Operations with QString and int

[Swati777999]

I have written the following code to increase the last digit of the QString by 1

QString myStr = "31415";
int myStr_sz = myStr.size();
int last_dig=myStr[myStr_sz-1].digitValue();
last_dig+=1;
debug()<<"New String is =" <<myStr;

This has not modifed myStr . Can someone tell me where I am going wrong?

Thanks in advance!

[Sivan]

@Swati777999 said in Operations with QString and int:

You are assigning ASCII value of 6 to the last character of string. Please refer to ASCII chart online. To simply answer your question, you can add '0' to ur last_dig. That should work.

 myStr[myStr_sz-1] = last_dig + '0'; 

But why are u doing like this? What if the last digit is 9? When u add 1, you will need to add the second last digit as well. A better solution would be to completely convert the string to digit, do the arithmetric operation, and then convert it back to QString

[jsulm]

@Swati777999 said in Operations with QString and int:

This has not modifed myStr

Why should it? You do not modify it.
You also need to replace the last character in myStr with the new number.
Use https://doc.qt.io/qt-5/qstring.html#operator-5b-5d

Also, think about what should happen if last number is 9?

[KroMignon]

@Swati777999 said in Operations with QString and int:

This has not modifed myStr . Can someone tell me where I am going wrong?

Qt is a C++ framework, so please take time to learn some C++ basics.
It looks to me as you are expected Qt/C++ working as another language like Python or JavaScript.
But that's not the case, and will never be!

Each programming language has his own way to work, so please learn C++ if you want to use Qt or maybe PySide (Qt binding for Python) is a better alternative for you.

[Swati777999]

@KroMignon
Thanks for your suggestion. I was working in Python before starting to work in Qt, even though I have studied c++ before, I feel my reasoning is biased towards Python's logic and syntax.

[KroMignon]

@Swati777999 said in Operations with QString and int:

I was working in Python before starting to work in Qt, even though I have studied c++ before

Again Qt is not a programming language, Qt is a framework developped in C++.
Like django is a web framework developped in Python.

[Swati777999]

@jsulm

QString myStr = "31415";
 int myStr_sz = myStr.size();
 int last_dig=myStr[myStr_sz-1].digitValue();
 last_dig+=1;
 myStr[myStr_sz-1] = last_dig;
 qDebug()<<"New String is =" <<myStr;

With this modified code , I got the following result-

Though it's not the desired output , yet I loved the appearance of Spade from nowhere!

[Swati777999]

@KroMignon Yes, I am acquainted with the fact that Qt is the framework of C++ but I struggle to work with its classes and functions.

[J.Hilk]

@Swati777999

what's wrong with

QString myStr = "31415";
int myInt = myStr.toInt();
myStr = QString::number(++myInt);

?

[Swati777999]

@jsulm said in Operations with QString and int:

@Swati777999 said in Operations with QString and int:

Also, think about what should happen if last number is 9?

Yes, this is in my mind, it's the next stage of this trial but before that, I need to be sure about the working of incrementation.

[KroMignon]

@Swati777999 said in Operations with QString and int:

With this modified code , I got the following result-

It is doing what you have coded.

QString myStr = "31415";
 int myStr_sz = myStr.size(); // => myStr = 5
 int last_dig=myStr[myStr_sz-1].digitValue(); // => last_dig = 5
 last_dig+=1; /// => last_dig = 6
 myStr[myStr_sz-1] = last_dig;  // change  myStr[myStr_sz-1] from '5' to '\x06'
 qDebug()<<"New String is =" <<myStr;

EDIT: you have to understand that 5 != '5', the ASCII code for 5 is 57 or '\x35'

[Swati777999]

@J-Hilk Yes, it works absolutely fine. I was trying this with real values say - myStr = " 314.19"

[KroMignon]

@Swati777999 said in Operations with QString and int:

Yes, it works absolutely fine. I was trying this with real values say - myStr = " 314.19"

Please again, don't program in C++ as you would do it in python, this will not work!
There is no implicit convertion from string <=> real or string <=> integer in C++.
I you want to program in python style, use python not C++ or you will only go frustrated.

[J.Hilk]

@Swati777999

template<typename T>
QString addOne(const QString &string)
{
    auto value = std::is_floating_point<T>::value ? string.toDouble() : string.toInt();
    return QString::number(++value);
}

int main(int argc, char *argv[])
{
    qDebug() << addOne<int>("12345") << addOne<float>("1234.56");
}

[mchinand]

It's not entirely clear, but from the wording of the original post (increase the last digit by one) and then in a subsequent message saying it should work with strings that contain floating-point numbers too, I think @Swati777999 wants '35.456' to become '35.457', for example.

[Swati777999]

@mchinand said in Operations with QString and int:

It's not entirely clear, but from the wording of the original post (increase the last digit by one) and then in a subsequent message saying it should work with strings that contain floating-point numbers too, I think @Swati777999 wants '35.456' to become '35.457', for example.

Yes, actually my true intention for putting this Question was about dealing with rounding of real values but I also wanted to be clear about the string and int conversion.

[Swati777999]

@KroMignon
My current project requirement expects me to code in Qt C++ [ As I've mentioned earlier that my mind always approaches the solution of a program in Python first because of spending more time in coding in Python than C++]

[Swati777999]

@KroMignon said in Operations with QString and int:

@Swati777999 said in Operations with QString and int:

With this modified code , I got the following result-

It is doing what you have coded.

QString myStr = "31415";
 int myStr_sz = myStr.size(); // => myStr = 5
 int last_dig=myStr[myStr_sz-1].digitValue(); // => last_dig = 5
 last_dig+=1; /// => last_dig = 6
 myStr[myStr_sz-1] = last_dig;  // change  myStr[myStr_sz-1] from '5' to '\x06'
 qDebug()<<"New String is =" <<myStr;

EDIT: you have to understand that 5 != '5', the ASCII code for 5 is 57 or '\x35'

EXAMPLE-1

int x = 56;
  QString str = QString::number(x);
  qDebug()<<"x = " <<str; // x ="56"

Using Example 1 -
I modified the code

 ```
 QString myStr = "31415";
 qDebug()<<"old String is =" <<myStr;  //31415
 int myStr_sz = myStr.size(); // => myStr = 5
 int last_dig=myStr[myStr_sz-1].digitValue(); // => last_dig = 5
 last_dig+=1; // => last_dig = 6
 qDebug()<<"incremented last dig is="<<last_dig;  // 6
 QString str_last =QString::number(last_dig);  
 qDebug()<<"incremented last dig after converting to string is="<<last_dig; // ==> 6
 myStr[myStr_sz-1] = last_dig; 
 qDebug()<<"New String is =" <<myStr;

I am getting the same old result.

Another way I'm thinking is that, popping out the last digit from the string and appending the incremented last digit . I tried it but didn't work. Please give suggestion .

[Sivan]

@Swati777999 said in Operations with QString and int:

You are assigning ASCII value of 6 to the last character of string. Please refer to ASCII chart online. To simply answer your question, you can add '0' to ur last_dig. That should work.

 myStr[myStr_sz-1] = last_dig + '0'; 

But why are u doing like this? What if the last digit is 9? When u add 1, you will need to add the second last digit as well. A better solution would be to completely convert the string to digit, do the arithmetric operation, and then convert it back to QString

[KroMignon]

@Swati777999 said in Operations with QString and int:

My current project requirement expects me to code in Qt C++ [ As I've mentioned earlier that my mind always approaches the solution of a program in Python first because of spending more time in coding in Python than C++]

So we are totally in opposite about the way to do: programming C++ but thinking the program with Python in mind is, for me, the worst way to do.

C++ and python are so far apart in almost every aspect of programming, I can't see how this can work.
If we are only talking about validating an algorithm, why not. But transposing from Python to C++ is not a simple matter. That alone amounts to a new development.

[Swati777999]

@Sivan said in Operations with QString and int:

@Swati777999 said in Operations with QString and int:

You are assigning ASCII value of 6 to the last character of string. Please refer to ASCII chart online. To simply answer your question, you can add '0' to ur last_dig. That should work.

 myStr[myStr_sz-1] = last_dig + '0'; 

But why are u doing like this? What if the last digit is 9? When u add 1, you will need to add the second last digit as well. A better solution would be to completely convert the string to digit, do the arithmetric operation, and then convert it back to QString

Yes, your suggestion works perfectly well.

    QString myStr = "31415";
    qDebug()<<"old String is =" <<myStr;
    int myStr_int = myStr.toInt();
    myStr_int +=1;
    qDebug()<<"New int value is =" <<myStr_int;

Trying with real vlaues

    QString myStr = "99.99";
    qDebug()<<"old String is =" <<myStr;
    int myStr_int = myStr.toDouble();   //  And:100  [.toFloat() ]
    myStr_int +=1;
    qDebug()<<"New int value is =" <<myStr_int;