endl;
And the result of printing the string is this:
A string: 0x3188b00
that I guess is the address of the string.
Here it is a capture of the debugger and the contents of the y cl_object. y->string.self type is an ecl_character.
Debug
(Starting from #coredump's answer that the string.self field provides the result.)
The string.self field is defined as type ecl_character* (ecl/object.h), which appears to be given in ecl/config.h as type int (although I suspect this is slightly platform dependent). Therefore, you will not be able to just print it as if it was a character array.
The way I found worked for me was to reinterpret it as a wchar_t (i.e. a unicode character). Unfortunately, I'm reasonably sure this isn't portable and depends both on how ecl is configured and the C++ compiler.
// basic check that this should work
static_assert(sizeof(ecl_character)==sizeof(wchar_t),"sizes must be the same");
std::wcout << "A string: " << reinterpret_cast<wchar_t*>(y->string.self) << std::endl;
// prints hello, as required
// note the use of wcout
The alternative is to use the lisp type base-string which does use char (base-char in lisp) as its character type. The lisp code then reads
(defun return-a-base-string ()
(coerce "Hello" 'base-string))
(there may be more elegant ways to do the conversion to base-string but I don't know them).
To print in C++
cl_object y2 = lisp("(return-a-base-string)");
std::cout << "Another: " << y2->base_string.self << std::endl;
(note that you can't mix wcout and cout in the same program)
According to section 2.6 Strings of The ECL Manual, I think that the actual character array is found by accessing the string.self field of the returned object. Can you try the following?
std::cout << y->string.self << std::endl;
std::string str {""};
cl_object y2 = lisp("(return-a-base-string)");
//get dimension
int j = y2->string.dim;
//get pointer
ecl_character* selv = y2->string.self;
//do simple pointer addition
for(int i=0;i<j;i++){
str += (*(selv+i));
}
//do whatever you want to str
this code works when the string is build from ecl_characters
from the documentation:
"ECL defines two C types to hold its characters: ecl_base_char and ecl_character.
When ECL is built without Unicode, they both coincide and typically match unsigned char, to cover the 256 codes that are needed.
When ECL is built with Unicode, the two types are no longer equivalent, with ecl_character being larger.
For your code to be portable and future proof, use both types to really express what you intend to do."
On my system the return-a-base-string is not needed, but I think it could be good to add for compatibility. I use the (ecl) embedded CLISP 16.1.2 version.
The following piece of code reads a string from lisp and converts to C++ strings types - std::string and c-string- and store them on C++ variables:
// strings initializations: string and c-string
std::string str2 {""};
char str_c[99] = " ";
// text read from clisp, whatever clisp function that returns string type
cl_object cl_text = lisp("(coerce (text-from-lisp X) 'base-string)");
//cl_object cl_text = lisp("(text-from-lisp X)"); // no base string conversions
// catch dimension
int cl_text_dim = cl_text->string.dim;
// complete c-string char by char
for(int ind=0;i<cl_text_dim;i++){
str_c[i] = ecl_char(cl_text,i); // ecl function to get char from cl_object
}
str_c[cl_text_dim] ='\0'; // end of the c-string
str2 = str_c; // get the string on the other string type
std::cout << "Dim: " << cl_ text_dim << " C-String var: " << str_c() << " String var << str2 << std::endl;
It is a slow process as passing char by char but it is the only way by the moment I know. Hope it helps. Greetings!
I would like to peek the next characters of a QTextStream reading a QFile, in order to create an efficient tokenizer.
However, I don't find any satisfying solution to do so.
QFile f("test.txt");
f.open(QIODevice::WriteOnly);
f.write("Hello world\nHello universe\n");
f.close();
f.open(QIODevice::ReadOnly);
QTextStream s(&f);
int i = 0;
while (!s.atEnd()) {
++i;
qDebug() << "Peek" << i << s.device()->peek(3);
QString v;
s >> v;
qDebug() << "Word" << i << v;
}
Gives the following output:
Peek 1 "Hel" # it works only the first time
Word 1 "Hello"
Peek 2 ""
Word 2 "world"
Peek 3 ""
Word 3 "Hello"
Peek 4 ""
Word 4 "universe"
Peek 5 ""
Word 5 ""
I tried several implementations, also with QTextStream::pos() and QTextStream::seek(). It works better, but pos() is buggy (returns -1 when the file is too big).
Does anyone have a solution to this recurrent problem? Thank you in advance.
You peek from QIODevice, but then you read from QTextStream, that's why peek works only once. Try this:
while (!s.atEnd()) {
++i;
qDebug() << "Peek" << i << s.device()->peek(3);
QByteArray v = s.device()->readLine ();
qDebug() << "Word" << i << v;
}
Unfortunately, QIODevice does not support reading single words, so you would have to do it yourself with a combination of peak and read.
Try disable QTextStream::autoDetectUnicode. This may read device ahead to perform detection and cause your problem.
Set also a codec just in case.
Add to the logs s.device()->pos() and s.device()->bytesAvailable() to verify that.
I've check QTextStream code. It looks like it always caches as much data as possible and there is no way to disable this behavior. I was expecting that it will use peek on device, but it only reads in greedy way. Bottom line is that you can't use QTextStream and peak device at the same time.
There is a magic number associated with each binary file , does anyone know how to retrieve this information from the file?
file <file_name>
magic numbers are usually stored in (linux):
/usr/share/file/magic
also check this link, someone was trying to use libmagic to get the information in C program, might be useful if you're writing something yourself.
Use libmagic from the file package to try and sniff out the type of file if that's your goal.
There are no general "magic" numbers in binary files on unix, though different formats might define their own. The above library knows about many of those and also use various other heuristics to try and figure out the format/type of file.
The unix file command uses magic number. see the file man page for more.(and where to find the magic file )
Read this: http://linux.die.net/man/5/magic
It's complex, and depends on the specific file type you're looking for.
There is a file command which in turn uses a magic library, the magic library reads from a file found in /etc called magic (this is installation dependant and may vary), which details what are the first few bytes of the file and tells the file what kind of a file it is, be it, jpg, binary, text, shell script. There is an old version of libmagic found on sourceforge. Incidentally, there is a related answer to this here.
Hope this helps,
Best regards,
Tom.
Expounding on #nos's answer:
Example below uses the default magic database to query the file passed on the command line. (Essentially an implementation of the file command. See man libmagic for more details/functions.
#include <iostream>
#include <magic.h>
#include <cassert>
int main(int argc, char **argv) {
if (argc == 1) {
std::cerr << "Usage " << argv[0] << " [filename]" << std::endl;
return -1;
}
const char * fname = argv[1];
magic_t cookie = magic_open(0);
assert (cookie !=nullptr);
int rc = magic_load(cookie, nullptr);
assert(rc == 0);
auto f= magic_file(cookie, fname);
if (f ==nullptr) {
std::cerr << magic_error(cookie) << std::endl;
} else {
std::cout << fname << ' ' << f << std::endl;
}
}